Muscle architecture has been known to be associated with performance and is adaptable to training (2,3,8). Increases in fascicle thickness have been observed with resistance training and are highly correlated with ability to produce force (26). Pennation angle has also been observed to increase because of training (2,3,8,19,26). This increase in pennation allows for a greater number of fibers to be present within a given cross-sectional area and thus is often associated with increased muscle and fascicle thickness and increased strength (21). However, when cross-sectional area is constant but angle of pennation differs, a deficit in strength can be observed along with an increase in pennation angle (13). This is because that angle of pull of the fibers is indirect to the pull of the muscle in total, and thus the pull of the muscle in total is diminished by cosine of the pennation angle. Fascicle length is a product of both pennation angle and fascicle thickness (Equation 1). As pennation angle decreases or fascicle thickness increases, fascicle length will also increase. A greater fascicle length represents either longer sarcomeres or more sarcomeres in line. Thus, as the length of the contractile element increases, so does the velocity of contraction and the force that can be applied at a high velocity. For a review of the role of muscle architecture read Blazevich (2). The influence of these structures has been associated with running performance, however, as of now, there is no known linkage to jump performance (1,17). Abe (1) found that trained sprinters were heavier, had less pennation, and had longer and thicker fascicles in the vastus lateralis (VL) and the medial and lateral gastrocnemius (MG and LG) than trained distance runners. This arrangement of longer less-pennated muscles was attributed to the sprinter's need for faster contraction velocity and distance runners. Kumagai et al (17) observed that 100-m sprinters with faster sprint times had smaller VL, MG, and LG pennation angles than those with slower sprint times. Furthermore, both groups had similar body mass, height, limb length, and VL and MG muscle thickness; however, LG muscle thickness was significantly greater in the faster group.
Counter movement jumping (CMJ) is a movement that involves a stretch shortening cycle (SSC) that allows the body to store and redirect energy through and an eccentric movement quickly followed by a concentric movement. Because of the SSC, more force and power can be performed during the concentric phase of the jump than if no eccentric was performed. This can easily be seen when comparisons are made between squat jumps (SJ) and CMJs (6,7). It is believed that this increase in performance occurs for several reasons including increased time of activation leading to greater work being produced by the muscle (5), increased muscle activation because of a potentiation effect (4), and storage of negative energy through a series of elastic components that is then transferred to the positive direction (4). However, it is also suggested that the increased premovement muscle activation can allow the muscle to retain a more optimal force-distance relationship and muscle fiber orientation (10,21).
Comparing depth drop jumps (DDJs) with CMJs has often been used to see how the body responds when an increased prestretched load is introduced into the jump. However, performance responses to DDJs differ between individuals. Previous research has demonstrated that jump height and maximal concentric power are generally found between drop heights of 30 and 45 cm (5,24,28). However, as drop height increases beyond these heights, peak force increases without any change in jump height or maximal power (22,28). Ruan and Li (24), when looking at drop heights of 15, 30, 45, and 60 cm, observed that each subject had their own optimal height for jumping and that jumps from heights beyond this height will no longer be able to produce more power and that in fact, power would decrease. He suggested that this ceiling is a point that is limited by the musculotendinous structures and properties of the legs. However, what these structures are and what aspects of jumping they affect have not been determined as of yet.
The contributions of the ankle and knee joints to the jump are affected by jump height (11,12,14). One study (28) compared drop heights of 20, 40, and 60 cm and found that knee moment force-max increased as drop height increased. Another study (24) observed that ankle power decreased when drop height increased from 45 to 60 cm. Gollhofer et al (11) found a similar pattern and explained this phenomenon as the inability of the plantarflexors to sufficiently decelerate the body and store before heel contact. The behavior of muscle also changes based on the muscle in question and the drop height. Ishikawa et al (12) observed that when drop jumps were performed from increasing drop heights but jump height remained constant, VL tendinous tissue stretch increased while fascicle lengthening increased but shortening decreased. In a follow-up study, Ishikawa (14) looked at both MG and VL during short contact DDJ from optimal drop height for vertical displacement and 10 cm above and below this height. In this study, the VL behaved quite differently than the MG. During DDJs for optimal drop height, the VL stretched then shortened, whereas the MG behaved isometerically or shortened, allowing greater stretch shortening from the triceps surae tendinous tissue. However, when a drop height higher than optimal was performed, the MG lengthened and then contracted. The researcher hypothesized that it is possible that at these drop heights, cross bridges within given sarcomeres are unable to tolerate the loading and the mechanical efficiency of the movement is compromised. Because of these observed individual differences in performance and the known alterations in the function of the musculature during increased prejump loading, it is the purpose of this study to investigate if jump performance can be predicted by the muscle structure of the lower body and if certain anatomical structures will allow individuals to increase performance when prestretch loading is increased.
