Locomotion requires coordination of leg joints to maintain stability and to maneuver. We studied ... more Locomotion requires coordination of leg joints to maintain stability and to maneuver. We studied leg joint function during constant-average-velocity running and the sagittal-plane maneuvers of step ascent and descent. We tested two hypotheses: (1) that leg joints perform distinct functions during locomotion; and (2) that humans select functional parameters to maximize intrinsic dynamic stability. We recorded whole-body kinematics and forces when participants stepped up or down a single vertical step, and found that leg joints show functional differences during both constant-average-velocity locomotion and maneuvers. The hip, knee and ankle function as a motor, damper, and spring, respectively. We therefore constructed a simplified computational model of a human leg with a motor, damper, and spring in series (MDS). The intrinsic dynamics of the model resulted in sustained locomotion on level ground within narrow parameter ranges. However, using parameters experimentally derived from humans, the model showed only short-term stability. Humans may not optimize intrinsic dynamic stability alone, but may instead choose mechanical and behavioral parameters appropriate for both constant-average-velocity locomotion and maneuvers. Understanding joint-level mechanical function during unsteady locomotion helps to understand how differential joint function contributes to whole-body performance, and could lead to improvements in rehabilitation, prosthetic and robotic design.
Background
Older adults are more susceptible to balance perturbations during walking than young a... more Background Older adults are more susceptible to balance perturbations during walking than young adults. However, we lack an individual joint-level understanding of how aging affects the neuromechanical strategies used to accommodate balance perturbations.
Research Question We investigated gait phase-dependence in and aging effects on leg joint kinematic variability during walking with balance perturbations. We hypothesized that leg joint variability would: 1) vary across the gait cycle and 2) increase with balance perturbations. We also hypothesized that perturbation effects on leg joint kinematic variability would be larger and more pervasive in older versus young adults.
Methods We collected leg joint kinematics in young and older adults walking with and without mediolateral optical flow perturbations of different amplitudes.
Results We first found that leg joint variability during walking is gait phase-dependent, with step-to-step adjustments occurring predominantly during push-off and early swing. Second, young adults accommodated perturbations almost exclusively by increasing coronal plane hip joint variability, likely to adjust step width. Third, perturbations elicited larger and more pervasive increases in all joint kinematic outcome measures in older adults. Finally, we also provide insight into which joints contribute more to foot placement variability in walking, adding that variability in sagittal plane knee and coronal plane hip joint angles contributed most to that in step length and step width, respectively.
Significance Taken together, our findings may be highly relevant to identifying specific joint-level therapeutic targets to mitigate balance impairment in our aging population.
Whole-body kinematics and kinetics impact pitching performance, and when coordination of momentum... more Whole-body kinematics and kinetics impact pitching performance, and when coordination of momentum transfer is impacted, throwing-related injury risks increase. Although correlations between overhead throwing velocity and lower-body power measured by jump tests have been reported by previous studies, further research is needed to identify these relationships to better understand pitching mechanics and the validity and application of jump tests for the assessment of baseball pitchers. This review communicates effective whole-body pitching mechanics, including throwing arm, trunk, and pelvis, and lower body, as well as discusses the applicability of strength and power field tests to deepen training insights in establishing more efficient motor patterns. If both lower-body power and coordination of momentum transfer are optimized, baseball pitchers may experience reduced throwing arm stress. The overarching insight to be gained from this review article is that high performance in baseball pitching is multifactorial, and attention to both strength and skill coaching is paramount. As a result, the intersection of lower-body power testing and biomechanical analysis can play an important role in uniting strength and conditioning professionals, clinicians, pitching coaches, and biomechanics experts to advance pitching performance and health in the sport of baseball.
Locomotion in a complex environment is often not steady, but the mechanisms used by animals to po... more Locomotion in a complex environment is often not steady, but the mechanisms used by animals to power and control unsteady locomotion (stability and maneuverability) are not well understood. We use behavioral, morphological, and impulsive perturbations to determine the compensations used during unsteady locomotion. At the level both of the whole-body and of joints, quasi-stiffness models are useful for describing adjustments to the functioning of legs and joints during maneuvers. However, alterations to the mechanics of legs and joints often are distinct for different phases of the step cycle or for specific joints. For example, negotiating steps involves independent changes of leg stiffness during compression and thrust phases of stance. Unsteady locomotion also involves parameters that are not part of the simplest reduced-parameter models of locomotion (e.g., the spring-loaded inverted pendulum) such as moments of the hip joint. Extensive coupling among translational and rotational parameters must be taken into account to stabilize locomotion or maneuver. For example, maneuvers with morphological perturbations (increased rotational inertial turns) involve changes to several aspects of movement, including the initial conditions of rotation and ground-reaction forces. Coupled changes to several parameters may be employed to control maneuvers on a trial-by-trial basis. Compensating for increased rotational inertia of the body during turns is facilitated by the opposing effects of several mechanical and behavioral parameters. However, the specific rules used by animals to control translation and rotation of the body to maintain stability or maneuver have not been fully characterized. We initiated direct-perturbation experiments to investigate the strategies used by humans to maintain stability following center-of-mass (COM) perturbations. When walking, humans showed more resistance to medio-lateral perturbations (lower COM displacement). However, when running, humans could recover from the point of maximum COM displacement faster than when walking. Consequently, the total time necessary for recovery was not significantly different between walking and running. Future experiments will determine the mechanisms used for compensations during unsteady locomotion at the behavioral, joint, and muscle levels. Using reduced-parameter models will allow common experimental and analytical frameworks for the study of both stability and maneuverability and the determination of general control strategies for unsteady locomotion.
