Anaerobic power and capacity Karim CHAMARIa,1 Anis CHAOUACHIa and Sebastien RACINAIS b a Research Unit ‘’Evaluation, Sport, Health’’, National Center of Medicine and Science in Sport, Tunis, TUNISIA b ASPETAR – Qatar Orthopaedic and Sports Medicine Hospital, Doha, QATAR E-mail address:
[email protected] Introduction The ability to develop power is important in many sports in which the body or a tool should accelerate at the fastest possible speed, in the shortest possible time. The capacity to sustain this power is also a major determinant of sports performance. If the assessment of power can be used to track performance improvements or decrements, the duration and characteristics of the testing procedure will influence the qualities investigated by the test (i.e. muscle force, power, and anaerobic capacity). This chapter presents the standard ways to investigate mechanical power output as an index of muscle power and anaerobic function. The effects of training and fatigue will be presented as well as the reliability of anaerobic evaluations. 1. Which precision for which measure? 1.1. Power output versus anaerobic power Coaches commonly use the terminologies of anaerobic power and anaerobic capacity. However, all of the tests presented in this chapter are measuring an output (e.g. power output) but not a metabolism (e.g. anaerobic power). Consequently, in most of the case, the terminology anaerobic function could be replaced by muscle function. The following paragraphs present how to obtain indexes of anaerobic function. 1.2. How to record anaerobic metabolism? Capacity of energy metabolism may be defined as the sum of all work that can be gained from energy stored in chemical form. Analogously, power may be defined as the sum of the maximal metabolic rates of the different energy transfer systems. All substrate needed for anaerobic processes are located inside the muscle cell. These substances can only be measured directly by laboratory methods - for example, muscle biopsy or 31P-MR spectroscopy (see Chapter 4 of Section 1 for the latter method). The 1
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determination of intermediate or end-products of energy metabolism such as lactate or pyruvate in the peripheral blood only allows an indirect assessment owing to the complex dynamics of diffusion and elimination processes. It remains that the above mentioned methods are invasive or not easily accessible, and that even if one assesses the potential anaerobic energy system stored in the muscle, the subject’s skill and the intervention of other metabolisms such as the aerobic metabolism will interfere during the on-field performance. 1.3. Indirect evaluation of anaerobic metabolism 1.3.1. Lactate and substrates during short duration testing There is at the moment no simple testing procedure to precisely determine anaerobic power and anaerobic capacity subdivided into their alactic and lactic components. Because of the complex interactions between these various components of anaerobic and aerobic energy metabolisms, a detailed assessment of these components can only be achieved using a battery of tests. This test battery should include an all-out 5-s test for the assessment of alactic power, an all-out 10-s test for the determination of lactic power, and an all-out 40-100 s test, with post-test measurement of maximal blood lactate concentration, for the assessment of lactic capacity even though it has been shown that post-exercise lactate depends upon several factors [1]. This approach is criticized as it has been shown that even if alactic anaerobic energy is predominant during a 5-s all out effort [2], lactic energy pathway is activated at the very onset of very intensive effort as short as the vertical jump [3]. And even in a maximal sprint of 10 s, aerobic metabolism starts to contribute to energy turnover [4]. 1.3.2. Maximal accumulated oxygen deficit The maximal accumulated oxygen deficit is a laboratory method aiming to assess from a supra-maximal burst of exercise the oxygen deficit which reflects the anaerobic contribution to the performed task [5]. The detailed procedure is presented in the Chapter 2 of Section 6.