Experimental Approach to the Problem
This study used a cross-sectional experimental design in which 25 men performed 2 separate testing sessions within 24 hours of the other. The first testing session was used to collect anthropometrics and ultrasonography measures. Jump testing was done during the second testing session to diminish any edema related to eccentric muscle damage that may have been produced from the jump testing. During the first testing session, the following measurements were taken in order: height, body mass, VL fascicle thickness and pennation angle, and LG fascicle thickness and pennation angle.
During the second testing session, vertical jump performance was determined. During this session, data were obtained using 3 distinct types of jump: SJ, CMJ, and DDJ. These jumps were used to categorize a minimal, moderate, and large prestretch loads. The peaks for each jump were compared for concentric relative and absolute power and jump height. Peak force data were discarded because it did not correlated with jump height, the main variable of interest of the study. Finally, regression analysis was used to determine if a subject's ability to use the SSC during the different types of jumps could be determined based off of the subject's lower-body muscle structure.
Twenty-five trained subjects, between the ages of 18 and 35, volunteered for the study (Table 1). Each subject had the risks and the benefits of the study explained to them before its initiation and signed an informed consent document approved by the University of Connecticut's Institutional Review Board. Each subject self-reported that they had been involved in at least 6 months of strength training and were familiar and trained with speed, plyometric, or power training or all as a part of their routines. Although familiar with plyometrics, our subjects were not necessarily jump trained but rather all being resistance trained as a key independent variable. Subjects completed a medical questionnaire, and a physician medically approved them before their participation as having no clinical conditions that would confound the results of the study or present a subject risk (e.g., musculotendon or musculoskeletal or neuromuscular pathologies or injury). Each subject was fully familiarized and allowed to practice with each test with jumping technique verified fr proper form and technique before participation in the study.
Muscle architecture was measured using ultrasonography (Table 2 and Figure 1) sampled subcutaneously at 16 Hz (Aspen Advanced; ACUSON, Monsey, NY, USA). All ultrasonography measures were performed on the subject's self-reported dominate side (20,23). Longitudinal probe positioning, as described in previous literature, was used, and standard positioning with equal contact pressure was maintained when sampling (15). Vastus lateralis fascicle thickness and pennation angle were measured at one-half mid thigh length with the subject lying supine with leg straight in a resting position (23,25). Lateral gastrocnemius fascicle thickness and pennation angle were taken at two-thirds lower leg length while the subject rested, lying in a prone position, with their dominate foot unloaded and unsupported (9,29). Based off of the fascicle length and pennation angle, an estimated muscle fiber length was calculated based on the equation used by Fukunaga (10) (Equation 1). The same technician did all ultrasound measures to increase construct validity.
Vertical Jump Testing
A second testing session was performed by subjects within 24 hours of the first. Vertical jump performance was analyzed using force plate and video analysis for the SJ, CMJ, and DDJ. As with the first testing session, subjects were required to refrain from working out for at least 24 hours. Before testing, the subjects all performed a standardized warm-up consisting of moderate cycle ergometry and dynamic lower-body stretches. No static stretching was done because of its known possible interference with power production and the disruption of the elastic component. The dynamic stretches included body weight squats, knee hugs, walking lunges, walking quadriceps stretch, high kicks, and lateral lunges.
All 3 jump types were performed in 2 nonconsecutive sets of 3 jumps in a balanced and randomized order (i.e., a subject may perform 3 CMJ, 3 SJ, 3 DDJ, and then repeat this for the second set). Subjects rested 3 minutes between sets to allow complete recovery (16). Subjects performed their first set of all 3 types of jumps before performing a second set of any specific jump type and then repeated this order for the second set in an effort to minimize any ordering effect. All jumps were performed with the hands on the hips to limit any upper body influence on the jumps. Subjects wore visual markers (Dell, Cheshire, CT, USA) on their left side at the inferior aspect of the lateral malleolus or the tibia, the joint line of the knee, the superior aspect of the greater trochanter of the femur, and the lateral aspect of the acromioclavicular joint. These markers provide a 3-piece model of the lower leg, upper leg, and torso.
