MUSCLE AND TENDON INTERACTION OF THE HUMAN GASTROCNEMIUS MEDIALIS DURING BODY WEIGHT SUPPORTED RUNNING USING THE VERTICAL TREADMILL FACILITY

Abstract

INTRODUCTION: To attenuate physiological deconditioning due to a lack of gravity, current ISS crew members perform a daily exercise countermeasure program, including frequent treadmill running. However, the maximum vertical loading applied is usually not more than 70-80% of body weight (BW) [1] leading to a reduced requirement of the gastrocnemius medialis (GM) muscle-tendon unit (MTU) to generate force and work to support and propel the human body [2]. As lower forces are acting on the Achilles tendon during unloading, we expect a smaller tendon elongation but its impact on fascicle and MTU length is still unknown. Therefore, the aim of this study was to confirm a smaller tendon elongation and to determine, whether it is compensated by a shorter MTU (different joint angles), longer fascicles or smaller pennation during running at 70% and 38% BW (Mars G). METHODS: Eight male subjects (31.9 ± 4.7 yrs) ran at 125% of their preferred walk-to-run transition speed at 70% and 38% BW on the vertical treadmill facility, and with 100% BW. GM fascicle length and pennation were measured via ultrasonography. Further, joint kinematics (goniometers) were analyzed to determine GM MTU length. Tendon length was estimated by using a MTU model [3]. Plantar pressure (loadsol) was measured to determine stance phase. One-way repeated measures ANOVA for dependent samples (38% vs 70% vs 100% BW) followed by post-hoc tests with Holm-Sidaks corrections for multiple comparisons were used to test for significant differences in joint angles, MTU and tendon elongation, fascicle length and pennation when running at different loadings. RESULTS: Compared to 100% BW, maximum ankle dorsiflexion and knee flexion during early stance was significantly smaller at 38% BW (- 6.4 ± 3.6°, p = 0.003; -8.1 ± 3.8°, p = 0.001), whereas for 70% BW no significant differences were found. Tendon and MTU elongation was significantly smaller at 70% BW (18.70 ± 13.7%, p = 0.005; 25.50 ± 20.3%, p = 0.009) and 38% BW (35.2 ± 11.5%, p < 0.001; 47.3 ± 17.3%, p < 0.001). Although, GM fascicles operated at a significantly longer length and smaller pennation at 70% BW (8.9 ± 5.8%, p = 0.024; 8 ± 4.4%, p = 0.015) and 38% BW (12.4 ± 6.4%, p < 0.001; 9.5 ± 6%, p = 0.015), fascicle shortening showed no significant differences. CONCLUSION: Reduced tendon elongation proves that less force is acting on the Achilles tendon resulting in a reduced storage of elastic energy. The reduced tendon elongation has shown to be compensated by a smaller MTU elongation, smaller pennation and longer fascicles, which leads to a more economical force generation due to the force-length relationship. These effects are observed at 70% and exaggerated at 38% BW. To determine if running with 70% BW is sufficient to maintain muscle mass and function, additional measurements of torque and neuromuscular activation are required to estimate the strain on the GM.