DIFFERENCES IN GASTROCNEMIUS ARCHITECTURE BETWEEN SPRINTERS AND NON-SPRINTERS: IMPLICATIONS FOR MUSCLE FUNCTION Sabrina S.M. Lee1 and Stephen J. Piazza1,2,3 Depts. of 1Kinesiology, 2Mechanical & Nuclear Engineering, and 2Orthopaedics & Rehabilitation, The Pennsylvania State University, University Park, PA, USA,
[email protected] INTRODUCTION
METHODS AND PROCEDURES
The role of a muscle in producing a specific movement is determined in part by the muscle’s force-generating capacity. This capacity depends on muscle architecture parameters and the muscle’s fiber length and shortening velocity. The relationships between these parameters and muscle function have been explored in animal models (Lieber and Boakes, 1988; Lutz and Rome, 1994), but the influence of muscle and joint structure on in vivo function in humans has received less consideration. The results of previous investigations have suggested that sprinting is an activity that is especially well suited to elucidating these relationships. The fascicle lengths of gastrocnemius in sprinters have been found to be greater than those of non-sprinters (Abe et al., 2000; Abe et al., 2001) and gastrocnemius fascicle length has been found to positively correlate with sprint performance while its pennation angle has been shown to negatively correlate with performance (Kumagai et al., 2000). No studies, however, have reported these parameters in sprinters along with Achilles tendon moment arm, which should critically influence F-L and F-V characteristics.
Fascicle length, pennation angle, and plantarflexion moment arm of the lateral gastrocnemius were measured in 12 male collegiate sprinters (age 19.4± 1.2 y, height 1.81 ± 0.08 m, body mass 77.5 ± 6.5 kg) and 12 young adult males (27.3± 5.7 y, 1.80 ± 0.07 m, 76.9 ± 9.2 kg) who were not engaged in competitive sports. All subjects gave informed consent prior to testing and all procedures were approved by our university’s Institutional Review Board. Fascicle length and pennation angle were determined from images captured using B-mode ultrasonography (Aloka 1100; transducer: SSD-625, 7.5 MHz). These images were made of the central region of the muscle while subjects were standing in anatomical position. To measure moment arm, the foot was rotated from 10º dorsiflexion to 20º plantarflexion using a potentiometerinstrumented rotating foot platform while ultrasound images of the musculotendinous junction were captured. The experimenter manually controlled plantarflexion during each trial while each subject plantarflexed maximally against the foot plate with the knee in full extension. Tendon excursion versus angle data were well approximated by linear fits; plantarflexion moment arms were taken to be the slopes of these lines. Two-tailed ttests were used to test for differences between sprinters and non-sprinters for each parameter. Pearson correlation coefficient was calculated to test for correlation between moment arm and fascicle length.
The purposes of this study were (1) to compare lateral gastrocnemius fascicle length, pennation angle, and Achilles moment arm measured in sprinters and non-sprinters; and (2) to use a simple model to investigate the mechanisms by which these factors influence sprint performance.
A one-muscle, 2-DOF, planar model was used to simulate a sprint push-off subject to covariation in muscle fiber length and plantarflexion moment arm. This simulation was developed using SIMM/Dynamics Pipeline and SD/FAST.
vmax, the intercept of the force-velocity curve, when each of these is expressed in unnormalized, absolute terms (Abe et al., 2001). Smaller moment arms result in smaller length changes for a given ankle rotation, resulting in more favorable operating ranges on the force-length and force-velocity curves.
Figure 1. Sprint push-off model. RESULTS Fascicle lengths were significantly larger and moment arms were significantly smaller in sprinters compared to non-sprinters (Table 1). The pennation angles of sprinters were smaller on average than those of nonsprinters, but this difference was not found to be significant. The work done by the plantarflexor muscle during simulated sprint push-off increased with fiber length but maximal work was produced when the muscle moment arm was between 2 –3 cm (Figure 2). parameter
SPR
Non-SPR
p
31.0 (3.7)
41.6 (5.5)
< 0.001
fascicle length (cm)
7.0 (0.6)
6.1 (0.6)
0.001
pennation angle (°)
12.8 (1.1)
14.1 (1.8)
0.055
moment arm (cm)
Table 1. Mean architecture parameters of lateral gastrocnemius of sprinters (SPR) and non-sprinters. (SD values in parentheses) DISCUSSION The architecture parameters measured for the lateral gastrocnemius in sprinters enhance the force generating capacity of this muscle, enabling sprinters to produce higher ground reaction forces for longer periods of time during push-off. Longer fascicle lengths, which may correspond to longer optimal fiber lengths will correspond to a wider plateau of the active force-length curve and a increase in
Figure 2. Simulated push-off work. SUMMARY Differences between sprinters and nonsprinters in gastrocnemius-Achilles architecture seem to permit sprinters to operate over more favorable ranges on the force-length and force-velocity curves for this muscle. The present study offers insights into the determinants of muscle function during sprinting that may also be relevant during push-off during walking and stair-climbing. REFERENCES Abe, T et al. (2000). Med Sci Sports Exerc, 32:1125-1129. Abe, T et al. (2001). J Physiol Anthropol, 20(2):141-147 Kumagai, K et al. (2000). J Appl Physiol, 88:811-816. Lieber, RL and Boakes, JL (1988). Am J Physiol Cell Physiol, 254:C769-C722., Lutz, GJ and Rome, LC (1994). Science, 263:370-372. ACKNOWLEDGMENT Supported by NSF BES-0134217.