2. Muscle power 2.1. Jumping Tests Vertical jump tests are commonly used as a means to assess lower limbs “muscle power.” However, vertical jump performance scores may be considerably different depending on the test used. Single leg vertical jumps have also been used to assess both legs independently [6,7,8,9,10]. The most popular vertical jump used is the Sargent’s test with a close version, i.e. the Abalakow test. But more recently two laboratory/field vertical jump tests gained a lot of attention from scientists, i.e. Squat Jump and Counter movement Jump
2.1.1. Sargent and Abalakow procedures The Sargent’s test (1924; cited in [11]), also known as the jump-and-reach test is simple to use, requiring only a wall or board and chalk powder to make marks with the fingers while jumping. The subject stands along the wall and marks his initial position on the wall with the chalk, arm fully extended vertically. When ready to jump she/he jumps as high as possible by performing a rapid flexion/extension (counter movement) of the legs, aiming at touching/marking the wall when maximal jumping height is attained. The Abalakow jump (AJ) test is somewhat similar to the Sargent’s test but the measurement method is different. Indeed, it is performed with a tape attached to a special hip belt. The measuring tape is then put underneath a ruler fixed on the floor [12]. The subjects put their feet as close to the ruler as possible. The measuring tape is straightened and the number above the ruler on the measuring tape is noted. After the jump the number where the measuring tape stopped is also written-down allowing deducing the jumping height by the difference between the two numbers. As an attempt to obtain a higher accuracy or more credibility to the measurement method, many scientists, have presented other methods using video systems or landing mats, to be able to measure jumping height without an arm swing or under more natural settings. 2.1.2. Squat Jump (SJ) and Counter movement Jump (CMJ) The CMJ starts with the subject standing in an upright position, hands on the hips to avoid the contribution of the arms to the jumping performance (see figure 1). CMJ is performed by a fast downward movement to about 90° knee flexion immediately followed by a fast upward vertical movement as high as possible, all in one sequence. The arms can also be free, i.e. free CMJ to be closer to several natural sports’ jumping techniques such as in volley-ball for example. SJ is almost the same jump, but the movement starts from an immobile knee-90° flexion position (see figure 2). The absence of the counter movement is more appropriate to isolate the contribution of the lower leg muscle extensors’ power.
Figure 1: Counter movement Jump CMJ and SJ are reliable tests to evaluate the explosive strength in the lower extremity extensor muscles in athletes [13], often performed by using a contact mat that measures flight time and calculates jump height from the formula : h=g.tflight.
Figure 2 : Squat Jump Nonetheless, to ensure measurement accuracy and reliability during performance of SJ and CMJ, athletes must use a consistent landing technique with the legs and hips extended until contact is made with the mat [12]. It must be pointed-out that contact mat testing is more efficient than jump and- reach or belt tests because there is no need to measure height [13]. The Gold Standard measurement method for evaluating the performance of such jumps is the use of force plates that allow obtaining jump height, force, power and speed of movement. The difference between the CMJ-SJ performances in the same subject allows the possibility of discriminating leg from leg+ arm contributions and the effect of pre-stretch, i.e. muscle-tendon unit elasticity [13]. 2.1.3. The multi-jumps tests Mainly Bosco and colleagues [14] presented the 60 s consecutive vertical jumping with a subsequent 30-s version [15]. It consists for the subject to perform consecutive CMJ for the pre-determined duration. As to the Wingate Test (see below), a peak Power is calculated on the basis of the best jumps observed at the very beginning of the test and a fatigue index is also calculated on the basis of the decrement in jumping performance throughout the test. While single jumps are purely anaerobic performances, continually executed maximum effort jumps may exhibit a substantial aerobic component depending, of course, on the duration of the test. 2.1.4. The drop jump tests The drop jump is also a recognized version of vertical jumps that allows power assessment [16,17]. It consists of jumping from pre-determined height (30 to 60 cm generally) allowing increasing the load on the lower limbs in order to increase the power applied to the lower limb’s extensors. This type of test is appropriate for the athletes that undergo a high load of eccentric muscle actions in their practiced sports.
