Isometric Contraction

The repeatability of subjective and objective assessments of neck muscle fatigue is very important with regard to the clinical applicability of these methods.To establish between-days reliability, 33 healthy volunteers performed a 60% maximum voluntary isometric contraction test from a standing position in all neck movements. Cervical muscle fatigue was assessed on three separate occasions from the spectral (median frequency, MF) and amplitude (root mean square, RMS) analysis of the electromyogram (EMG) signal recorded from the cervical paraspinal group, splenius capitis, levator scapulae and sternocleidomastoid. Subjective assessment of fatigue was rated by employing the Borg scale. Intraclass correlation coefficient ICC(1,1), standard error of measurement (SEM), smallest detectable difference (SDD) indices and Pearson’s correlation co-efficient were calculated for the analysis of the results.Normalised median frequency (MF) slope had low repeatability and large between-day error (ICC(1,1) = 0.28–0.61; SEM = 0.33–0.60%/s; SDD = 132.7–703.2%) for the protagonist muscles of each movement. Initial median frequency (IMF) had moderate to good reliability and small error (ICC(1,1) = 0.64–0.81; SEM = 2.8–8.8 Hz; SDD = 19.9–38.5%). The RMS slope yielded also poor repeatability. The Borg assessment was more reliable than the EMG estimate though variability between sessions was still quite high (SDD = 29.2–136.5%). No correlation was found between the EMG and Borg assessment of neck muscle fatigue (r = −0.01–0.39).The protocol used for assessing neck muscle fatigue proved to be reliable only for the IMF but the clinical usefulness of this measure remains questionable. The lack of correlation between objective and subjective estimation of neck muscle fatigue was possibly a consequence of the poor measurement repeatability. Further research is needed to identify the factors responsible for these results on neck area.

Background. High-intensity resistance training plays an essential role in the prevention and rehabilitation of musculoskeletal injuries and disorders. Although resistance exercises with heavy weights yield high levels of muscle activation, the efficacy of more user-friendly forms of exercise needs to be examined. Objective. The aim of this study was to investigate muscle activation and per-ceived loading during upper-extremity resistance exercises with dumbbells com-pared with elastic tubing. Design. A single-group, repeated-measures study design was used. Setting. Exercise evaluation was conducted in a laboratory setting. Participants. Sixteen female workers (aged 26–55 years) without serious mus-culoskeletal diseases and with a mean neck and shoulder pain intensity of 7.8 on a 100-mm visual analog scale participated in the study. Measurements. Electromyographic (EMG) activity was measured in 5 selected muscles during the exercises of lateral raise, wrist extension, and shoulder ex...

The effect of elevated human muscle temperature on energy turnover was investigated during cycling exercise (at 85 % of (VO(2)max)) at a contraction frequency of 60 revs min(-1). Muscle temperature was passively elevated prior to exercise by immersion of the legs in a hot water bath (42 degrees C). During exercise at this low pedalling rate, total energy turnover was higher (P<0.05) when muscle temperature was elevated compared with normal temperature (70.4+/-3.7 versus 66.9+/-2.4 kJ min(-1), respectively). Estimated net mechanical efficiency was found to be lower when muscle temperature was elevated. A second experiment was conducted in which the effect of elevated human muscle temperature on energy turnover was investigated during cycling exercise (at 85 % of (VO(2)max)) at a contraction frequency of 120 revs min(-1). Under the conditions of a high pedalling frequency, an elevated muscle temperature resulted in a lower energy turnover (P<0.05) compared with the normal muscle...

Empirical evidence suggests that our nervous system considers many objectives when performing various tasks. With the progression of fatigue, researchers have noted increase in both joint moment variability and muscular cocontraction during isometric force production tasks. Muscular cocontraction increases joint stability, but is metabolically costly. Thus, our nervous system must select a compromise between joint stability and energy efficiency. Interestingly, the continuous increase in cocontraction with fatigue suggests there may be a shift in the relative weighting of these objectives. Here we test the notion of dynamic objective weightings. Using multi-objective optimization, we found a shift in objective weighting that favoured joint stability at the expense of energy efficiency during fatigue. This shift was highly correlated with muscular cocontraction (R(2)=0.78, p<0.001) and elbow moment variability in the time (R(2)=0.56, p<0.01) and frequency (R(2)=0.57, p<0.01)...

