PERSPECTIVES ON AEROBIC AND STRENGTH INFLUENCES ON MILITARY PHYSICAL READINESS: REPORT OF AN INTERNATIONAL MILITARY PHYSIOLOGY ROUNDTABLE KARL E. FRIEDL,1 JOSEPH J. KNAPIK,1,2 KEIJO HÄKKINEN,3 NEAL BAUMGARTNER,4 HERBERT GROELLER,5 NIGEL A.S. TAYLOR,5 ANTONIO F.A. DUARTE,6,7 HEIKKI KYRÖLÄINEN,3 BRUCE H. JONES,8 WILLIAM J. KRAEMER,9 AND BRADLEY C. NINDL2,10 1 ORISE Knowledge Preservation Program, U.S. Army Research Institute of Environmental Medicine, Natick, Massachusetts; U.S. Army Public Health Center (Provisional), Aberdeen Proving Ground, Aberdeen, Maryland; 3Department of Biology of Physical Activity, University of Jyva¨skyla¨, Jyva¨skyla¨, Finland; 4USAF Fitness Testing and Standards Unit, Joint Base San Antonio-Randolph, San Antonio, Texas; 5Centre for Human and Applied Physiology, School of Medicine, University of Wollongong, Wollongong, New South Wales, Australia; 6Brazilian Army Research Institute of Physical Fitness, IPCFEx, Rio de Janeiro, Brazil; 7Department of Applied Physiology and Kinesiology, University of Florida, Gainesville, Florida; 8Directorate of Epidemiology and Disease Surveillance, US Army Public Health Command, Aberdeen Proving Ground, Aberdeen, Maryland; 9 Department of Human Sciences, The Ohio State University, Columbus, Ohio; and 10Neuromuscular Research Laboratory/ Warrior Human Performance Research Center, Department of Sports Medicine and Nutrition, School of Health and Rehabilitation Science, University of Pittsburgh, Pittsburgh, Pennsylvania 2 ABSTRACT Friedl, KE, Knapik, JJ, Häkkinen, K, Baumgartner, N, Groeller, H, Taylor, NAS, Duarte, AFA, Kyröläinen, H, Jones, BH, Kraemer, WJ, and Nindl, BC. Perspectives on aerobic and strength influences on military physical readiness: Report of an international military physiology roundtable. J Strength Cond Res 29(11S): S10–S23, 2015—Physical fitness training of military recruits is an enduring focus of armies. This is important for safe and effective performance of general tasks that anyone may have to perform in a military setting as well as preparation for more specialized training in specific job specialties. Decades of studies on occupationally specific physical requirements have characterized the dual aerobic and strength demands of typical military tasks; however, scientifically founded strategies to prepare recruits with a good mix of these 2 physiologically opposing capabilities have not been well established. High levels of aerobic training can compromise resistance training gains and increase injury rates. Resistance training requires a greater commitment of time and resources as well as a greater understanding of the Disclaimer: The views, opinions, and/or findings contained in this publication are those of the authors and should not be construed as an official Department of the Army position, policy, or decision unless so designated by official documentation. Address correspondence to Karl E. Friedl, friedlke@gmail.com. 29(11S)/S10–S23 Journal of Strength and Conditioning Research Ó 2015 National Strength and Conditioning Association S10 the science to produce true strength gains that may be beneficial to military performance. These are critical issues for modern armies with increased demands for well-prepared soldiers and fewer injury losses. The actual physical requirements tied to metrics of success in military jobs are also under renewed examination as women are increasingly integrated into military jobs previously performed only by men. At the third International Congress on Soldiers’ Physical Performance, a roundtable of 10 physiologists with military expertise presented comparative perspectives on aerobic and strength training. These topics included the physiological basis of training benefits, how to train effectively, how to measure training effectiveness, considerations for the integration of women, and the big perspective. Key discussion points centered on (a) the significance of findings from research on integrated training, (b) strategies for effective strength development, and (c) injury reduction in training as well as the benefits of improved fitness to injury reduction across the force. KEY WORDS physical endurance/physiology, muscle strength/ physiology, physical fitness/physiology, employment/standards, military personnel, sex factors INTRODUCTION hysical fitness of military recruits is an enduring focus of armies worldwide. Although there is a growing presence of cyber warriors serving the military from a computer console indoors, there is also an unremitting need for physically capable men and P TM Journal of Strength and Conditioning Research Copyright © National Strength and Conditioning Association Unauthorized reproduction of this article is prohibited. the TM Journal of Strength and Conditioning Research women to fight for and hold ground, sea, and air space. Even for chair-bound cyber warriors, there is mounting evidence of the importance of regular physical exercise to cognitive performance, although these exercise requirements have not yet been defined. Traditionally, new military recruits are given a general physical preparation for the demands of military service in a compressed training period of 2–3 months. Modern concepts have shifted the focus of this training from a rite of passage ordeal or test of motivation to scientifically based preparation of recruits with capabilities thought to be necessary for success in their subsequent military duties. Motivated by economic and manpower pressures, new evidence-based training seeks to improve efficiency with faster results and fewer injuries. The actual physical requirements tied to metrics of success in military jobs are also under renewed examination as women are increasingly integrated into military jobs previously performed only by men. One clear observation from decades of studies on occupationally specific physical requirements is that military tasks have both aerobic and strength demands, with general categories involving carrying, lifting, pushing, and pulling (70,75,87,99). Although strength is important, how much strength an individual needs is a point of debate. An earlier response of the U.S. military to a perceived need to better match job demands was to create a strength test to classify individuals at Military Entry Processing Stations (this testing concept was later abandoned) (97). There has never been a comparable concern about aerobic trainability, with an assumption that any healthy young man or woman with proper motivation can be trained to an acceptable level of running and marching capacity. If they fail the standards of a timed run test, they simply receive more aerobic training until they can achieve arbitrary minimum standards. There is, in fact, a genetic component to this trainability as demonstrated by Bouchard and Rankinen (5), but this is generally overlooked either because minimum standards (e.g., timed running tests) are so low or because failure of these individuals in basic training is ascribed to motivational failure (66). Strength trainability has had the opposite problem, with an assumption that many individuals simply will not be strong enough for standard military tasks, and even after consideration to poorly designed equipment and tasks, a lack of appreciation for what strength training might do. Women as a group were put in this category based on the known differences in upper-body strength but also with an assumption that without a strong androgenic hormone