All jumps were performed on a calibrated force plate (AccuPower; Athletic Republic, Fargo, ND, USA) with a sampling rate of 200 Hz. The force plate was synchronized and integrated using a video analysis system (DartFish ProSuit 5.0; DartFish, Alpharetta, GA, USA) that sampled at 35 frames per second. Subjects were filmed from the frontal plane to analyze sagittal movement during the jumps. A reference distance marker of 1 m was placed 20 cm from the plate in direct line from the center of the force plate, and the video was leveled and balanced to give an internal framework to all distance measurements.
Countermovement jumps were defined as those jumps in which the subject started in a standing position then dropped down to a volitional depth and jumped as high as possible. Subjects were asked to reset and pause for a second in a standing position between jumps. Static SJs were assigned as those jumps in which the subject would lower themselves to the bottom position of their jump pause for 1-2 seconds and then jump from a dead stop. The bottom position of this jump was assigned off of the natural position observed during the CMJ and practiced before the first SJ set. Depth drop jumps were defined as jumps in which the subjects would step off the box with 1 foot, land on 2 feet, and jump as high as possible. Subjects were told to drop to the most natural position before jumping while keeping the jumps as fluid as possible, but no other instructions were given for any jumps.
Joint angles at the bottom position of the jump and vertical jump height were measured using the video analysis. Jump height was measured as the vertical displacement of the marker at the base of the hips from standing position to peak of jump. Peak force in the z-axis and concentric power were calculated by the DartPower software using information from the force plate. Using the center of mass velocity, center of mass displacement was calculated via the newtonian impulse equation.
The data are presented as mean ± SE. All assumptions for linear statistics were met before the analysis schemes. A paired T-test was used to analyze the differences between jump performance parameters. A Bonferroni correction was used with these tests where appropriate. It was determined that an n of 25 in the sample group was adequate to defend the 0.05 alpha level of significance with a Cohen probability level of at least 0.80 for each dependent variable (nQuery AdvisorÒ software; Statistical Solutions, Saugus, MA, USA) and use in regression analyses for stable beta coefficients. Intraclass Rs ≥ 0.80 were determined for test-retest reliability of the dependent variables. Simple and multiple regressions were used to determine the relationships between and among the dependent variables. For this investigation, significance was defined as p ≤ 0.05.
Jump height, peak power, and relative power were compared using a paired T-test to determine a difference in means between jump types. Then, using a stepwise regression, links between body structure and muscle architecture have been made. Lastly, a comparison of performance parameters between SJ, countermovement (CMJ), and DDJ and the structural measures was made, and regressions were made to determine prediction variables.
Ankle, knee, and hip joint angles were similar between SJ and CMJ conditions. However, DDJ joint angles were significantly different than SJ and CMJ conditions. A series of paired T-tests were used to compare performance measures between jump types. The results showed that jump heights for CMJ and DDJ were significantly greater than for SJs. However, there was no significant difference between CMJ and DDJ (Figure 2).
Muscle Structure and Performance Measures
Muscle architecture was checked as viable predictors for jump performance measures using a descriptive stepwise regression (Table 3). Lateral gastrocnemius pennation angle was the only significant predictor for jump height or relative power for all 3 jump types. For all 3 jump types, LG fascicle thickness was the most significant predictor of absolute power (SJ: r2 = 0.181, p = 0.034; CMJ: r2 = 0.201, p = 0.014; DDJ: r2 = 0.122, p = 0.049) and LG pennation angle was the most significant predictor of power relative to body weight (SJ: r2 = 0.212, p = 0.021; CMJ: r2 = 0.186, p = 0.018; DDJ: r2 = 0.263, p = 0.005).
Differences in Jump Types
Stepwise regression results from comparing individual subject's response to prejump loading is displayed in Table 4. No predictors could be found for the difference between CMJ and SJ in jump height or peak force. When comparing DDJ and CMJ jump heights, LG fascicle length could be used as a weak predictor of jump height (R2 = 0.187). Furthermore, LG thickness could be used to predict the change in peak power at increasing preloads (CMJ-SJ: r2 = 0.201, p = 0.014; DDJ-CMJ: r2 = 0.146, p = 0.034).
However, when comparing the changes in power relative to body weight, LG pennation angle by itself could weakly predict relative power difference (CMJ-SJ: r2 = 0.391, p = 0.000; DDJ-CMJ: r2 = 0.136, p = 0.039).
