Isokinetic Strength and Plantarflexor Structure in Sprinters and Non-sprinters

Open Access
Kalkbrenner, Ryan Christopher
Area of Honors:
Bachelor of Science
Document Type:
Thesis Supervisors:
  • Stephen Jacob Piazza, Thesis Supervisor
  • Steriani Elavsky, Honors Advisor
  • Plantarflexor (PF)
  • Gastrocnemius lateralis (GL)
INTRODUCTION Studies of the musculoskeletal structure of sprinters suggest that sprinters differ from their non-sprinting counterparts in several ways that have the potential to affect sprinting performance. The plantarflexor (PF) muscles have received the bulk of this attention. Sprinters have been shown to have longer PF muscle fascicles, lesser PF pennation, and shorter moment arms for the Achilles tendon [1,2,3]. Characteristics such as these should facilitate the production of work at high shortening velocities, and this has been demonstrated using musculoskeletal computer simulations [2,3]. The relationship of these differences to actual human performance, however, is the subject of some controversy. Sprint performance has been shown to correlate with PF fascicle length [4], but another study showed neither differences in PF properties between fast and slow sprinters nor correlation between these properties and performance [5]. The purpose of this study was to test (1) whether sprinters are less affected by the effects of speed than non-sprinters when generating maximal PF moments in a dynamometer; and (2) if enhanced isokinetic PF strength among sprinters is associated with variation in musculoskeletal structure. METHODS The participants in this study were 6 club-level collegiate sprinters (178±2 cm; 74±5 kg; 22±2 y) and 9 non-sprinters (179±6 cm; 79±16 kg; 21±2 y). There were no significant differences between the sprinter and non-sprinter groups in terms of stature, mass, BMI, or age (all p ≥ 0.541). All participants provided informed consent and all experimental procedures were approved by the Institutional Review Board of The Pennsylvania State University. Photographs were made of each participant’s right foot in order to derive foot anthropometric measurements, including the distance from the lateral malleolus posterior to the Achilles tendon, which we used as a proxy for Achilles tendon moment arm. B-mode ultrasonography (Aloka 1100; transducer: SSD-625, 7.5 MHz and 39 mm scan width) was used to capture still images of the gastrocnemius lateralis (GL) as each participant stood. From these images we obtained measures of GL thickness t and pennation angle α; the fascicle length was calculated from these measures according to lF = t / sin(). Plantar flexor strength was measured with subjects seated in a System 3 isokinetic dynamometer (Biodex Medical Systems) with the right foot unshod. Maximal plantar flexor torque was measured under isometric conditions and isokinetic conditions as the foot plate was rotated in the plantar flexion direction at 30 °/s, 120 °/s, and 210 °/s. Plantarflexor moment was assessed as the ankle passed through its neutral position at each speed in order to minimize length-dependent effects. T-tests were performed to identify sprinter-non-sprinter differences in the measured variables. A mixed model ANOVA was used to test for the influence of speed and group on maximum isokinetic moment normalized by isometric moment. Finally, simple regressions were done to identify correlations between isokinetic strength and fascicle length, moment arm, and pennation angle. The level of significance for these tests was set at p ≤ 0.05.   RESULTS AND DISCUSSION ANOVA revealed a main effect for speed (p < 0.001); subjects in both groups saw significant reductions in strength at each level as the speed increased. The sprinters were stronger than non-sprinters at all speeds tested (Figure 1), but these differences were not statistically significant. There was a trend toward a significant interaction between group and speed (p = 0.113), suggesting that sprinters’ PF torque generation at higher speeds may be less susceptible to force velocity effects than that of the non-sprinters. Similar to the previous reports, we found sprinters to have smaller GL pennation angles (p = 0.018) and longer GL fascicles (p = 0.006). There was no difference between the groups in moment arm (as approximated by heel length), however (p = 0.905). Sprinters and non-sprinters exhibited different relationships between isokinetic strength and structural properties. For example, there was no significant correlation between maximum PF torque at 210 °/s for non-sprinters (p = 0.709), but there was a strong (R2 = 0.579) and nearly significant (p = 0.079) correlation for sprinters (Figure 2). This apparent association suggests that sprinters may owe some of their maintenance of PF strength at high speeds to their longer fascicles. CONCLUSIONS It is important to note that several of these findings based on our current data analysis do not rise to the level of statistical significance. With only n = 6 sprinters and n = 9 non-sprinters, this is perhaps not surprising. Data collection in this study is ongoing and it remains to be seen if the nearly significant findings will become significant after more participants are tested. The dynamometer measurements of PF strength are not direct measures of sprinting performance, but they are measurements of performance that are very likely related to sprinting ability. We elected to study PF strength rather than sprinting performance because of the myriad of other variables (including technique and reaction time) that influence sprint times. Future work will address how (and whether) sprinters adapt to sprint training to achieve musculoskeletal properties favorable for torque generation during rapid shortening. REFERENCES 1. Abe T, et al. Med Sci Sports Exc 32, 1125-9, 2000. 2. Lee SSM and Piazza SJ. J Exp Biol 212, 3700-7, 2009. 3. Baxter JR et al. Proc Royal Soc B 279, 2018-24, 2012. 4. Abe T, et al. J Phys Anthrop 20, 141-7, 2001. 5. Karamanidis K, et al. Gait Posture 34, 138-141, 2011.