influence, they might not be able to change strength capacity. Bill Kraemer et al disproved this myth with their landmark studies on female strength trainability, demonstrating remarkable improvements in strength capabilities (26,49). Knapik (38) also demonstrated strength task improvements even in a relative fit population of women. Translation of these observations to strength training of men and women in basic training has been slow, and | www.nsca.com strength gains are not typically observed as a result of physical training programs (86). Clearly, elite world-class marathoners and powerlifters are not interchangeable in their physical capabilities, and it can be argued that neither one of these athletic extremes would be ideally suited to a squad on foot march patrol duties. Achieving a mix of training to yield a product that is “just right” for military duties is a key goal for basic training. Recent physiological studies with military recruit populations highlight a precarious physiological balance between aerobic and strength training effects. Running and marching has long been a standard part of recruit training, but Santtila (80) has demonstrated that a high volume of aerobic training in Finnish male conscripts attenuated strength and muscle hypertrophy gains that would otherwise have been produced by a resistance training program. This puts front and center questions of what balance of these 2 elements is most appropriate relative to a desired fitness outcome, and to answer this, the actual desired outcome must be defined (i.e., how strong and how aerobically fit?). We put these questions to a panel of ten internationally recognized physiologists with expertise in military physical training and performance requirements, asking them to argue for one or the other of these 2 essential components. Nevertheless, there was universal agreement among the panel and the audience that a combination of both aerobic and resistance components is essential in recruit training. Five questions were posed to the 10 panelists and addressed in the order presented here (Figure 1). MILITARY READINESS BENEFITS LIKELY DERIVED FROM STRENGTH OR AEROBIC FITNESS TRAINING TO BE Aerobic Training (Joseph Knapik) To describe the most relevant training adaptations derived from aerobic training and how these relate to soldier physical readiness, this section will (a) provide definitions of aerobic fitness and aerobic training, (b) describe physiological adaptations induced by aerobic training, (c) note common soldering activities that benefit from a high level of aerobic fitness, and finally (d) describe other health benefits soldiers derived from a higher level of aerobic fitness. Aerobic fitness can be defined as the ability to sustain long-term low-power physical activity. For this type of activity, energy is primarily derived from glucose and fat (with small contributions from amino acids), oxygen is used in proportion to the energy produced, and thus the rate of oxygen consumption (V_ O2) can be directly linked to the amount of energy produced (18,46). Aerobic fitness is improved by constant moderateintensity activity (e.g., running, bicycling, cross-country skiing) that increases the circulation of oxygen through the body and increases the rate of breathing. Progressively and systematically increasing the intensity of this type of activity over time induces central and local (primarily muscle) VOLUME 29 | NUMBER 11 | SUPPLEMENT TO NOVEMBER 2015 | S11 Copyright © National Strength and Conditioning Association Unauthorized reproduction of this article is prohibited. Physical Readiness Roundtable Figure 1. Members of the roundtable surrounding the conference host, Colonel Thomas Eccles, Commander of the U.S. Army Research Institute of Environmental Medicine, include (left to right) Brad Nindl, Herbert Groeller, Keijo Haakinen, Heikki Kyrolainen, Karl Friedl, Joe Knapik, Eccles, Bruce Jones, Neal Baumgartner, Antonio Duarte, Bill Kraemer, and Nigel Taylor. physiological adaptations that have been well studied. These include central cardiovascular effects that consist of an increase in cardiac output, stroke volume, and blood volume, and a decrease in peripheral vascular resistance. At the local muscular level, there is an increase in myoglobin and muscle glycogen content, a slower rate of muscle glycogen use, an increase in the number of capillaries, and an increase in the number and size of the mitochondria with concomitant increases in oxidative enzymes within the mitochondria. These factors result in an increase in maximal aerobic power _ O2max) brought about primarily by the mechanisms that (V improve cardiac output (central) and arteriovenous oxygen difference (local) (4,63). Aerobic training can also result in minor muscle hypertrophy, at least in previously untrained individuals (47). These and other factors result in an increased capacity for, and faster recovery from, long-term physical activity. Many common soldiering tasks require longer-term moderate-intensity physical activity. For example, longterm tasks involving activities like tactical road marches; preparing fighting positions; filling and emplacing sandbags; constructing emplacements; loading and unloading trucks; evacuating casualties over long distances; erecting camouflage; moving over, through, and around obstacles; land navigation; and the like are often performed for long periods of time (88,89). Individuals with higher aerobic fitness (i.e., higher V_ O2max), perform these activities at a lower fraction or percentage of their maximal capacity. More aerobically fit individuals can perform tasks for longer periods of time, fatigue less rapidly, recover faster, and S12 the have greater reserve capacity for subsequent tasks (34). A certain level of muscle strength may be necessary to initiate some occupational soldiering tasks, but a high level of aerobic fitness allows a greater capacity to sustain it. Besides the direct importance for physical soldiering activities, higher aerobic capacity has been shown to be important for injury prevention and the general health and life span of soldiers. Given similar levels of physical activity, individuals with higher aerobic power have been consistently shown to have lower risk of injury (43). Higher levels of aerobic fitness are also associated with lower risk of cardiovascular disease, lower risk of certain types of cancers, greater bone health, better lipid profiles, and more effective functioning immune systems (22,63). In summary, aerobic fitness, derived from adaptations induced from aerobic training, are important to sustain the long-term physical activities that soldiers commonly perform, allow for faster recovery from physical effort, and provide a cardiorespiratory reserve for subsequent tasks. Aerobic fitness is also associated with lower injury risk and enhanced overall health. Thus, a high level of aerobic fitness is an indispensable and necessary aspect of soldier physical readiness, and aerobic training should be considered a major component of soldier physical training programs. Strength Training (Keijo Hakkinen) Modern scientifically based preparation of military recruits requires a much greater emphasis on resistance training. The TM Journal of Strength and Conditioning Research Copyright © National Strength and Conditioning Association Unauthorized reproduction of this article is prohibited. the TM Journal of Strength and Conditioning Research benefits of this