Locomotion in a complex environment is often not steady state, but unsteady locomotion (stability... more Locomotion in a complex environment is often not steady state, but unsteady locomotion (stability and maneuverability) is not well understood. We investigated the strategies used by humans to perform sidestep cutting turns when running. Previous studies have argued that because humans have small yaw rotational moments of inertia relative to body mass, deceleratory forces in the initial velocity direction that occur during the turning step, or 'braking' forces, could function to prevent body over-rotation during turns. We tested this hypothesis by increasing body rotational inertia and testing whether braking forces during stance decreased. We recorded ground reaction force and body kinematics from seven participants performing 45 deg sidestep cutting turns and straight running at five levels of body rotational inertia, with increases up to fourfold. Contrary to our prediction, braking forces remained consistent at different rotational inertias, facilitated by anticipatory changes to body rotational speed. Increasing inertia revealed that the opposing effects of several turning parameters, including rotation due to symmetrical anterior-posterior forces, result in a system that can compensate for fourfold changes in rotational inertia with less than 50% changes to rotational velocity. These results suggest that in submaximal effort turning, legged systems may be robust to changes in morphological parameters, and that compensations can involve relatively minor adjustments between steps to change initial stance conditions.
Sakurai, M, Qiao, M, Szymanski, DJ, and Crotin, RL. Countermovement jump and momentum generation ... more Sakurai, M, Qiao, M, Szymanski, DJ, and Crotin, RL. Countermovement jump and momentum generation associations to fastball velocity performance among Division I collegiate pitchers. J Strength Cond Res 38(7): 1288-1294, 2024-The current study explored the relationships between countermovement jump (CMJ) profiles and baseball pitching performance. Nineteen Division I collegiate pitchers performed in-laboratory pitching and bilateral CMJs. Whole-body kinematics and ground reaction force were collected during both pitching and CMJ evaluations. Statistically significant correlations of concentric impulse and peak power in the CMJ test with fastball velocity were observed (r 5 0.71 and 0.68). Concentric impulse in CMJ also showed a statistically significant correlation with linear momentum in the anterior-posterior direction during pitching (r 5 0.68). Lean body mass and body mass showed statistically significant correlations with both of the 2 linear momentums during pitching (r 5 0.71;0.83), and concentric impulse in CMJ (r 5 0.71 and 0.81). Pelvis and trunk pitching mechanics did not correlate with any of the CMJ variables at the statistically significant level, whereas the direction of the correlations varied (|r| , 0.45). Assessment of a baseball pitcher's CMJ should focus on concentric impulse and peak power because only these showed meaningful relationships with fastball velocity or momentum generation during pitching. An increase in lean body mass is also suggested to be able to generate more impulse and momentum. Baseball coaches, strength coaches, and clinicians are encouraged to include lower-body explosive training to enhance the force and power output capacity of baseball pitchers.
A B S T R A C T Background: Older adults are at an exceptionally high risk of falls, and most fal... more A B S T R A C T Background: Older adults are at an exceptionally high risk of falls, and most falls occur during locomotor activities such as walking. Reduced local dynamic stability in old age is often interpreted to suggest a lessened capacity to respond to more significant balance challenges encountered during walking and future falls risk. However, it remains unclear whether local dynamic stability during normal, unperturbed walking predicts the response to larger external balance disturbances. Research question: We tested the hypothesis that larger values of local dynamic instability during unperturbed walking would positively correlate with larger changes thereof due to optical flow balance perturbations. Methods: We used trunk kinematics collected in subjects across a spectrum of walking balance integrity – young adults, older non-fallers, and older fallers – during walking with and without mediolateral optical flow perturbations of four different amplitudes. Results: We first found evidence that optical flow perturbations of sufficient amplitude appear capable of revealing independent effects of aging and falls history that are not otherwise apparent during normal, un-perturbed walking. We also reject our primary hypothesis; a significant negative correlation only in young adults indicated that individuals with more local dynamic instability during normal, unperturbed walking exhibited smaller responses to optical flow perturbations. In contrast, most prominently in older fallers, the response to optical flow perturbations appeared independent of their baseline level of dynamic instability. Significance: We propose that predicting the response to balance perturbations in older fallers, at least that measured using local dynamic stability, likely requires measuring that response directly.
The purpose of this pilot study was to establish the efficacy and feasibility of a single-session... more The purpose of this pilot study was to establish the efficacy and feasibility of a single-session treadmill-based stance-slip perturbation program on preventing slip-related falls while walking over the ground among young adults. Two groups (training vs. control) of healthy young participants were respectively exposed to a treadmill-based stance-slip perturbation training protocol and a placebo training protocol. Post training, both groups experienced an unexpected overground gait-slip. Our results indicated that 28.6% of individuals in the training group and 55.0% of controls fell when responding to the overground slip. In comparison with the control group, the training group exhibited better control over the compensatory step and dynamic stability at the instant immediately prior to recovery touchdown. The improved dynamic stability control in the training group likely resulted from the enhanced capability of harnessing the slip kinematics of the base of support. Dynamic stability did not display any significant group-associated difference at slipping foot touchdown and recovery foot liftoff. This implies that a stance-slip perturbation training protocol with eight slips may not provide enough and very task-specific incentive to the Central Nervous System to form the capability of sufficiently modifying regular gait pattern after an unexpected gait slip. However, given its ease of use, stance-perturbation could be a practical option to train individuals in clinical settings as a simple push or pull could exert a perturbation to a standing individual. The findings from this study provide information for developing future studies based on large-scale samples.