There are 3 major findings of this study. The first is that significant predictions of jump performance can be made by accounting for the lower leg muscle architecture of the jumper. Second, as preload increases, having specific LG muscle architecture can predict an individual's response to the increased load. Lastly, the muscle architecture most beneficial for jumping is not necessarily the same for sprinting, suggesting that training specificity be a goal of strength and conditioning coaches when prescribing exercises for athletes. The subjects in this study were resistance trained, not jump trained as a group. Thus, the interpretations of our data must be put in this context. Furthermore, strength levels and training state might have well influenced jumping performance and muscle architecture. In addition, some subjects may use more strength for their jumping actions than others and some may use more explosiveness. Thus, such factors will also influence our data. We had no measures of muscle activation for jumping performance in this study, and prior studies have shown this to be important and differ with regards to one's training history. Therefore, one needs to put these concerns into context when examining our data. Although we admit that relationships to jumping are difficult to interpret, we had very high reliability of the measures used in this investigation.
An interesting finding of this study was the observation that only LG muscle structure and not VL muscle structure could predict performance during jumping. This is despite that when comparing jump types, it has been well established that as preload increases, the contribution of the knee joint increases, whereas that of the ankle decreases (24,28). However, the importance of LG architecture at increased preloads is not surprising. Although ankle moment may decrease at these high preloads because of the inability to decelerate, the body or spontaneous elongation of the fascicles because of high external forces having optimal muscle architecture should allow the ankle to contribute better in these situations, thus increasing the strength of the “weak link in the chain.”
The main variable of interest of this study (jump height) could be significantly predicted by LG pennation angle for all 3 jump types. The influence of muscle pennation on performance can be both negative and positive (2). At a given muscle volume greater pennation, force per cross-sectional area has been reported to decrease with an increased pennation (13). Furthermore, it is generally agreed upon that at high contraction velocities, a muscle with greater pennation will see a greater diminishment of force as per the force-velocity relationship (1-3,17). However, with greater pennation, it is believed that a greater number of fibers can be fit within a given cross-sectional area, which increases the physiological cross-sectional area of the muscle allowing for greater force to be developed (3).
An interesting finding of this study was that absolute power was best predicted by LG muscle thickness, and this relationship was not significantly strengthened when taking into account pennation angle or fascicle length. However, when power was calculated relative to the subject's body mass LG pennation angle was the only significant predictor of power. This was not only the case when looking at each individual type of jump but also when comparing jumps with increased preloads. It can be extrapolated from this data that LG muscle thickness, which was used as an estimate of muscle size, was more important when looking at the body's ability to produce force over a high velocity (the definition of power). However, when the force needed to overcome increases by either increasing the mass of the body or adding negative momentum to be overcome, pennation angle is much more important.
It is postulated from this data that pennation, in addition to allowing for high instantaneous force by increasing the physiological cross-sectional area, may also play an important role during eccentric muscle actions in the gastrocnemius. The gastrocnemius has a long tendon, which efficiently can store and redirect forces during stretch shortening movements (18,21). However, the gastrocnemius must act isometerically or concentrically at a relatively slow velocity for this to occur optimally (20,21). A highly pennated muscle may be better equipped to handle these high eccentric loads through several mechanisms. Increased passive resistance of nonparallel contractile components and dissipation of forces by indirect force transmissions based on the cosine of the angle of pennation may lead to increased eccentric performance, increased muscular stiffness, or more isometeric-like qualities during muscle lengthening. Furthermore, with greater pennation, muscle fascicles will lengthen or shorten at a velocity higher than that of the whole muscle. This may have possible influences on stretch reflexes or allow for possible previous modulation of the force-velocity relationship for active muscle tissue. However, these explanations are only speculative and further research needs to be done into the role of muscle pennation during eccentric or lengthening contraction.
The jump height differences between CMJ and DDJ could be weakly predicted by fascicle length. In the current study, subjects with shorter fascicle lengths were better able to handle the increased downward forces of the DDJ, suggesting that longer fascicles cannot withstand high eccentric forces as well as shorter fascicles. One possible explanation is that longer fascicles have more potential places of fascicle disruption leading to greater fascicle instability. While this explanation is possible to date no direct evidence has linked fascicle length to the number of sarcomeres in series (14).
Understanding the role of muscle structure during jumping can be beneficial to coaches and athletes. While knowing that an athlete can be at an anatomical advantage for a given movement may be important for talent identification. However, a more direct implication for this study is for athlete development. Previous research has shown pennation angle adaptations in the VL from traditional strength training (3) and in the gastrocnemius (8) from eccentric strength training in young healthy men. However, more research is needed to better understand the muscle architectural adaptations of the gastrocnemius from other training modalities and in other populations.