strength preparation are significant enhancement of military task performance and reduced injury rates. Most military occupation specialties include tasks that require muscle strength and explosive power. Common military tasks include loaded marching, repetitive lifting, digging, and carrying (31). Each of these four fundamental soldier tasks can be substantially improved with strength training programs. However, excessive aerobic training interferes with the development of this critical strength and explosive power ability (25,29). Maximal strength is increased by resistance training with high loads: first producing neural adaptations followed by muscle hypertrophy in a later phase of continued training. Muscular strength improvements are easily attained in untrained recruits (24). Resistance training improves body composition and power production as well as occupational task performance (49,81,102). For example, studies on the British Army basic training have demonstrated the value of progressive resistance training to material handling tasks, including loaded march performance, and to overall physical fitness including increase in fat-free mass and reduction in body fat (102). Low repetitions with high loads promote development of maximal strength, whereas high repetitions and lower loads (expressed as percent of repetition maximum) tend to promote muscle endurance. Intermediate loads and repetitions typically are used to promote muscle hypertrophy. Low and medium loads are used in explosive strength training, but high or maximal action velocity of each repetition is required. As an example of this approach, bench press training is a classic resistance exercise involving multimuscle recruitment that can be used to enhance upperbody power to improve military activities involving lifting and carrying (82). Improved strength capability in individual soldiers not only improves performance but is also thought to reduce injury risk. This has been difficult to demonstrate because of the multifactorial causes of injuries, with the highest prevalence of military injuries typically involving the running component of physical training unrelated to occupational strength demands. Recently, Roy et al. (78) concluded that load carriage injuries in Afghanistan are related to mismatches between strength capability and strength demands. Trunk strength in Swiss recruits has strong predictive value for injury during basic training (104). There is also an association between lost duty time due to illness/injury and muscle fitness (54). Strength capacity is determined by neuromuscular adaptations and muscle hypertrophy that increase fiber crosssectional area. High loading conditions that activate type II motor unit and muscle fibers produce the greatest neuromuscular adaptations. Military field training conditions are inadequate to activating type II motor units. Detraining is a significant problem when the strongest soldiers are not challenged to maintain a high level of resistance training during recruit training and, for all soldiers, when they deploy | www.nsca.com to field environments where there may be sedentary periods without a resistance training stimulus. Muscle force production depends on the availability of phosphocreatine as the primary fuel; this is produced endogenously but is also provided by the consumption of red meat and fish and other exogenous sources of creatine. Mechanisms of muscle hypertrophy are understood, but emerging findings further support an understanding of training benefits to strength performance. For example, a new study demonstrates that resistance exercise stimulates an increase in androgen receptor binding and intracellular signaling even without an acute increase in serum testosterone, explaining potential trophic effects at the molecular level (91). Nevertheless, exercise-induced acute anabolic hormone response is of importance because greater muscle hypertrophic adaptation to strength training may occur in those subjects whose endocrine system adapts to produce a greater acute response during hypertrophic resistance exercise (100). Moreover, the maintenance of basic serum levels of endogenous anabolic and catabolic hormones within the normal physiological range is of importance for gains in strength and muscle mass during prolonged strength training, and especially to optimize the individual training process and to avoid overtraining (24). In summary, training and detraining adaptations in maximal voluntary activation, electromyographic muscle fiber, and force production characteristics of human muscles are one of the most important areas of research for the military because strength performance is important to military task performance and mission success, including the reduction of injury and illness, and strength training can also improve cardiorespiratory capacity. By contrast, aerobic training should be applied periodically and sparingly as too much can be harmful to strength and explosive power training benefits that are so important to the military. UTILITY AND MANNER IN WHICH STRENGTH OR AEROBIC FITNESS TRAINING SHOULD BE CONDUCTED IN THE MILITARY Aerobic Training (Neal Baumgartner) Myriad military tasks call on anaerobic energy supply to meet the demands of high-intensity short-burst physical actions. A well-trained anaerobic system and a fit musculature are necessary for a soldier executing tactical movements or a pilot performing anti-G straining maneuvers; however, the ability to sustain an adequate level of performance is highly dependent on aerobic energy supply. Rarely are military missions short in duration or limited in bursts of activity, rather they often call for steady-state effort and repetitive tasks over protracted periods; therefore, aerobic energy production is indispensable to successful military operations. Oxidative phosphorylation supplies the majority of energy for operational tasks and supplies the energy for “recovery or reset” of anaerobic energy systems as well. Recovery of anaerobic energy systems is necessary because VOLUME 29 | NUMBER 11 | SUPPLEMENT TO NOVEMBER 2015 | S13 Copyright © National Strength and Conditioning Association Unauthorized reproduction of this article is prohibited. Physical Readiness Roundtable of hydrogen ion buildup, i.e., anaerobic energy production is limited; it is a capacity. In contrast, aerobic energy production is stated in terms of rate, and it is limited only by substrate supply or limitations in the musculoskeletal or integumentary systems. Contemporary oxygen kinetic research on energy system contribution across activities of varying durations confirms the aerobic system supplies the high proportion of ATP production for tasks greater than 60 seconds. For example, the long held 20th century “classical” model stated that the ATP source for a 2-minute highintensity activity, e.g., 800-m run, was 35% aerobic and 65% anaerobic, whereas current research shows the breakout is actually 60% aerobic and 40% anaerobic (1,8,16,30,65). Therefore, physical training should focus on the primary source of energy, i.e., train the aerobic rate system so one has less dependence on the limited system, anaerobic capacity. Sound exercise training principles, e.g., overload, progression, and regularity, must undergird the physical training program. Aerobic training should elicit cardiovascular, metabolic, and muscular adaptations that result in improvements in the 3 primary determinants of performance: maximal oxygen uptake (V_ O2max), lactate threshold, and movement economy (2,61). To elicit these physiological training effects, optimally reach goals, reduce injury/illness rates, and best prescribe rest periods, program designers must address several variables in program design:  Time Cycles: lay out physical training stimuli in a progressive stepwise fashion over multiyear, annual, macro, meso, and micro cycles (61).  