This study tested two hypotheses on the nature of unintentional force drifts elicited by removing... more This study tested two hypotheses on the nature of unintentional force drifts elicited by removing visual feedback during accurate force production tasks. The role of working memory (memory hypothesis) was explored in tasks with continuous force production, intermittent force production, and rest intervals over the same time interval. The assumption of unintentional drifts in referent coordinate for the fingertips was tested using manipulations of visual feedback: young healthy subjects performed accurate steady-state force production tasks by pressing with the two index fingers on individual force sensors with visual feedback on the total force, sharing ratio, both, or none. Predictions based on the memory hypothesis have been falsified. In particular, we observed consistent force drifts to lower force values during continuous force production trials only. No force drift or drifts to higher forces were observed during intermittent force production trials and following rest intervals. The hypotheses based on the idea of drifts in referent finger coordinates have been confirmed. In particular, we observed superposition of two drift processes: a drift of total force to lower magnitudes and a drift of the sharing ratio to 50:50. When visual feedback on total force only was provided, the two-finger forces showed drifts in opposite directions. We interpret the findings as evidence for the control of motor actions with changes in referent coordinates for participating effectors. Unintentional drifts in performance are viewed as natural relaxation processes in the involved systems; their typical time reflects stability in the direction of the drift. The magnitude of the drift was higher in the right (dominant) hand, which is consistent with the dynamic dominance hypothesis.
Although constant-average-velocity walking has been extensively studied, less is known about walk... more Although constant-average-velocity walking has been extensively studied, less is known about walking maneuvers that change speed. We investigated the function of individual leg joints when humans walked at a constant speed, accelerated or decelerated. We hypothesized that leg joints make different functional contributions to maneuvers. Specifically, we hypothesized that the hip generates positive mechanical work (acting like a “motor”), the knee generates little mechanical work (acting like a “strut”), and the ankle absorbs energy during the first half of stance and generates energy during the second half (consistent with “spring”-like function). We recorded full body kinematics and kinetics, used inverse dynamics to estimate net joint moments, and decomposed joint function into strut-, motor-, damper-, and spring-like components using indices based on net joint work. Although overall leg mechanics were primarily strut-like, individual joints did not act as struts during stance. The hip functioned as a power generating “motor,” and ankle function was consistent with spring-like behavior. Even though net knee work was small, the knee did not behave solely as a strut but also showed motor-, and damper-like function. Acceleration involved increased motor-like function of the hip and ankle. Deceleration involved decreased hip motor-like function and ankle spring-like function and increased damping at the knee and ankle. Changes to joint mechanical work were primarily due to changes in joint angular displacements and not net moments. Overall, joints maintain different functional roles during unsteady locomotion.
Journal of Biomechanical Engineering-Transactions of the ASME, 2020
Falling backward can lead to injuries including hip fracture, back injury, and traumatic brain im... more Falling backward can lead to injuries including hip fracture, back injury, and traumatic brain impact among older adults. A loss of consciousness is associated with falling backward and accounts for about 13% of all falls among older adults. Little is known about the dynamics of backward falls, such as the falling duration, the impact severity, and how the fall dynamics are affected by the biomechanical properties of the lower limb joints, particularly the rotational stiffness. The purpose of this study was to investigate the influence of the stiffness of individual leg joints on the dynamics of backward falls after losing consciousness in terms of the falling duration and impact velocities. Based on a 15-segment human model, we simulated the process of falling backwards by sweeping the parameter space of ankle, knee, and hip's stiffness varying from 0 to 8.73 Nm/deg (or 500 Nm/rad). The results revealed that the falling duration and impact speeds of the head and hip ranged from 0.27 to 0.63 s, 2.65 to 7.88 m/s, and 0.35 to 3.36 m/s, respectively, when the stiffness of the leg joints changed within their limits. Overall, the influence of the joint stiffness on the falling dynamics (falling duration and impact speed) is comparable between hip and knee joints. Whereas, ankle stiffness showed little influence on the backward falling dynamics. Our findings could provide references for designing protective devices to prevent impact-induced injuries after a backward fall.
Maneuverability is essential for locomotion. For animals in the environment, maneuverability is d... more Maneuverability is essential for locomotion. For animals in the environment, maneuverability is directly related to survival. For humans, maneuvers such as turning are associated with increased risk for injury, either directly through tissue loading or indirectly through destabilization. Consequently, understanding the mechanics and motor control of maneuverability is a critical part of locomotion research. We briefly review the literature on maneuvering during locomotion with a focus on turning in bipeds. Walking turns can use one of several different strategies. Anticipation can be important to adjust kinematics and dynamics for smooth and stable maneuvers. During running, turns may be substantially constrained by the requirement for body orientation to match movement direction at the end of a turn. A simple mathematical model based on the requirement for rotation to match direction can describe leg forces used by bipeds (humans and ostriches). During running turns, both humans and ostriches control body rotation by generating fore-aft forces. However, whereas humans must generate large braking forces to prevent body over-rotation, ostriches do not. For ostriches, generating the lateral forces necessary to change movement direction results in appropriate body rotation. Although ostriches required smaller braking forces due in part to increased rotational inertia relative to body mass, other movement parameters also played a role. Turning performance resulted from the coordinated behavior of an integrated biomechanical system. Results from preliminary experiments on horizontal-plane stabilization support the hypothesis that controlling body rotation is an important aspect of stable maneuvers. In humans, body orientation relative to movement direction is rapidly stabilized during running turns within the minimum of two steps theoretically required to complete analogous maneuvers. During straight running and cutting turns, humans exhibit spring-mass behavior in the horizontal plane. Changes in the horizontal projection of leg length were linearly related to changes in horizontal-plane leg forces. Consequently, the passive dynamic stabilization associated with spring-mass behavior may contribute to stability during maneuvers in bipeds. Understanding the mechanics of maneuverability will be important for understanding the motor control of maneuvers and also potentially be useful for understanding stability.
We explored a recently discovered phenomenon that smooth transient perturbations applied to the h... more We explored a recently discovered phenomenon that smooth transient perturbations applied to the hand can lead to violations of equifinality. Healthy subjects occupied an initial hand position against a bias force and tried not to interfere with hand displacements produced by changes in the force. The force changes were smooth and transient (ending up with the same bias force value), with or without a time interval (dwell time) between the force change application and removal. They could lead to an increase or a decrease in the bias force. The subjects performed the task with eyes open and closed. After the force change was over, the hand stopped consistently short of the initial position only when the initial force change increased the bias force. No consistent positional errors were seen for the opposite force change direction. These results were consistent across trials with and without dwell time performed with and without vision. We conclude that the positional errors were not due to muscle properties but reflected a drift in the hand referent coordinate within the central nervous system triggered by the perturbation and driven by the difference between the actual and referent hand coordinates during the dwell time.