2.2. Cycling sprint tests Sprints on cycle ergometer has been used to investigate maximal muscle power (as presented in the paragraphs below) as well as repeated sprint ability (as presented in the Chapter 3 of Section 6). 2.2.1. The force-velocity Test (F-v) This test, which has been described by Peres et al. [18], allows the measurement of short term (6 s) peak Power and is usually performed on a cycle ergometer even if it can be performed in rowing or on a non-motorized treadmill. The F-v test consists of repetitive short maximal sprints against increasing braking forces (Fb). The duration of each sprint is fixed at 6-s, the maximal time it takes for the vigorously motivated subject to attain maximal velocity (Vmax) for each sprint after the starting signal. The duration of each recovery period is fixed at 5-min. The subjects remain in a sitting position for the sprints and all the recovery periods; their feet are fixed to the pedals with toe-clips to avoid slipping and maximal power attainment. The Vmax and F-v relationships are assessed during the test by automatic system. The test begins against an F of 20-N (2 kg). Thereafter, F is increased by 20-N for each consecutive sprint, until the pedaling frequency drops below 130 rpm. Then F is increased by 10-N or 5-N to obtain a peak Power as precisely as possible. For each sprint, the power output (P) is obtained by calculating the product F.Vmax. As it has been pointed out by Vandewalle et al. [19], the relationship between F and v can be expressed as follows: v=b-aF or v=vo-voF/Fo=Vo(1–F/ Fo) , where vo is the intercept with the velocity axis, i.e. the Vmax for an F equal to zero, and Fo the intercept with the force axis, i.e. the maximal F corresponding to a v equal to zero. These force and velocity indexes (Fo and Vo) are calculated by extrapolation from the linear relation linking F and v at a pedaling frequency greater than 90-rpm. Given the linear F-v relationship, the power-force relationship is parabolic. The peak Power is defined as the highest P calculated for the different Fb. It is assumed that the subject had attained peak power if an additional load induces a power decrease. (See figure 3)
Figure 3: Force-velocity relationship (linear regression line) and Power-force relationship (parabolic line).
2.2.2. Evaluation of maximal power with a single cycling sprint Even if widely used over the last 20-years, the F-v test presents two important limits. Firstly, the realizations of five to seven sprints require time and can induce fatigue affecting the results. Secondly, the maximal power is not measured but calculated. Integration of the flywheel inertia [20] into power calculation allows to go over these limitations and to measure the peak power production with a test based on a single sprint. Interestingly, if the power developed to accelerate the flywheel is taken into account, the total power recorded is independent of the braking resistance [21,22,23], except for extremely low or high resistance [24]. Applying on the flywheel a braking resistive force between 50 and 75-g per-kg of body mass [24] and up to 86-g per-kg of body mass [22] was suggested to be adapted for the determination of maximal power in young adults. In children (i.e., 8 to 15 y old), the braking resistance should be limited to 50-g per-kg of body mass or less [24]. During this test, young adults are generally developing values close to 1000 watts [22, 26]. From a methodological point of view, this method requests to calculate the moment of inertia of the flywheel (see Lakomy [20] or Martin et al. [25] for two different methods of calculation). The speed and acceleration of the flywheel need also to be recorded with a higher precision and at a faster rate than with during a classical Fv test. Recent cycle ergometers propose this test as standard. 2.2.3. The Wingate Test The 30-s Wingate cycling Test [27] is a popular and reliable test for determining athletes’ power performance capabilities in a laboratory setting. The test has been demonstrated to possess strong associations with other estimates of anaerobic potential such as accumulated oxygen deficit (see Chapter 2 of the Section 6 for further details of its estimate), oxygen debt, post-exercise blood lactate, and fast-twitch muscle fiber cross-sectional area [28,29,30]. It consists of performing an all-out 30-s sprint against a braking load calculated according to the subject’s body mass (approximately two to four times the maximal aerobic capacity’ power) Vandewalle et al. [19]. The most common measures determined from the Wingate Test are: 1) Peak power; the highest mechanical power produced during any of the six 5-s periods (generally the first 5-s period). Originally, peak power was assumed to reflect alactic (phosphagen) anaerobic processes, but subsequent research has shown that muscle lactate rises to extremely high values as early as 10 seconds into the test (Sands et al. 2004 [31]). 2) Mean power; the average power sustained throughout the 30-s period. Originally it has been proposed as a measure of anaerobic capacity, but subsequent research has shown that the aerobic contribution is as high as 28% in sprinters and even 45% in middle distance runners over the 30-s effort [32]. 3) Fatigue index; the percent decrease in power between the peak power at the start of the sprint and the lowest power measured during the last 5-s period of the 30-s effort. 2.3. Overview of some other tests Numerous other tests exist in order to assess the ‘’so called’’ anaerobic power and capacity, that is the ability to perform intense short or relatively long bursts of supramaximal exercise. Some of them are: maximal anaerobic running test (treadmill or track version; [33,34]), running time to exhaustion [35], 300 m shuttle test [36]. Some other tests are described below.