Modality: select aerobic exercise modalities like crosscountry skiing, running, cycling, swimming, rowing, indoor aerobic exercise machines, or “aerobic rotations”—a series of repeat muscle fitness exercises accomplished with short or no rest between exercise stations.  Exercise Prescription: determine and balance the training load—a product of training volume (duration 3 frequency) and intensity (61). Prescribe intensity as percentages of physiological variables—%V_ O2max, % HRmax, and lactate levels. One well-accepted method recommends 77% of training volume at “easy” intensity (59–74% V_ O2max) or “moderate” intensity (75–84% V_ O2max), 10% of training volume at “threshold” intensity (83–88% V_ O2max), 8% of training volume at “interval” intensity (95–100% V_ O2max), and 5% of training volume at “repetition” intensity (.100% V_ O2max) (4). This balance of training quantity and quality will elicit improvements in V_ O2max, lactate threshold, and movement economy. An anaerobic-centric counterargument states that one should work on developing lactate tolerance. The goal is not to tolerate lactic acid buildup, rather the goal is to lessen lactic acid (hydrogen ion) production at a given work rate (or race pace in athletics) (8). This is accomplished by training at moderate, threshold, and interval intensities—training S14 the at or near V_ O2max—with short rest intervals, resulting in an increased aerobic contribution to energy production and a higher lactate threshold, the threshold where hydrogen ion accumulates (2,8,12,20,42).  Ability Groups: must use these for both aerobic and muscle fitness (strength/endurance) training. Ability groups are especially needed for high-risk unfit sedentary groups, to prevent overtraining (relative to baseline fitness).  Progression: gradually and consistently apply an escalating training stimulus. Finally, for a given physical task, the military member with a higher level of aerobic fitness will operate at a lower percent of V_ O2max, which lends to reduced injury and illness risk, less absenteeism, better situational awareness and cognitive decisions (101), and more reserve to handle unknown stressors. Therefore, military physical training must primarily develop and maintain aerobic fitness, but not singularly. The program must also include muscle strength and endurance training, maintenance of sound body composition, and training of other physical fitness components as necessary to maintain health, general fitness, and achieve success in military operations. Strength Training (Herbert Groeller) The occupational environment for military personnel is a loaded one (15). Manual material handling and the carriage of loads over prolonged distances are critical tasks within a military environment and necessary for successful combat performance. For example, consider Australian Naval Clearance Divers, a military trade that on initial examination may not appear to be associated with exposure to significant external physical loads. In contrast, the divers engage in substantial and critical manual material handling tasks for deployment, moving over 1500 items, 9 of which have a mass greater than 200 kg (96). However, the carriage and lifting of external loads is associated with the highest incidence of injury during deployment, having a direct influence on combat utility (78). Furthermore, increased loads significantly elevate extrinsic task demands, such that in some situations, they approach the intrinsic physical capacity of the soldier. Indeed, elevations in external load relative to body mass, lift height, and carry duration are associated with significant and marked increases in the risk of injury (56,78). Under these circumstances, when there is a minimal gap between inherent physical capacity of a soldier and the physiological demand of the task, overexertion is the most likely mechanism to cause injury (57). There are 2 approaches to resolve this situation, the first and most desirable is to reengineer the task, such that the physiological demands are reduced, thereby decreasing the gap between the demand of the task and physical capacity of the soldier. Although technological advances have been consistently proposed to be a mechanism to reduce the physical strain for the soldier, from TM Journal of Strength and Conditioning Research Copyright © National Strength and Conditioning Association Unauthorized reproduction of this article is prohibited. the TM Journal of Strength and Conditioning Research a historical perspective, it appears unlikely that the physiological burden will be reduced significantly in the near future. As an example, a 3-fold increase in soldier load carriage mass in the last 150 years has been observed despite significant technological gains (42). Thus, with limited ability to reengineer soldiering duties to reduce the demands of the task, a gain in physical capacity is the only means that will allow soldiers to maintain combat effectiveness within an increasingly loaded military environment. What therefore is the manner in which soldier physical capacity should be increased to tolerate loaded tasks? Resistance training is the most effective method for developing musculoskeletal strength suitable to meet the physiological burden of an increased external load (48). However, the manner in which resistance regimens are delivered within a military context requires careful deliberation. Five areas for consideration are listed below. Facilities. Military personnel can be considered similar in many respects to “industrial or tactical athletes.” As tactical athletes, military personnel should have access to modern facilities that allow large cohorts to simultaneously and safely engage in contemporary and efficacious resistance training regimen. Training time. An increase in the training time allocated to resistance exercise is recommended. Thus, as a consequence and because of the fixed nature of the total available physical training time, endurance training volume would be reduced. The resistance training regimen should also be periodized with distinct transitions or phases in training load and volume (50). Specificity. A central focus should be upon engaging the large muscles of the body in compound-specific multijoint activities, functionally relevant to military duties. Progression of these activities to enhance soldier movement skill, speed, and agility would be incorporated (7). In contrast, there should be less emphasis placed upon single-joint and isolated muscle activations for the purposes of military training. Loading. Higher load and lower volume resistance training regimen is recommended. Such a training approach would require increased emphasis upon movement quality rather than the total number of repetitions achieved. Rest must be structured and determined a priori; such planned breaks in training are not a sign of weakness, and on the contrary, rest is a critical prerequisite for optimal adaptation to occur. Structure. Increased consistency in the configuration of daily physical training