Our results for jumping differ from those of sprinting. Whereas previous research showed that faster 100-m sprinters typically had longer fascicle lengths and smaller pennation angle in the gastrocnemius (17), our results show the opposite that having a greater pennation angle will allow subjects to jump higher and produce more power in all of our jump conditions. Furthermore, that as preloading increased between CMJs and DDJs, having a longer fascicle length was inversely related to jump height. The disparity in these results most likely plays into the role of the gastrocnemius during jumping and running. However, it should be noted that greater LG muscle thickness has now been related to both greater running velocity and jump performance.
The gastrocnemius is a biarticular muscle because it crosses 2 joints and is involved in 2 types of movement: plantarflexion and knee flexion. Before the foot strike of the maximal velocity phase of sprinting, the ankle starts in a relatively dorsiflexed position and the hips and knee are being extended. However, after foot strike occurs, the role of the knee rapidly changes from extension to flexion. This rapid extension of the hips, flexion of the knee, and rebound from the ankle are vital to the propulsion needed during sprinting (27). In contrast, during jumping, the gastrocnemius works in a separate manner. The movement of jumping is a much simpler model than sprinting in that center of mass remains relatively constant and over the base of support. During the negative phase of a jumping movement, the ankle dorsiflexes, whereas the knee and hips flex. Then during the positive phase, the hips and knee extend, whereas the ankle plantar flexes.
The disparity between the plantarflexion and knee flexion that is seen during sprinting and the plantarflexion and knee extension seen during jumping emphasizes that the anatomy most beneficial to the movement differs. For sprinting, both the origin and insertion of the gastrocnemius quickly accelerate toward each other. In this situation, a long fascicle length and low pennation angle will be most beneficial for the contraction speed (1,27). However, during jumping, being able to maintain the tension in the gastrocnemius during knee extension and successfully transfer that energy to the Achilles tendon is far more important. This suggests that strength coaches and sport coaches should focus on similar movement patterns to those observed in the sport of the athlete. For example, 100-m sprinters may be better served by focusing on plyometeric exercises such as ankling, bounding, or pulling drills, whereas volleyball players should focus on vertical jumping drills such as CMJs or DDJs.
In summary, muscle architecture can be used to predict jump performance for SJ, CMJ, and DDJ, as well as an individual's response to increase preloads. Lateral gastrocnemius pennation and thickness were positively related to jump performance and can possibly be trained to allow for better performance of by improving the “weak link in the chain.” Shorter LG fascicle length could also weakly predict the change in jump height between CMJ and DDJ, but more research is needed to understand this relationship. Lastly, sport specificity should be a focus for coaches when prescribing exercises because of the role of the gastrocnemius as a biarticular muscle.
1. Abe, T, Kumagai, K, and Brechue, WF. Fascicle
length of leg muscles is greater in sprinters than distance runners. Med Sci Sports Exerc
32: 1125-1129, 2000.
2. Blazevich, AJ. Effects of physical training and detraining, immobilization, growth, and aging on human fascicle
geometry. Sports Med
36: 1003-1017, 2006.
3. Blazevich, AJ, Cannavan, D, Coleman, DR, and Horne, S. Influence of concentric and eccentric resistance training on architectural adaptation in human quadriceps muscles. J Appl Physiol
103: 1565-1575, 2007.
4. Bobbert, MF, Gerritsen, KG, and Litjens, MC. Why is countermovement jump height greater than squat jump height. Appl Sci
28: 1402-1412, 1996.
5. Bobbert, MF, Huifing, PA, and Schenau, GJ. Drop jumping 1 the influence of jumping technique on the biomechanics of jumping. Med Sci Sports Exerc
19: 332-338, 1987.
6. Cavagna, GA. Force platforms as ergometers. J Appl Physiol
39: 174-179, 1985.
7. Cavagna, GA, Dusman, B, and Margaria, R. Positive work done by the previously stretch muscle. J Appl Physiol
24: 21-31, 1968.
8. Duclay, J, Martin, A, Duclay, A, Cometti, G, and Pousson, M. Behavior of fascicles and the myotendinous junction of human medial gastrocnemius following eccentric strength training. Muscle Nerve
39: 819-827, 2009.
9. Fredberg, U, Bolvig, L, Andersen, NT, and Stengaard-Pendersen, K. Ultrasonography in evaluation of Achilles and patella tendon thickness. Ultraschall Med
29: 60-65, 2008.