Multiple sclerosis and related disorders, Jan 9, 2018
Mobility impairment is common in people with multiple sclerosis (MS). Gait speed has shown strong... more Mobility impairment is common in people with multiple sclerosis (MS). Gait speed has shown strong correlations with other mobility measures in MS. The purpose of this study was to assess the relative importance of a battery of factors in determining gait speed among people with MS. Thirty-one individuals with MS (the mean (standard deviation) of the Patient Determined Disability Steps: 3.68(1.70)) participated in this cross-sectional observational study. Their gait speed was assessed using the Timed-25-Foot Walking test. Six factors which could slow gait speed in MS, including the strength capacity at knee joints, functional mobility, body balance, dorsiflexion range of motion of ankle joints, bilateral foot cutaneous sensation level, and the fear of falling, were also assessed. Multiple regression and relative weight analysis were used to identify the relative importance of each factor in explaining the gait speed variation. All six factors together accounted for about 86% of the o...
BACKGROUND: Treadmill-based gait-slip training shows to be effective in reducing the risk of slip... more BACKGROUND: Treadmill-based gait-slip training shows to be effective in reducing the risk of slip-related falls. In previous relevant studies, the number of repeated slip perturbations ranged from 12 to 30. RESEARCH QUESTION: It is unclear if a reduced number of treadmill-slips can still yield adaptive strategies to lower the likelihood of falls after a slip over ground. This study examined if eight repeated slips on a treadmill reduced the risk of falls among young adults when they were exposed to a novel overground slip. METHODS: Forty-three healthy young adults were randomized into either training or control group. The training group underwent an 8-slip perturbation training procedure on a treadmill while the control group received the same number of normal walking trials on the same treadmill. Following the training, both groups were exposed to an unrehearsed slip during overground walking. Their body's reactions to the novel overground slip were collected by a motion capture system. RESULTS: The training group exhibited significantly better reactions to the slip than did the control group, evidenced by the lower fall proportion and improved dynamic stability at recovery foot touchdown during the overground slip. No improvement in dynamic stability was detected in the training group at the slipping foot touchdown and recovery foot liftoff. SIGNIFICANCE: The results suggested that the shortened perturbation training program may be efficacious in improving responses to a novel overground slip but may not be as effective as protocols using greater number of slips. This study could provide guidance for selecting the number of slips for future perturbation-based training protocols.
Multidirectional ground reaction forces (GRFs) and jump tests within baseball pitchers provide in... more Multidirectional ground reaction forces (GRFs) and jump tests within baseball pitchers provide insight into athletic ability and coordination to produce lower-body force and power. Lowerbody power is a biomechanical feature that denotes physiological capacity through dynamic and passive tissue stretch-shortening in transferring energy from the ground through the kinetic chain. Optimized lower-body power may lessen the magnitude of forces on the upper extremity. Insufficient lower-body power may create a greater risk of upper-body injury. Lower-body power and its relationship to ball velocity have been minimally investigated, yet some research points to a correlation between jumping ability and fastball velocity. Because pitching is unilateral, practitioners should consider unilateral jumps to determine the extent of bilateral asymmetry or stride to drive leg differences that can guide training to remediate deficiencies. The purposes of this brief review are to (a) examine factors that influence vertical jump performance among baseball players, (b) examine research on pitching multidirectional GRFs, and (c) examine literature concerning jump performances to baseball pitching performance. Collectively, this review can assist coaches and practitioners in lower-body power testing and training for baseball pitchers.
The development of performance, such as learning a new motor skill, can be represented in a perfo... more The development of performance, such as learning a new motor skill, can be represented in a performance curve. The shape of the performance curve is both of theoretical and practical relevance. Here, the author studied the interday performance of juggling over a period of 17 days in 112 college students. The results showed that 60% of participants followed an S-shaped performance curve with the inflection date on the 11th day, followed by a decelerated (20%), accelerated (14%), and linear curve (6%). As expected, except on Day 1, male participants performed at least 33% better than female participants on each practice day. Also as expected, learning performance was found to depend on the type of performance curve with the best learning performance exhibited by the linear group. The results further revealed that pooling all participants' performance together without considering the percentage of each underlying type of performance curve would lead to biased, nonrepresentative results. Given the variety of the observed performance curves and the dominance of the S-shaped performance curve among them, coaches should continuously monitor the shape of an individual's performance curve.
Locomotion requires coordination of leg joints to maintain stability and to maneuver. We studied ... more Locomotion requires coordination of leg joints to maintain stability and to maneuver. We studied leg joint function during constant-average-velocity running and the sagittal-plane maneuvers of step ascent and descent. We tested two hypotheses: (1) that leg joints perform distinct functions during locomotion; and (2) that humans select functional parameters to maximize intrinsic dynamic stability. We recorded whole-body kinematics and forces when participants stepped up or down a single vertical step, and found that leg joints show functional differences during both constant-average-velocity locomotion and maneuvers. The hip, knee and ankle function as a motor, damper, and spring, respectively. We therefore constructed a simplified computational model of a human leg with a motor, damper, and spring in series (MDS). The intrinsic dynamics of the model resulted in sustained locomotion on level ground within narrow parameter ranges. However, using parameters experimentally derived from humans, the model showed only short-term stability. Humans may not optimize intrinsic dynamic stability alone, but may instead choose mechanical and behavioral parameters appropriate for both constant-average-velocity locomotion and maneuvers. Understanding joint-level mechanical function during unsteady locomotion helps to understand how differential joint function contributes to whole-body performance, and could lead to improvements in rehabilitation, prosthetic and robotic design.