sessions is suggested. Improved familiarization of soldiers in the organization of physical training lessons will allow military physical training experts and leaders to dedicate more time to facilitate improvements in movement quality and individual soldier exercise progression, | www.nsca.com with less time expended on lesson coordination and maintenance of exercise cadence. IMPORTANCE OF OPTIMAL STRENGTH OR AEROBIC FITNESS TRAINING TO SUCCESSFUL INTEGRATION OF WOMEN INTO COMBAT OCCUPATIONS Aerobic Training (Nigel Taylor) There is no doubt that high-level aerobic fitness and strength are essential attributes for combat-centric military occupations. It is also appropriate for employment opportunities to be expanded by eliminating discriminatory employment practices. However, such opportunities must not occur at the expense of either operational capability or personal health and safety, for not all men and women possess the physiological attributes essential for those occupations (58). With this caveat in mind, several facets to this topic require consideration. For instance, have the aerobic fitness requirements of the combat trades been validly defined? Are there aerobic demands of critical tasks that are beyond the capacity of women? Do women have unique aerobic strengths or weaknesses? Do these attributes vary cyclically? How does one best quantify aerobic fitness for military occupations? Since a discussion of any of these topics would exceed the available space, then only critical considerations that may prompt further examination are noted. Few countries have objectively identified and characterized the aerobic demands of the critical tasks performed within the combat trades (95). Until that has occurred, legally defensible answers cannot be assumed to exist. However, based on observation and first principles, one would predict that a long-term capability for carrying loads up to 40 kg would be required (85,96). Because it is well established that the metabolic impact of load carriage is proportional to the ratio of the load to one’s body mass (93), then the aerobic demand of load carriage is greater for smaller men and women. This has two immediate consequences, each a different side of the same coin. First, when carrying the same absolute load and working at a constant pace, as is military convention, the average woman is closer to her maximal capability. This implies a reduced stress tolerance reserve. Thus, it is not sufficient just to evaluate short-term capability, one must also consider the longer-term and repeated impact of work through increased and protracted fatigue (71), longer recovery times (59), physiological overreaching (55), and acute dysfunction across environments (83). Second, the stimulus to adapt is elevated for smaller individuals, provided the homeostatic disturbance is not excessive. As a consequence, stress tolerance is now elevated. Thus, one must initially focus on protecting smaller individuals during acute endurance exposures, and if that protection is effective, one can then watch them thrive as aerobic adaptation progresses. For acute exposures before adaptation, there is ample evidence to support the proposition that, on average, the aerobic potential of women is lower. For instance, blood VOLUME 29 | NUMBER 11 | SUPPLEMENT TO NOVEMBER 2015 | S15 Copyright © National Strength and Conditioning Association Unauthorized reproduction of this article is prohibited. Physical Readiness Roundtable volume is typically .5 ml$kg21 lower (62) and is attributable to a lower red cell count (19). As a consequence, the capacity to transport oxygen is lower (103). For the same stature, women also have smaller hearts, resulting in a lower stroke volume but a higher cardiac frequency for a given flow of oxygen to the exercising muscle mass (9). Accordingly, during maximal exercise, the aerobic power of the average man exceeds that of the average woman (11). However, occupational requirements rarely, if ever, approach these levels, so the only aerobic limitation of relevance is that imposed by the occupation. In this regard, it is oxygen delivery that is important. Although oxygen carriage may be lower at a given blood flow, evidence exists showing greater intramuscular vasodilatation during exercise (45). This implies a buffering of the central limitation at the periphery. Moreover, it is now well established that substrate metabolism during endurance exercise favors lipid use in women (92), potentially improving endurance across most work intensities. Therefore, if allowed to work at equivalent relative intensities, women may display superior endurance and fatigue resistance. Finally, the menstrual cycle appears to have minimal impact on muscle fatigue and strength (33), and there appears to be little, if any, effect on aerobic performance (21). Against this background, we must now consider how aerobic fitness for work should be measured. Typically, this is performed using unloaded field methods, such as shuttle running. These tests favor lighter and discriminate against heavier individuals. Accordingly, such tests are neither scientifically valid nor legally defensible. Instead, occupational endurance tests for the military should involve load carriage work simulations (94). Strength Training (Bradley Nindl) Opening up more physically demanding and combat-centric occupations to women is an important issue for militaries interested in human capital investment. Moving forward in the 21st century, human dimension efforts must be formulated to ensure that all soldiers are trained, developed, taught, coached, and mentored to reach their individual potential. We know that the physiological differences between men and women put women at a physical disadvantage when it comes to occupational and military performance (49,50). These disadvantages are most pronounced for strength and power, especially for the upper body. For example, the sex differences for aerobic fitness range from 20 to 30%, whereas for strength fitness, they range from 30 to 50% (49,50,67). Thus, it is clear that efforts should be more focused in strength and power development to best successfully prepare women for physically demanding combat-centric military duties (64,68). The 2013 National Strength and Conditioning Associations’ Blue Ribbon Panel on Military Physical Readiness, comprised of 30 subject matter experts from the military, academia, and the strength and conditioning fields, S16 the conducted a task analysis and reported that strength and power were the fitness components considered of greatest relative importance for being able to successfully accomplish warrior tasks and battle drills (those tasks considered most essential for soldier war-fighting duties) (64). It would therefore be most effective for militaries to place a premium on optimal strength and power development for women to maximize the likelihood of seamless integration into combat-centric military occupations (49,50,69). A greater emphasis on strength and power fitness could also have a profound effect on mitigating musculoskeletal injury risk among military women. The dose-response relationship between running and injuries is crystal clear having been firmly established by the pioneering work of Drs. Bruce Jones and Joe Knapik (23,41,43). Quite simply, more running leads to greater incidence rates of lower extremity musculoskeletal injuries. Similar dose-response injury rates for resistance training have