Research on the focus of attention has begun exploring the physiological changes that underlie the difference between internal and external foci of attention. However, previous electromyography studies have used dynamic tasks, making it difficult to interpret electrophysiological data. The authors analyzed how the focus of attention affects a subject's ability to perform an isometric force production task (focus was directed either at the force platform or the muscles responsible for force production). Subjects received practice without attentional focus instructions and then completed blocks of trials with an external and internal attentional focus separately. An external focus led to significantly less error overall and reduced surface electromyography activity with lower median power frequencies in the antagonist muscle, but attentional focus had no effects on the agonist muscle. Thus, an external focus of attention led to more efficient motor unit recruitment patterns (reduced cocontraction) and improved performance. Posttest surveys revealed subjects were aware of their improved performance with an external focus.

Research is limited and controversial on the effectiveness of various stretching methods on muscle lengthening. Studying the effectiveness of static stretch and muscle energy technique on the flexibility of hamstring muscles. Thirty young females (20-25 yrs old) participated voluntarily in this study. Subjects were randomly assigned to two groups: static stretch group (controls; n=15) and muscle energy technique (MET) group (experimental; n=15). The control group underwent ten sessions of static stretches of hamstring muscle, and the experimental group was assigned to ten sessions of MET at 50% voluntary isometric contraction. Hamstring flexibility was evaluated by passive knee extension test at baseline and again following interventions. Both treatment methods were capable of significantly improving the shortness of hamstring muscles (P< 0.01); and no considerable difference was observed between the methods. Ten sessions of hamstring stretches in normal young women using either ...

The aim of this study was to investigate the effect of a multi-directional soccer-specific fatigue protocol on sprinting kinematics in relation to hamstring injury risk. Nine semi-professional soccer players (Mean +/- SD: Age: 21.3 +/- 2.9 year; Height 185.0 +/- 8.7 cm; Body Mass 81.6 +/- 6.7 kg) completed the SAFT(90); a multi-directional, intermittent 90 min exercise protocol representative of soccer match-play. The 10m sprint times and three-dimensional kinematic data were recorded using a high-speed motion capture system (Qualisys Track Manager) every 15 min during the SAFT(90). A significant time dependent increase was observed in sprint time during the SAFT(90) (P<0.01) with a corresponding significant decrease in stride length (P<0.01). Analysis of the kinematic sprint data revealed significantly reduced combined maximal hip flexion and knee extension angle, indicating reduced hamstring length, between pre-exercise and half-time (P<0.01) and pre-exercise and full-time (P<0.05). These findings revealed that the SAFT(90) produced time dependent impairments in sprinting performance and kinematics of technique which may result from shorter hamstring muscle length. Alterations in sprinting technique may have implications for the increased predisposition to hamstring strain injury during the latter stages of soccer match-play.

This study evaluated changes in shoulder function in patients with frozen shoulder after 4-week rehabilitation combining exercise with electrical therapy and massage. Ten patients with frozen shoulder (mean +/- standard error (SE) age 50.2+/-4.6 years) and 10 control subjects (49.8+/-4.6 years) participated in the study. Standard goniometric measurements were used to assess shoulder flexion, extension, abduction, adduction, internal and external rotation active range of motion. Isometric maximal force of the shoulder flexors, abductors, adductors, internal and external rotators was measured by hand-held dynamometer. Shoulder muscle isometric endurance was characterized by net impulse assessed during weight (30% of maximal force) holding in hand till exhaustion. Shoulder pain was assessed by visual analogue scale. Before rehabilitation, patients with frozen shoulder had less (p<0.05) active range of motion and shoulder muscle maximal force for all measured directions, and less (p&...

Context: Proper conditioning of the neck muscles may play a role in reducing the risk of neck injury and, possibly, concussions in contact sports. However, the ability to reliably measure the force-time–based variables that might be relevant for this purpose has not been addressed. Objective: To assess the between-days reliability of discrete force-time–based variables of neck muscles during maximal voluntary isometric contractions in 5 directions. Design: Cohort study. Setting: University research center. Patients or Other Participants: Twenty-six highly physically active men (age  =  21.6 ± 2.1 years, height  =  1.85 ± 0.09 m, mass  =  81.6 ± 9.9 kg, head circumference  =  0.58 ± 0.01 m, neck circumference  =  0.39 ± 0.02 m). Intervention(s): We used a custom-built testing apparatus to measure maximal voluntary isometric contractions of the neck muscles in 5 directions (extension, flexion, protraction, left lateral bending, and right lateral bending) on 2 separate occasions separa...