Background
Older adults are more susceptible to balance perturbations during walking than young a... more Background Older adults are more susceptible to balance perturbations during walking than young adults. However, we lack an individual joint-level understanding of how aging affects the neuromechanical strategies used to accommodate balance perturbations.
Research Question We investigated gait phase-dependence in and aging effects on leg joint kinematic variability during walking with balance perturbations. We hypothesized that leg joint variability would: 1) vary across the gait cycle and 2) increase with balance perturbations. We also hypothesized that perturbation effects on leg joint kinematic variability would be larger and more pervasive in older versus young adults.
Methods We collected leg joint kinematics in young and older adults walking with and without mediolateral optical flow perturbations of different amplitudes.
Results We first found that leg joint variability during walking is gait phase-dependent, with step-to-step adjustments occurring predominantly during push-off and early swing. Second, young adults accommodated perturbations almost exclusively by increasing coronal plane hip joint variability, likely to adjust step width. Third, perturbations elicited larger and more pervasive increases in all joint kinematic outcome measures in older adults. Finally, we also provide insight into which joints contribute more to foot placement variability in walking, adding that variability in sagittal plane knee and coronal plane hip joint angles contributed most to that in step length and step width, respectively.
Significance Taken together, our findings may be highly relevant to identifying specific joint-level therapeutic targets to mitigate balance impairment in our aging population.
Whole-body kinematics and kinetics impact pitching performance, and when coordination of momentum... more Whole-body kinematics and kinetics impact pitching performance, and when coordination of momentum transfer is impacted, throwing-related injury risks increase. Although correlations between overhead throwing velocity and lower-body power measured by jump tests have been reported by previous studies, further research is needed to identify these relationships to better understand pitching mechanics and the validity and application of jump tests for the assessment of baseball pitchers. This review communicates effective whole-body pitching mechanics, including throwing arm, trunk, and pelvis, and lower body, as well as discusses the applicability of strength and power field tests to deepen training insights in establishing more efficient motor patterns. If both lower-body power and coordination of momentum transfer are optimized, baseball pitchers may experience reduced throwing arm stress. The overarching insight to be gained from this review article is that high performance in baseball pitching is multifactorial, and attention to both strength and skill coaching is paramount. As a result, the intersection of lower-body power testing and biomechanical analysis can play an important role in uniting strength and conditioning professionals, clinicians, pitching coaches, and biomechanics experts to advance pitching performance and health in the sport of baseball.
Locomotion in a complex environment is often not steady, but the mechanisms used by animals to po... more Locomotion in a complex environment is often not steady, but the mechanisms used by animals to power and control unsteady locomotion (stability and maneuverability) are not well understood. We use behavioral, morphological, and impulsive perturbations to determine the compensations used during unsteady locomotion. At the level both of the whole-body and of joints, quasi-stiffness models are useful for describing adjustments to the functioning of legs and joints during maneuvers. However, alterations to the mechanics of legs and joints often are distinct for different phases of the step cycle or for specific joints. For example, negotiating steps involves independent changes of leg stiffness during compression and thrust phases of stance. Unsteady locomotion also involves parameters that are not part of the simplest reduced-parameter models of locomotion (e.g., the spring-loaded inverted pendulum) such as moments of the hip joint. Extensive coupling among translational and rotational parameters must be taken into account to stabilize locomotion or maneuver. For example, maneuvers with morphological perturbations (increased rotational inertial turns) involve changes to several aspects of movement, including the initial conditions of rotation and ground-reaction forces. Coupled changes to several parameters may be employed to control maneuvers on a trial-by-trial basis. Compensating for increased rotational inertia of the body during turns is facilitated by the opposing effects of several mechanical and behavioral parameters. However, the specific rules used by animals to control translation and rotation of the body to maintain stability or maneuver have not been fully characterized. We initiated direct-perturbation experiments to investigate the strategies used by humans to maintain stability following center-of-mass (COM) perturbations. When walking, humans showed more resistance to medio-lateral perturbations (lower COM displacement). However, when running, humans could recover from the point of maximum COM displacement faster than when walking. Consequently, the total time necessary for recovery was not significantly different between walking and running. Future experiments will determine the mechanisms used for compensations during unsteady locomotion at the behavioral, joint, and muscle levels. Using reduced-parameter models will allow common experimental and analytical frameworks for the study of both stability and maneuverability and the determination of general control strategies for unsteady locomotion.
Locomotion in a complex environment is often not steady state, but unsteady locomotion (stability... more Locomotion in a complex environment is often not steady state, but unsteady locomotion (stability and maneuverability) is not well understood. We investigated the strategies used by humans to perform sidestep cutting turns when running. Previous studies have argued that because humans have small yaw rotational moments of inertia relative to body mass, deceleratory forces in the initial velocity direction that occur during the turning step, or 'braking' forces, could function to prevent body over-rotation during turns. We tested this hypothesis by increasing body rotational inertia and testing whether braking forces during stance decreased. We recorded ground reaction force and body kinematics from seven participants performing 45 deg sidestep cutting turns and straight running at five levels of body rotational inertia, with increases up to fourfold. Contrary to our prediction, braking forces remained consistent at different rotational inertias, facilitated by anticipatory changes to body rotational speed. Increasing inertia revealed that the opposing effects of several turning parameters, including rotation due to symmetrical anterior-posterior forces, result in a system that can compensate for fourfold changes in rotational inertia with less than 50% changes to rotational velocity. These results suggest that in submaximal effort turning, legged systems may be robust to changes in morphological parameters, and that compensations can involve relatively minor adjustments between steps to change initial stance conditions.