not been reported and are likely nonexistent. As women have an injury rate of nearly twice that of men, injury risk mitigation strategies would seem especially prudent for the military to implement (43,69). A training paradigm shift away from long-distance running and more toward strength fitness training will have the dual advantage of enhancing physical performance and reducing injury (23,28,69). For example, recent published results have reported that more weekly resistance training imparts a protective effect for injuries in operational soldiers. Grier et al. (23) reported that soldiers from the fourth Infantry Division who participated in resistance training for as little as 1 time a week were observed to half the injury risk when compared with those reporting no participation in resistance training. Perhaps, the most physically demanding task a soldier must accomplish when deployed in a combat theater is maneuvering under loaded conditions (i.e., load carriage) (28,41,49,68). In one of the most well-designed studies to date examining the effects of various physical training regimens on military occupational performance, Kraemer et al. (49,50) demonstrated that women who underwent 6 months of periodized heavy resistance training performing sets in the 5–8 repetition maximum range had the most improvement and significantly attenuated the gender gap in load carriage (time to complete 2 miles with a 75-lb rucksack) and repetitive box-lifting capacity (maximum of number of 45-lb boxes that could be lifted from the ground to a height of 1.32 m in 10 minutes) and that those women who only conducted aerobic training did not demonstrate any improvements in load carriage ability. Furthermore, Paavolainen et al. (72) demonstrated that when endurance trained runners conducted explosive strength and power training, their 5-km running time and running economy were significantly improved. Thus, once a baseline aerobic fitness level is established, it appears that an emphasis on strength and power training may be preferable to exhibit additional neuromuscular training adaptations while also protecting against injury risk (28). TM Journal of Strength and Conditioning Research Copyright © National Strength and Conditioning Association Unauthorized reproduction of this article is prohibited. the TM Journal of Strength and Conditioning Research From a neuromuscular perspective, it is essential to recruit and activate type II motor unit and muscle fibers (those fibers that produce the most force and have the greatest capacity to hypertrophy), and this only occurs under loaded conditions (51). Most callisthenic-based military field training fails to provide the necessary overload to activate type II motor units and thus falls considerably short of truly optimizing neuromuscular training adaptations. The failure of military leaders and physical training policymakers to fully understand the size principle (the physiological principle governing muscle fiber recruitment) and an undue emphasis on imposing artificial constraints on field-expedient bodyweight exercises requiring minimal or no equipment remain a significant threat to optimally physically training women for the rigors of combat-centric occupations. TESTING AND ASSESSMENT ASPECTS AEROBIC FITNESS OF STRENGTH OR Aerobic Training (Antonio Duarte) The assessment of soldiers’ fitness status is an important issue among the militaries. The performance of military duties, in great part, requires an optimum level of physical fitness for both male and female soldiers. To accomplish military missions, the basic fitness needs include cardiorespiratory capacity, muscle strength, and endurance, and flexiblity and mobility is often added to these components (17). At a first glance, the evaluation of the soldiers’ physical performance with usual fitness tests could be a way to guarantee that they are ready for deployment, but this is a matter that deserves further considerations. Published findings document the importance of specific job requirements for cardiorespiratory and neuromuscular fitness, with defined minimum physical requirements (98). However, the appropriate characterization of the standards should include measures of the subjects’ physiological responses during the execution of the critical tasks (32), and this is not always feasible. This is a key point in the definition of the so-called performance predictive tests, designed to measure specific military capacities. In an attempt to adequately deal with these issues, most of the Armed Forces around the globe choose to perform standardized tests (e.g., aerobic, strength, agility, flexibility, etc.) to obtain sufficient feedback about the soldiers’ physical conditioning levels. This aerobic fitness assessment provides commanders with insight into several important components of the readiness and health status of their armies. First, because the energy that is provided through aerobic metabolism is the primary mechanism fueling these tasks, aerobic fitness sustains the performance of the major body systems during dynamic work (74). Aerobic fitness therefore determines the ability to perform continuous physical activity, and it is one of the key components for optimal militaryrelated performance (17). Commonly expressed by means of the maximal oxygen consumption (V_ O2max), the evaluation | www.nsca.com of the aerobic fitness provides the feedback about the general capacity of the soldiers to use aerobic energy to accomplish a broad range of military tasks. Therefore, aerobic power measures provide an overarching fitness assessment, which is at the root of all military physical performance and clearly related to work capacity. Another important information component derived from the measurement of the V_ O2max is based on its good relationship with health outcomes. Several studies indicate the association between aerobic fitness and global or cardiovascular health (3,6). The expected age-related decline in aerobic power that may guarantee a healthy life to an individual is also well recognized (73). Taken together, these data provide the commander with a global soldier health status indicator. Aerobic testing can also be very efficiently conducted in the field, and there is a high correlation between the assessment of the cardiorespiratory fitness in the laboratory setting and in the field (10). In general, military physical fitness tests need to be able to be administered in distributed and remote regions, including places in which there are no laboratory facilities where V_ O2max can be measured directly. Moreover, there are usually large groups to be evaluated at the same time and field tests of aerobic fitness are suitable to be used in such conditions. Among the most popular running tests used around the world, the following are highlighted: the 12-minutes test and the 2-miles, the 1.5-miles, and the 3000-m tests (37,70). Standard tests in the laboratory usually involve a treadmill or cycle test of maximal aerobic power. Timed runs used by the military are highly correlated with maximal aerobic power. Each of these tests performed by motivated individuals have correlations to treadmill tested maximal aerobic power of 0.85 or better (60,99). In all these assessments, the examiners do not need any special equipment to perform the tests. It is time saving and simple. One examiner may apply the test in a large group of soldiers at a time, confident on the validity of the results. The