2.3.1. The Margaria step running test In the Margaria step running test, the subject runs up several steps at maximal speed, determining peak mechanical power of legs in a period of approximately one second. Power is calculated based on body mass, vertical distance, and time. The test has some limitations: it requires some skill and arm performance cannot be measured [37]. 2.3.2. The running sprints Sprinting performance is a common determinant of numerous sports such as team sports like soccer [38]. Measuring sprinting performance over varying distances is common across sports coaches and scientists. For example the 30-m sprint with 5 and 10-m lap-times is often used in professional soccer players. Such testing has a validity criteria as a strong correlation exists between sprinting performance and maximal strength measured by the 1-RM method [39], or anaerobic measures of vertical jumping [39,40]. The logical validity relies on the fact that this performance represents the integral ability of the assessed subject to cover a predetermined distance in the shortest time possible, often found in a high number of sports. It is crucial that timing be performed with photo-electrical cells as manual timing allows a too high error. 2.3.3. The horizontal jumps In terms of measuring functional power of the lower body the single hop for distance [6,7,41,42,43], triple hop for distance [6,10,41,42], 5-jump test for distance [44] 6-m timed hop [7,9,41,42], crossover hop [6,9,41,42], seem the assessment methods most widely used. Other jump assessments that have been used to measure lower body power also include the stair hop [9] and adapted crossover hop [42]. All these different types of short-term performance tests are usable by coaches/scientists as a function of the athletes assessed. Indeed, a volley-ball player will be evaluated by the way of vertical jumping while run and track athlete or team player (e.g. soccer) will be assessed by horizontal of multiple horizontal jumps/hops, that are closer to their movement performed in their respective sports. 2.4. Reproducibility and coefficient of variation Reliability can be defined as the consistency of measurements for an individual’s performance for a given test; or ‘the absence of measurement error’ [45]. It is an important measure as it gives an indication of the biological and technical variation of the protocol [46] and could be used to calculate the necessary sample size for a given effect size, therefore reducing the risk of a type II error occurring [45]. In the case of tests of physical performance, the reliability refers to the consistency or reproducibility of performance (accurate measurement) when someone performs the test repeatedly. Hopkins [47] suggested a high level of reliability means a sports scientist can confidently detect small changes in an athlete’s performance and use smaller sample sizes in research. In contrast, a test with poor reliability is unsuitable for tracking changes in performance between trials, and it lacks precision for the assessment of performance in a single trial Hopkins [47]. Accordingly, it is always recommended that Researchers and practitioners who assess performance of study participants, patients, athletes or other clients should therefore use tests with high reliability.
Methods used to describe ‘reliability’ are different and include coefficient of variation (CV), Pearson's product moment correlation (r), Intraclass correlation coefficients (ICC), limits of agreement and the standard error of measurements (SEM). In the case of anaerobic tests, ICC are used commonly and are based upon the results of an analysis of variance, which separates the error into variability between individuals and variability within an individual (error due to repeated measures) [48] with a value of 0.7-0.8 being questionable, and >0.9 being high reliability [49]. Also commonly used is that of the coefficient of variation (CV) which is utilized to determine the similarity of measurement among trials [50]. The CV expresses the standard deviation (SD) of the measure as a percentage of the mean, making it easier to compare the amount of variation between different protocols. The results of reliability studies of some of the anaerobic (sprint and jump measures) tests described in this chapter are summarized in table I. Test Vertical jumps Sargent vertical jump Abalakov jump Squat Jump
