Title: Muscle Fascicle Dynamics Explain Differences In Tendon-Aponeurosis Elongation
Abstract: It has been reported that the aponeurosis stretches more than the free tendon during contraction, presumably because of shortening-induced radial expansion of muscle fascicles. Fascicle pennation angle (PA) also increases during contraction, which reduces the longitudinal fascicle radius. Together, the combined impact of fascicle length (Lf) and PA changes during contraction in explaining the ‘extra’ aponeurosis stretch has not been evaluated. PURPOSE: To quantify fascicle radius change due to Lf and PA, independently, to explain the difference in tendon and aponeurosis elongation during contraction. METHODS: 11 male and female participants performed steady-state isometric plantarflexions of their dominant ankle at various intensities from 20 to 100% of maximum. Two ultrasound probes were placed on the medial gastrocnemius (MG) muscle belly and myotendinous junction (MTJ), respectively, to quantify stretch of the aponeurosis, 5 mm proximal to the MTJ, and Achilles tendon. MG Lf and PA were measured to calculate fascicle radius change, assuming fascicle constant volume and transverse and longitudinal diameters were equal to 1.2 mm at PA = 90°. Data are reported as mean ± SD. RESULTS: Two-way, repeated measures ANOVA revealed a significant location (tendon vs aponeurosis) x force level (%MVC) interaction (p = 0.02) and a significant main effect of location (p < 0.001). Across all force levels, aponeurosis stretch was significantly greater than tendon stretch (mean difference = 2.7 ± 1.9 mm, 14.5 ± 12.0%, p < 0.0001). Radial expansion due to Lf shortening was 0.76 ± 0.08 mm/fascicle (27.0 ± 13.6%) and radial contraction due to increased PA was -0.18 ± 0.13 mm/fascicle (-20.7 ± 11.8%). The additional aponeurosis length change was significantly correlated with Lf radial expansion (r2 = 0.31, p = 0.02), and PA radial contraction (r2 = 0.27, p = 0.047). Combined, Lf radial expansion and PA radial contraction explained all of the additional aponeurosis stretch (mean difference 0.3 ± 2.0 mm). A one sample t-test showed this mean difference was not significantly different from 0 mm (p = 0.52). CONCLUSIONS: MG aponeurosis stretch is greater than tendon stretch, which can be explained by combined effects of radial expansion due to fascicle shortening and radial contraction due to increases in fascicle pennation angle.