reliability of the aerobic fitness assessments is also an advantage. The tests are simple to conduct and are examiner independent. If conducted under similar environmental conditions and using the same facilities (i.e., track, itinerary) test-retest performances are comparable. In summary, the assessment of aerobic fitness in the military can bring to the Armed Forces leadership important information about the health and operational status of the troops with simple, valid, and reliable tests that can be applied in large groups simultaneously. These are some of the characteristics that make aerobic fitness evaluation one of the most used tools worldwide to monitor the physical conditioning level of the soldiers. Strength Training (Heikki Kyrolainen) The National Strength and Conditioning Association’s (NSCA’s) Second Blue Ribbon Panel on Military Physical Readiness: Military Physical Performance Testing (64) VOLUME 29 | NUMBER 11 | SUPPLEMENT TO NOVEMBER 2015 | S17 Copyright © National Strength and Conditioning Association Unauthorized reproduction of this article is prohibited. Physical Readiness Roundtable identified muscular strength as the most critical fitness component necessary to successfully execute physically demanding military occupational and functional tasks. The importance of muscular fitness assessment is universally recognized because all countries include some form of muscular fitness assessment in their regular physical fitness tests. These muscular fitness assessments typically include repetitive push-ups (timed), sit-ups (timed), and pull-ups (maximum number) (64). Although these field-expedient tests are practical, inexpensive, and readily scorable, these tests are a greater reflection of muscular endurance than muscle strength. As muscular strength is defined by the maximal force exerted for a given movement, a 1 repetition maximum (1RM) test is the gold standard for strength assessment. The 1-RM tests require equipment and thus pose logistical and resourcing requirements for practical implementation. Historically, the U.S. Army evaluated the efficacy of assessing muscle strength using a 1RM incremental dynamic lifting devise during the 1980s (3). The incremental dynamic lift was essentially a freestanding upright weight stack machine in which the maximum amount that could be lifted from the floor to a height of 1.52 cm (the height of the bed of a 2.5 ton military truck) was measured. The movement was similar to the power clean. The incremental dynamic lift was originally developed by the Air Force. Teves et al. reported that the incremental dynamic lift was superior to handgrip strength, isometric 38 cm upright pull strength, body composition, and submaximal prediction of oxygen uptake when predicting job related criterion performance tasks (job requirements involving pulling, pushing, lifting, carrying, and applying torque) (27). Although it was recommended that the Army adopt this test to assess muscle strength before job assignment, the Army did not implement it as a strength test. Of interest, the Air Force does currently use the incremental dynamic lift as a screening assessment at military entrance processing centers. The utility of the incremental dynamic lift as a strength measure for widespread implementation deserves to be revisited. The singular exception to an existing 1RM strength measure appears to be the inclusion of the 1RM deadlift in the ranger athlete warrior (RAW) assessment used with U.S. Army Rangers. The incremental dynamic lift and isometric dynamometry were identified as viable strength measurements from NSCA’s Second Blue Ribbon Panel subject matter experts. Isometric dynamometers have the advantage of being relatively inexpensive, portable, reliable, and valid. Although isometric dynamometers have routinely been used in research studies measuring muscle strength in military populations, they have not gained widespread use in militaries for strength assessment. To mitigate potential safety concerns concerning injury risk for maximal effort 1RM strength tests, surrogate 3RM maximum strength tests have been used to accurately predict 1RM strength. Although 1RM strength measures are not routinely evident in generalized military physical fitness tests, tasks S18 the involving muscle strength are routinely observed in tier II tests (tests designed to assess occupational or tactical functional performance). Examples include a 20- to 30-kg sand and box lift (Canada, Australia, United Kingdom, Germany, and U.S. Marine Corps) and casualty (50–80 kg) drag (15–30 m) (Canada, United Kingdom, and Germany) (27). A common belief is that soldiers will “train to the test.” If this is true and given the lack of pure strength tests within military physical fitness assessments, a significant gap and concern remains about the extent to which soldiers and military leaders may be incentivized to strength train. Although the importance of strength to military occupation performance is clear and well supported by the scientific literature, one must ponder the potential for cultural change and for physical performance transformation by including strength measures as doing so may help to drive physical training toward greater strength fitness. SUMMARY COMMENTS FITNESS TEAMS FOR STRENGTH AND AEROBIC Aerobic Training (Bruce Jones) Aerobic fitness, defined as the ability to sustain long-term low-power physical activity, is an essential component of health- and performance-related physical fitness and underlies most activities of daily life and performance of military tasks. Physical training to improve and maintain aerobic fitness is easy to conduct for individuals and large groups. Aerobic activities such as running or marching with or without loads can be performed anywhere, with little or no equipment in large groups or by individuals. Adaptations to aerobic exercise include central cardiovascular effects (increases in cardiac output and blood volume and a decrease in peripheral vascular resistance) and effects in the muscles (increases in myoglobin and muscle glycogen content and number and size of the mitochondria). These and other factors increased exercise capacity and provide for faster recovery. Some military tasks such as marching with predetermined absolute loads will have a bigger physiologic impact on women and other individuals of smaller stature and with lower absolute maximum oxygen utilization. Because women are smaller on average than men and have a lower maximal oxygen power (V_ O2max),at a similar workload, they will be using a greater percentage of their maximal capacity. This puts women at a disadvantage when performing military tasks such a road marching or other repetitive or sustained lifting, carrying, digging, dragging, pushing, or pulling activities. Such discrepancies between men and women and others of small stature can be mitigated through a combination of selection and physical training. Aerobic fitness can be easily assessed in large groups using running tests. Running and other tests of aerobic fitness have been shown to be valid and highly reliable. By contrast, strength testing is muscle group specific and provides different maximal values depending on the muscle group and type of test. TM Journal of Strength and Conditioning Research Copyright © National Strength and Conditioning Association Unauthorized reproduction of this article is prohibited. the TM Journal of Strength and Conditioning Research Other approaches