Countermovement Jump
Repeated vertical jump
Horizontal jumps Triple hop for distance Single hop for distance Crossover hop for distance Six-meter hop for time
Reference
n
Measure
ICC
CV (%)
[11] [13] [51] [13] [52] [53] [13] [51] [52] [53] [54] [14] [31] [52]
93 29 93 17 10 93 29 17 10 67 12 11 17
Height Height Height Height Height Height Height Height Height Height Force and power Power Power Height
0.93 0.96 0.87 0.97 0.89-0.91 0.98 0.93 0.93-0.95 0.89-0.91 0.95 0.87 -
3.0 3.3 5.4 2.4-2.6 2.8 6.3 2.1-2.4 6.7
[10] [55] [55]
21 18 18
Distance Distance Distance Distance Time
0.92-0.97 0.97 0.92 0.93 0.92
2.0-2.4
[56]
32
[53]
10
Sprint time (0-40m) Stride length Stride frequency Ground contact Sprint time
0.92-0.98 0.85-0.99 0.85-0.99 0.70-0.80 0.91-0.93
1.9-2.0
-
Sprint tests
3. Muscle force Power corresponds to a quantity of force needed to generate movement multiplied by the velocity at which the movement is produced. However, the velocity at which an
athlete can apply a force is directly dependent to the relative level of the force in comparison of the maximal force of this athlete. Consequently, most of the testing and training procedures are focusing on force development. 3.1. The different modes of muscle actions A muscle contraction generally induces a shortening of the muscle (concentric mode). However, if the resistive load equals the force developed by the muscle, the contraction does not induce any movement (isometric mode). If the external forces applied on the limb/articulation are higher than the one developed by the muscle, the muscle is stretching while contracting (eccentric mode). Maximal isometric (constant length) contraction failed to be specific for most of the sports activities but they allow standardized condition suitable for testing. When it turns to rehabilitation, physiotherapists are preferentially using isokinetic (constant speed) contraction allowing to progressively going back to a specific mode of contraction for the athlete. 3.1.1. The isometric contractions Muscle force is generally recorded in isometric mode during maximal voluntary contraction (MVC) lasting 4-6 s. Different commercially available ergometers allow to investigate the muscle acting on the main articulations (e.g. knee, ankle, hip, shoulder and elbow) as well as handgrip. Moreover, it is also possible to use strain gauge on sports equipment or in specifically designed device to investigate muscle force in positions/actions specific to a given sport. 3.1.2. The dynamic contractions Isometric maximal force is higher than concentric force, which is decreasing when the speed of contraction increases [57,58,59]. Eccentric force is higher than concentric force [60,61], but not always higher than isometric force [62]. Different brands are proposing commercially available ergometers to record isokinetic torque. These ergometers allow recording both eccentrically and concentrically the force produced by the muscle acting on the main articulations (e.g. knee, ankle, hip, shoulder and elbow) with a speed generally up to 300 deg.s-1. 3.2. Training effect 3.2.1. Central versus peripheral adaptations Resistance training is generally considered as inducing muscle hypertrophy but it also increases motor drive, i.e. the muscle electrical activity [63,64]. During the first weeks of training, neural adaptations are the first one to occur (Figure 1) and are the main cause for improving voluntary maximal force [65]. Indeed, training induces an increase in the quantity of neural drive to the trained muscle [60,66,67,68,69] suggesting that human are not able to fully activate their muscle before training. Training based on isometric [70], concentric [63] and eccentric [60,67] contractions are all increasing muscle electrical activity. When training is prolonged (e.g. above 4 weeks), the muscle structural adaptations (e.g. hyperthrophy) become the main source for improving muscle force (Figure 4).
Figure 4. Top left: Sedentary human are not able to fully activate their muscle. Top right: At the beginning of a resistance-training program, the increase in strength is mainly the results of neural adaptation (see text for details). Bottom left: The structural adaptation (e.g. muscle hyperthrophy) needs more time to occur than the neural adaptation. Bottom right: After a prolonged period of training, the rate of improvement is very slow.