10. Fukunaga, T, Ichinose, Y, Ito, M, Kawakami, Y, and Fukashiro, S. Determination of fascicle
length and pennation
in a contracting human muscle in vivo. J Appl Physiol
82: 354-358, 1997.
11. Gollhofer, A, Strojnik, V, Rapp, W, and Schweizer, L. Behavior of triceps surae muscle-tendon complex in different jump conditions. Eur J Appl Physiol
64: 283-291, 1992.
12. Ishikawa, M, Finni, T, and Komi, PV. Behaviour of vastus lateralis muscle-tendon during high intensity SSC exercises in vivo. Acta Physiol Scand
178: 205-213, 2003.
13. Ikegawa, S, Funato, K, Tsunoda, N, Kanehisa, H, Fukunaga, T, and Kawakami, Y. Muscle force per cross-sectional area is inversely related with pennation
angle in strength trained athletes. J Strength Cond Res
22: 128-131, 2008.
14. Ishikawa, M, Niemela, E, and Komi, PV. Interaction between fascicle
and tendinous tissues in short-contact stretch-shortening cycle exercise with varying eccentric intensities. J Appl Physiol
99: 217-223, 2005.
15. Klimstra, M, Dowling, J, Durkin, JL, and MacDonald, M. The effect of ultrasound
probe orientation on muscle architecture measurement. J Electromyogr Kinesiol
17: 504-514, 2007.
16. Kubo, K, Morimoto, M, Komuro, T, Tsunoda, N, Kanehisa, H, and Fukunaga, T. Influences of tendon stiffness, joint stiffness, and electromyographic activity on jump performances using single joint. Eur J Appl Physiol
99: 235-243, 2007.
17. Kumagai, K, Abe, T, Brechue, WF, Ryushi, T, Takano, S, and Mizuno, M. Spring performance is related to muscle fascicle
length in male 100-m sprinters. J Appl Physiol
88: 811-816, 2000.
18. Kurokawa, S, Fukunaga, T, and Fukashiro, S. Behavior of fascicles and tendinous structures of human gastrocnemius during vertical jumping. J Appl Physiol
90: 1349-1358, 2001.
19. Kraemer, WJ, Hatfield, DL, Volek, JS, Fragala, MS, Vingren, JL, Anderson, JM, Spiering, BA, Thomas, GA, Ho, JY, Quann, EE, Izquierdo, M, Häkkinen, K, and Maresh, CM. Effects of amino acid supplement on physiological adaptations to resistance training. Med Sci Sports Exerc
41: 1111-1121, 2009.
20. Maganaris, CN, Baltzopoulos, V, and Sargeant, AJ. In vivo measurements of the triceps surae complex architecture in man. J Physiol
512: 603-614, 1998.
21. Manal, K, Roberts, DP, and Buchanan, TS. Optimal pennation
angle of the primary ankle plantar and dorsiflexors. J Appl Biomech
22: 255-263, 2006.
22. McBride, JM, McCaulley, GO, and Cormie, P. Influence of preactivity and eccentric muscle activity on concentric performance during vertical jumping. J Strength Cond Res
22: 750-757, 2008.
23. Pang, BSF and Ying, M. Sonographic measurement of Achilles tendon in asymptomatic subjects. J Ultrasound Med
25: 1291-1296, 2006.
24. Ruan, M and Li, L. Influence of a horizontal approach on the mechanical output during drop jumps. Res Q Exerc Sport
79: 1-9, 2008.
25. Rutherford, OM and Jones, DA. Measurement of fibre pennation
in the human quadriceps in vivo. Eur J Appl Physiol
65: 433-437, 1992.
26. Seynnes, OR, Boer, MD, and Narici, MV. Early skeletal muscle hypertrophy and architectural changes in response to high-intensity resistance training. J Appl Physiol
102: 368-373, 2007.
27. Stafilidis, S and Arampatzis, A. Muscle-tendon unit mechanical and morphological properties and sprint performance. J Sports Sci
25: 1035-1046, 2007.
28. Walsh, M, Arapatzis, A, Schade, F, and Bruggemann, GP. The effect of drop jump starting height and contact time on power, work performed, and moment of force. J Strength Cond Res
18: 561-566, 2004.
29. Ward, SR, Eng, CM, Smallwood, LH, and Lieber, RL. Are current measurements of lower extremity muscle architecture accurate? Clin Orthop Relat Res
476: 1074-1082, 2009.