Sakurai, M, Qiao, M, Szymanski, DJ, and Crotin, RL. Countermovement jump and momentum generation ... more Sakurai, M, Qiao, M, Szymanski, DJ, and Crotin, RL. Countermovement jump and momentum generation associations to fastball velocity performance among Division I collegiate pitchers. J Strength Cond Res 38(7): 1288-1294, 2024-The current study explored the relationships between countermovement jump (CMJ) profiles and baseball pitching performance. Nineteen Division I collegiate pitchers performed in-laboratory pitching and bilateral CMJs. Whole-body kinematics and ground reaction force were collected during both pitching and CMJ evaluations. Statistically significant correlations of concentric impulse and peak power in the CMJ test with fastball velocity were observed (r 5 0.71 and 0.68). Concentric impulse in CMJ also showed a statistically significant correlation with linear momentum in the anterior-posterior direction during pitching (r 5 0.68). Lean body mass and body mass showed statistically significant correlations with both of the 2 linear momentums during pitching (r 5 0.71;0.83), and concentric impulse in CMJ (r 5 0.71 and 0.81). Pelvis and trunk pitching mechanics did not correlate with any of the CMJ variables at the statistically significant level, whereas the direction of the correlations varied (|r| , 0.45). Assessment of a baseball pitcher's CMJ should focus on concentric impulse and peak power because only these showed meaningful relationships with fastball velocity or momentum generation during pitching. An increase in lean body mass is also suggested to be able to generate more impulse and momentum. Baseball coaches, strength coaches, and clinicians are encouraged to include lower-body explosive training to enhance the force and power output capacity of baseball pitchers.
A B S T R A C T Background: Older adults are at an exceptionally high risk of falls, and most fal... more A B S T R A C T Background: Older adults are at an exceptionally high risk of falls, and most falls occur during locomotor activities such as walking. Reduced local dynamic stability in old age is often interpreted to suggest a lessened capacity to respond to more significant balance challenges encountered during walking and future falls risk. However, it remains unclear whether local dynamic stability during normal, unperturbed walking predicts the response to larger external balance disturbances. Research question: We tested the hypothesis that larger values of local dynamic instability during unperturbed walking would positively correlate with larger changes thereof due to optical flow balance perturbations. Methods: We used trunk kinematics collected in subjects across a spectrum of walking balance integrity – young adults, older non-fallers, and older fallers – during walking with and without mediolateral optical flow perturbations of four different amplitudes. Results: We first found evidence that optical flow perturbations of sufficient amplitude appear capable of revealing independent effects of aging and falls history that are not otherwise apparent during normal, un-perturbed walking. We also reject our primary hypothesis; a significant negative correlation only in young adults indicated that individuals with more local dynamic instability during normal, unperturbed walking exhibited smaller responses to optical flow perturbations. In contrast, most prominently in older fallers, the response to optical flow perturbations appeared independent of their baseline level of dynamic instability. Significance: We propose that predicting the response to balance perturbations in older fallers, at least that measured using local dynamic stability, likely requires measuring that response directly.
The purpose of this pilot study was to establish the efficacy and feasibility of a single-session... more The purpose of this pilot study was to establish the efficacy and feasibility of a single-session treadmill-based stance-slip perturbation program on preventing slip-related falls while walking over the ground among young adults. Two groups (training vs. control) of healthy young participants were respectively exposed to a treadmill-based stance-slip perturbation training protocol and a placebo training protocol. Post training, both groups experienced an unexpected overground gait-slip. Our results indicated that 28.6% of individuals in the training group and 55.0% of controls fell when responding to the overground slip. In comparison with the control group, the training group exhibited better control over the compensatory step and dynamic stability at the instant immediately prior to recovery touchdown. The improved dynamic stability control in the training group likely resulted from the enhanced capability of harnessing the slip kinematics of the base of support. Dynamic stability did not display any significant group-associated difference at slipping foot touchdown and recovery foot liftoff. This implies that a stance-slip perturbation training protocol with eight slips may not provide enough and very task-specific incentive to the Central Nervous System to form the capability of sufficiently modifying regular gait pattern after an unexpected gait slip. However, given its ease of use, stance-perturbation could be a practical option to train individuals in clinical settings as a simple push or pull could exert a perturbation to a standing individual. The findings from this study provide information for developing future studies based on large-scale samples.
This study tested two hypotheses on the nature of unintentional force drifts elicited by removing... more This study tested two hypotheses on the nature of unintentional force drifts elicited by removing visual feedback during accurate force production tasks. The role of working memory (memory hypothesis) was explored in tasks with continuous force production, intermittent force production, and rest intervals over the same time interval. The assumption of unintentional drifts in referent coordinate for the fingertips was tested using manipulations of visual feedback: young healthy subjects performed accurate steady-state force production tasks by pressing with the two index fingers on individual force sensors with visual feedback on the total force, sharing ratio, both, or none. Predictions based on the memory hypothesis have been falsified. In particular, we observed consistent force drifts to lower force values during continuous force production trials only. No force drift or drifts to higher forces were observed during intermittent force production trials and following rest intervals. The hypotheses based on the idea of drifts in referent finger coordinates have been confirmed. In particular, we observed superposition of two drift processes: a drift of total force to lower magnitudes and a drift of the sharing ratio to 50:50. When visual feedback on total force only was provided, the two-finger forces showed drifts in opposite directions. We interpret the findings as evidence for the control of motor actions with changes in referent coordinates for participating effectors. Unintentional drifts in performance are viewed as natural relaxation processes in the involved systems; their typical time reflects stability in the direction of the drift. The magnitude of the drift was higher in the right (dominant) hand, which is consistent with the dynamic dominance hypothesis.