to determining the importance of aerobic fitness for military performance include (a) examining the correlations of aerobic fitness with performance of military tasks and (b) determining the risks of injury associated with aerobic fitness and other health- and skillrelated components of physical fitness. A recent systematic review of literature on the correlation of the components of physical fitness with performance of 11 military tasks (e.g., lifting, lifting and lowering, casualty drags, stretcher carrying, road marching with loads, climbing, crawling, and digging) found that aerobic fitness was more highly correlated with more such tasks than muscle strength or muscle endurance (27). Strong and consistent associations of a component of physical fitness with musculoskeletal injury risk suggests that tasks requiring that component of fitness are either common or biomechanically stressful or both. Low levels of aerobic fitness as measured by run-times has consistently been shown to be strongly associated with higher risk of injuries in military populations (35,36,43,84). However, poorer muscular endurance as measured by push-ups and sit-ups are also associated with injuries, at least in basic training (43,44). In summary, aerobic fitness is more strongly associated with performance of more military tasks than muscle strength or other components of fitness. Consistently strong association of low levels of aerobic fitness suggests that soldiers are either exposed to more aerobically taxing activities or that aerobic activities are more biomechanically or physiologically stressful or both. This makes sense because weight-bearing activities are ubiquitous during military training and operations, and individuals with low levels of aerobic fitness perform at greater proportion of their maximal capacity for any given fixed task. Fortunately, aerobic fitness is easy to validly and reliably test in mass, unlike muscle strength which requires equipment that is harder to standardize and maintain. These observations regarding testing of aerobic fitness probably explain why aerobic fitness tests are so common historically in the U.S. Army and why there have been so few routine strength tests (40). The biggest challenges for soldier physical fitness development and testing will be how to reduce injuries associated with aerobic training and how to develop and test strength safely in large groups. Strength Training (William Kraemer) Strength training provides one of the most potent exercise modalities that can be incorporated into a warfighter’s physical training regime. Strength, power, and local muscular endurance are vital fitness parameters needed in each military occupational specialty, varying to some degree depending on the specific demands of the specialty. Combat-centric occupations typically require the warfighter to have greater strength and power capacity for optimal performance in a variety of tasks. The modern battlefield has evolved over time from being “aerobic” in nature to being more anaerobic | www.nsca.com in its different demands, requiring high force and quick and explosive movements. While aerobic training certainly plays a role in the overall fitness of the soldier, too often it constitutes the bulk of the warfighter’s physical training program, resulting in significant strength and power deficits that become apparent when the soldier is faced with heavy anaerobic demands on the battlefield. Lack of preparation for these anaerobic demands not only contributes to the overall stress of the task but also increases the likelihood of injury or even death. The bottom line is that strength training is a necessary component of physical training for today’s warfighter that simply cannot be overlooked. In contrast to cardiovascular or endurance training, strength training, also called resistance training, can “get at” or stimulate all the body’s musculature by using different resistance loading schemes in the training program (called periodization of training). By using appropriate loading schemes that are specific to the occupational demands (i.e., specific to the demands of heavy load carriage or anaerobic activities such as sprinting under load, conducting an ambush, or offensive/defensive maneuvers), favorable adaptations in terms of muscle mass and force production capability can be achieved (90). Heavy loading stimulates high threshold motor units and thus recruits the type II muscle fibers that are used during the performance of primarily anaerobic, strength, and power activities but are otherwise simply not activated through the use of light loading schemes (51). Most important to understand in the physical development of the warfighter is that light weight resistance exercise, even when performed for high repetitions, simply will not result in the optimal development of type II muscle fibers that is necessary for optimal performance in combatcentric occupations that are primarily anaerobic in their demands. Understanding the physical implications of the specific occupational specialty and optimizing training for those specific requirements, i.e., heavy load carriage, lifting and carrying heavy objects, and so on, is vital to warfighter performance (52,53). This is becoming increasingly important as women compete for assignment to combat-centric occupations that were previously closed to them. Upperbody physical demands are now greater than ever, which challenge everyone, especially female warfighters. A few early investigations have pointed to the importance of upper- and lower-body strength and power during the performance of field infantry tasks including: ambushes, offensive and defensive maneuvers, and casualty rescues (13,39). Other research has characterized the heavy demands load carriage places on the musculature and joints, specifically, the shoulders, spine, low back, and knees (42,76,77). Not surprisingly, lower back injury is the top contributor to musculoskeletal injury in soldiers (78). Given the stress on both the upper and lower body with load carriage, optimally training not only muscular development but also structural strength is vital in both men and women to enhance load VOLUME 29 | NUMBER 11 | SUPPLEMENT TO NOVEMBER 2015 | S19 Copyright © National Strength and Conditioning Association Unauthorized reproduction of this article is prohibited. Physical Readiness Roundtable carriage performance. This is critical as the typical loads carried by soldiers have become much heavier over time (14), easily representing well over half the soldier’s own bodyweight. Previous studies have demonstrated that load carriage performance can be significantly improved through a targeted physical training program, with additional emphasis on upper-body strengthening (26,41,49). With properly designed strength training programs, the neuromuscular system can be improved to meet the challenges of heavy load carriage and better withstand physical stressors. The neuromuscular development of the body to meet the physical demands of the specific occupational specialty while aiding in the prevention of injury stands as a hallmark of the strength training modality’s importance in overall physical conditioning. Beyond the targeted benefits for the various demands of combat-centric occupations, a welldesigned resistance training program provides other advantages for warfighter health and well-being over the life span. 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