3.2.2. Agonist / antagonist ratio The net output of a muscle contraction is the result of the force produced by the agonist muscle minus the resistive force produced by the antagonist muscle. This resistive force is higher during high velocity contraction (ballistic movement) and participates to the stabilization and protection of the articulation [71]. Antagonist muscle act for 17% of the net output during concentric contraction, 8% during eccentric contraction [72] and around 6% during isometric contraction [73]. This co-contraction of antagonist muscle can be reduced by training [74,75,76,77], as soon as the first weeks of training [76]. 3.2.3. Angular specificity Isometric contractions allow training with higher level of force than concentric contraction. However, the force produced as well as the training adaptations are dependent on the angle of the articulation involved. For example, leg extensors and elbow flexors produce their higher level of force when the knee or the elbow are flexed at around 70 and 90 degrees, respectively. These relations within force and angle are reflecting both muscle properties and the global participation of the synergist muscles acting on the articulations. After training, hypertrophy of one of the synergist muscles can modify these relations [78]. Even if the improvement in force is the bigger at the
training angle [79], there is also some improvement at the angle close to the training angle [80]. 3.2.4. Velocity specificity In line with the angular specificity of strength training, there is also specificity to the speed of contraction. Training adaptation to concentric training seems to be related to the angular velocity used during training [81,82,83]. The adaptations to eccentric training are linked to the contraction mode [84,85]. 3.3. Fatigue effect Fatigue is the result of progressive alterations occurring even before the task failure and is dependent on both peripheral and central modifications [86]. Fatigue occurs during long duration exercise and repeated exercise, but also during short duration exercise (e.g. 8 s, [87]). If fatigue is not reversed by rest, it was not induced by exercise but by overtraining or sickness. 3.3.1. Peripheral fatigue Reserves in phosphocreatine are the first one to decrease during exercise in order to maintain ATP levels. However, if high intensity contractions are prolonged, the rate of energy turnover is not sufficient and ATP content is decreasing leading to contractions cessation. As detailed in the Chapter 3 of Section 1, limitations in energy supply, intramuscular accumulation of metabolic by-products, impairment of Ca2+ kinetics and ion-shifts have often been implicated as a cause of muscular fatigue during highintensity exercise [88]. For example, Gaitanos et al. [2] have previously observed, in muscle tissue, a dramatic increase in metabolic by-products (i.e. lactate and inorganic Phosphate), a lowered pH, and a decrement in phosphocreatine concentration from the first to the last sprints of a repeated-sprint exercise. These changes in the intracellular environment of the recruited fibers have been associated with decrements in the powergenerating capacity of the skeletal muscle [88]. High intensity exercises can increase intracellular H+ and lower pH [89] possibly altering fiber contractile properties [90] and excitation-contraction coupling [91,92,93]. It has been proposed that accumulation of inorganic phosphate [94] and ADP decrease the amount of cross-bridges, their velocity and force [93,95]. 3.3.2. Central fatigue 3.3.2.1. Evidence for central fatigue If subjects are exhorted to produce an extra effort at the end of the contraction, the force decrement is lower [96], suggesting limitations from the central nervous system (CNS). 3.3.2.2. Origin of central fatigue during short duration exercise Electrical stimulations of the peripheral nerve were used to reveal the existence of fatigue perturbations both distally (e.g. contractile properties) and proximally (e.g. at spinal or supraspinal levels) to the stimulation point after short duration exercise [97]. In addition, decrements in the amplitude of electrically evoked H-reflex were observed after an isometric contraction maintained until exhaustion [98,99,100], suggesting the occurrence of spinal perturbations. Andersen et al. [101] confirmed with transcranial
magnetic stimulation that a large proportion of the spinal motoneuron pool becomes inaccessible to descending drive with fatigue. 3.3.3. Effect of training status and exercise modality Trained and untrained subjects can have different adaptation to fatigue. For example, fatigue can manifest by an increase of intermuscular co-activation [73,102] reducing the force/power output. However, in trained subject, an improvement in intramuscular coordination was proven to partly compensate for muscle fatigue [103]. Development of central fatigue may also vary according to the continuous/intermittent nature of the exercise task. Nybo and Nielsen [104] observed that voluntary activation during repeated intermittent contractions of a few seconds was not impaired by an exhaustive exercise, whereas muscle activation failed when the voluntary contraction was continuously prolonged for about ten or so seconds. This suggests that the CNS capacity to maximally activate the muscle may recover or be preserved for a few second but be altered in cases of continuous exercise mode. Bilodeau [105] recently confirmed this hypothesis by showing an earlier and larger central activation deficit during a continuous elbow extension task than during the same task performed intermittently. This has conducted elite sprinters coaches to suggest to their athletes to perform ‘’ins and outs’’ during short sprinting, e.g. 100 m. In order to avoid neural fatigue, the sprinters attain their maximum speeds at around 30-m and then alternate short periods of strong command to periods of relative rest, in order to avoid acute neural fatigue at the end of the sprint [106]. 3.3.4. Conclusion Fatigue is a complex phenomenon depending on perturbations occurring in different organs as well as interactions between these organs. When we consider the effect of exercise-induced fatigue on the muscle contraction, contractile and other peripheral alterations should be taken into account as well as spinal and supraspinal perturbations and/or modulations, but also factors "upstream" of the motor cortex. It is important to note that peripheral and central fatigues are both dramatically increased when short duration exercises have to be repeated (see Chapter 3 of Section 6).
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