Although constant-average-velocity walking has been extensively studied, less is known about walk... more Although constant-average-velocity walking has been extensively studied, less is known about walking maneuvers that change speed. We investigated the function of individual leg joints when humans walked at a constant speed, accelerated or decelerated. We hypothesized that leg joints make different functional contributions to maneuvers. Specifically, we hypothesized that the hip generates positive mechanical work (acting like a “motor”), the knee generates little mechanical work (acting like a “strut”), and the ankle absorbs energy during the first half of stance and generates energy during the second half (consistent with “spring”-like function). We recorded full body kinematics and kinetics, used inverse dynamics to estimate net joint moments, and decomposed joint function into strut-, motor-, damper-, and spring-like components using indices based on net joint work. Although overall leg mechanics were primarily strut-like, individual joints did not act as struts during stance. The hip functioned as a power generating “motor,” and ankle function was consistent with spring-like behavior. Even though net knee work was small, the knee did not behave solely as a strut but also showed motor-, and damper-like function. Acceleration involved increased motor-like function of the hip and ankle. Deceleration involved decreased hip motor-like function and ankle spring-like function and increased damping at the knee and ankle. Changes to joint mechanical work were primarily due to changes in joint angular displacements and not net moments. Overall, joints maintain different functional roles during unsteady locomotion.
Journal of Biomechanical Engineering-Transactions of the ASME, 2020
Falling backward can lead to injuries including hip fracture, back injury, and traumatic brain im... more Falling backward can lead to injuries including hip fracture, back injury, and traumatic brain impact among older adults. A loss of consciousness is associated with falling backward and accounts for about 13% of all falls among older adults. Little is known about the dynamics of backward falls, such as the falling duration, the impact severity, and how the fall dynamics are affected by the biomechanical properties of the lower limb joints, particularly the rotational stiffness. The purpose of this study was to investigate the influence of the stiffness of individual leg joints on the dynamics of backward falls after losing consciousness in terms of the falling duration and impact velocities. Based on a 15-segment human model, we simulated the process of falling backwards by sweeping the parameter space of ankle, knee, and hip's stiffness varying from 0 to 8.73 Nm/deg (or 500 Nm/rad). The results revealed that the falling duration and impact speeds of the head and hip ranged from 0.27 to 0.63 s, 2.65 to 7.88 m/s, and 0.35 to 3.36 m/s, respectively, when the stiffness of the leg joints changed within their limits. Overall, the influence of the joint stiffness on the falling dynamics (falling duration and impact speed) is comparable between hip and knee joints. Whereas, ankle stiffness showed little influence on the backward falling dynamics. Our findings could provide references for designing protective devices to prevent impact-induced injuries after a backward fall.
Maneuverability is essential for locomotion. For animals in the environment, maneuverability is d... more Maneuverability is essential for locomotion. For animals in the environment, maneuverability is directly related to survival. For humans, maneuvers such as turning are associated with increased risk for injury, either directly through tissue loading or indirectly through destabilization. Consequently, understanding the mechanics and motor control of maneuverability is a critical part of locomotion research. We briefly review the literature on maneuvering during locomotion with a focus on turning in bipeds. Walking turns can use one of several different strategies. Anticipation can be important to adjust kinematics and dynamics for smooth and stable maneuvers. During running, turns may be substantially constrained by the requirement for body orientation to match movement direction at the end of a turn. A simple mathematical model based on the requirement for rotation to match direction can describe leg forces used by bipeds (humans and ostriches). During running turns, both humans and ostriches control body rotation by generating fore-aft forces. However, whereas humans must generate large braking forces to prevent body over-rotation, ostriches do not. For ostriches, generating the lateral forces necessary to change movement direction results in appropriate body rotation. Although ostriches required smaller braking forces due in part to increased rotational inertia relative to body mass, other movement parameters also played a role. Turning performance resulted from the coordinated behavior of an integrated biomechanical system. Results from preliminary experiments on horizontal-plane stabilization support the hypothesis that controlling body rotation is an important aspect of stable maneuvers. In humans, body orientation relative to movement direction is rapidly stabilized during running turns within the minimum of two steps theoretically required to complete analogous maneuvers. During straight running and cutting turns, humans exhibit spring-mass behavior in the horizontal plane. Changes in the horizontal projection of leg length were linearly related to changes in horizontal-plane leg forces. Consequently, the passive dynamic stabilization associated with spring-mass behavior may contribute to stability during maneuvers in bipeds. Understanding the mechanics of maneuverability will be important for understanding the motor control of maneuvers and also potentially be useful for understanding stability.
We explored a recently discovered phenomenon that smooth transient perturbations applied to the h... more We explored a recently discovered phenomenon that smooth transient perturbations applied to the hand can lead to violations of equifinality. Healthy subjects occupied an initial hand position against a bias force and tried not to interfere with hand displacements produced by changes in the force. The force changes were smooth and transient (ending up with the same bias force value), with or without a time interval (dwell time) between the force change application and removal. They could lead to an increase or a decrease in the bias force. The subjects performed the task with eyes open and closed. After the force change was over, the hand stopped consistently short of the initial position only when the initial force change increased the bias force. No consistent positional errors were seen for the opposite force change direction. These results were consistent across trials with and without dwell time performed with and without vision. We conclude that the positional errors were not due to muscle properties but reflected a drift in the hand referent coordinate within the central nervous system triggered by the perturbation and driven by the difference between the actual and referent hand coordinates during the dwell time.
Multiple sclerosis and related disorders, Jan 9, 2018
Mobility impairment is common in people with multiple sclerosis (MS). Gait speed has shown strong... more Mobility impairment is common in people with multiple sclerosis (MS). Gait speed has shown strong correlations with other mobility measures in MS. The purpose of this study was to assess the relative importance of a battery of factors in determining gait speed among people with MS. Thirty-one individuals with MS (the mean (standard deviation) of the Patient Determined Disability Steps: 3.68(1.70)) participated in this cross-sectional observational study. Their gait speed was assessed using the Timed-25-Foot Walking test. Six factors which could slow gait speed in MS, including the strength capacity at knee joints, functional mobility, body balance, dorsiflexion range of motion of ankle joints, bilateral foot cutaneous sensation level, and the fear of falling, were also assessed. Multiple regression and relative weight analysis were used to identify the relative importance of each factor in explaining the gait speed variation. All six factors together accounted for about 86% of the o...
BACKGROUND: Treadmill-based gait-slip training shows to be effective in reducing the risk of slip... more BACKGROUND: Treadmill-based gait-slip training shows to be effective in reducing the risk of slip-related falls. In previous relevant studies, the number of repeated slip perturbations ranged from 12 to 30. RESEARCH QUESTION: It is unclear if a reduced number of treadmill-slips can still yield adaptive strategies to lower the likelihood of falls after a slip over ground. This study examined if eight repeated slips on a treadmill reduced the risk of falls among young adults when they were exposed to a novel overground slip. METHODS: Forty-three healthy young adults were randomized into either training or control group. The training group underwent an 8-slip perturbation training procedure on a treadmill while the control group received the same number of normal walking trials on the same treadmill. Following the training, both groups were exposed to an unrehearsed slip during overground walking. Their body's reactions to the novel overground slip were collected by a motion capture system. RESULTS: The training group exhibited significantly better reactions to the slip than did the control group, evidenced by the lower fall proportion and improved dynamic stability at recovery foot touchdown during the overground slip. No improvement in dynamic stability was detected in the training group at the slipping foot touchdown and recovery foot liftoff. SIGNIFICANCE: The results suggested that the shortened perturbation training program may be efficacious in improving responses to a novel overground slip but may not be as effective as protocols using greater number of slips. This study could provide guidance for selecting the number of slips for future perturbation-based training protocols.
Multidirectional ground reaction forces (GRFs) and jump tests within baseball pitchers provide in... more Multidirectional ground reaction forces (GRFs) and jump tests within baseball pitchers provide insight into athletic ability and coordination to produce lower-body force and power. Lowerbody power is a biomechanical feature that denotes physiological capacity through dynamic and passive tissue stretch-shortening in transferring energy from the ground through the kinetic chain. Optimized lower-body power may lessen the magnitude of forces on the upper extremity. Insufficient lower-body power may create a greater risk of upper-body injury. Lower-body power and its relationship to ball velocity have been minimally investigated, yet some research points to a correlation between jumping ability and fastball velocity. Because pitching is unilateral, practitioners should consider unilateral jumps to determine the extent of bilateral asymmetry or stride to drive leg differences that can guide training to remediate deficiencies. The purposes of this brief review are to (a) examine factors that influence vertical jump performance among baseball players, (b) examine research on pitching multidirectional GRFs, and (c) examine literature concerning jump performances to baseball pitching performance. Collectively, this review can assist coaches and practitioners in lower-body power testing and training for baseball pitchers.
The development of performance, such as learning a new motor skill, can be represented in a perfo... more The development of performance, such as learning a new motor skill, can be represented in a performance curve. The shape of the performance curve is both of theoretical and practical relevance. Here, the author studied the interday performance of juggling over a period of 17 days in 112 college students. The results showed that 60% of participants followed an S-shaped performance curve with the inflection date on the 11th day, followed by a decelerated (20%), accelerated (14%), and linear curve (6%). As expected, except on Day 1, male participants performed at least 33% better than female participants on each practice day. Also as expected, learning performance was found to depend on the type of performance curve with the best learning performance exhibited by the linear group. The results further revealed that pooling all participants' performance together without considering the percentage of each underlying type of performance curve would lead to biased, nonrepresentative results. Given the variety of the observed performance curves and the dominance of the S-shaped performance curve among them, coaches should continuously monitor the shape of an individual's performance curve.
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Older adults are more susceptible to balance perturbations during walking than young adults. However, we lack an individual joint-level understanding of how aging affects the neuromechanical strategies used to accommodate balance perturbations.
Research Question
We investigated gait phase-dependence in and aging effects on leg joint kinematic variability during walking with balance perturbations. We hypothesized that leg joint variability would: 1) vary across the gait cycle and 2) increase with balance perturbations. We also hypothesized that perturbation effects on leg joint kinematic variability would be larger and more pervasive in older versus young adults.
Methods
We collected leg joint kinematics in young and older adults walking with and without mediolateral optical flow perturbations of different amplitudes.
Results
We first found that leg joint variability during walking is gait phase-dependent, with step-to-step adjustments occurring predominantly during push-off and early swing. Second, young adults accommodated perturbations almost exclusively by increasing coronal plane hip joint variability, likely to adjust step width. Third, perturbations elicited larger and more pervasive increases in all joint kinematic outcome measures in older adults. Finally, we also provide insight into which joints contribute more to foot placement variability in walking, adding that variability in sagittal plane knee and coronal plane hip joint angles contributed most to that in step length and step width, respectively.
Significance
Taken together, our findings may be highly relevant to identifying specific joint-level therapeutic targets to mitigate balance impairment in our aging population.
Older adults are more susceptible to balance perturbations during walking than young adults. However, we lack an individual joint-level understanding of how aging affects the neuromechanical strategies used to accommodate balance perturbations.
Research Question
We investigated gait phase-dependence in and aging effects on leg joint kinematic variability during walking with balance perturbations. We hypothesized that leg joint variability would: 1) vary across the gait cycle and 2) increase with balance perturbations. We also hypothesized that perturbation effects on leg joint kinematic variability would be larger and more pervasive in older versus young adults.
Methods
We collected leg joint kinematics in young and older adults walking with and without mediolateral optical flow perturbations of different amplitudes.
Results
We first found that leg joint variability during walking is gait phase-dependent, with step-to-step adjustments occurring predominantly during push-off and early swing. Second, young adults accommodated perturbations almost exclusively by increasing coronal plane hip joint variability, likely to adjust step width. Third, perturbations elicited larger and more pervasive increases in all joint kinematic outcome measures in older adults. Finally, we also provide insight into which joints contribute more to foot placement variability in walking, adding that variability in sagittal plane knee and coronal plane hip joint angles contributed most to that in step length and step width, respectively.
Significance
Taken together, our findings may be highly relevant to identifying specific joint-level therapeutic targets to mitigate balance impairment in our aging population.