How do differences in Achilles' tendon moment arm lengths affect muscle-tendon dynamics and energy cost during running?

Abstract

Introduction: The relationship between the Achilles tendon moment arm length (AT_MA) and the energy cost of running (E_run) has been disputed. Some studies suggest a short AT_MA reduces E_run while others claim a long AT_MA reduces E_run. For a given ankle joint moment, a short AT_MA permits a higher tendon strain energy storage, whereas a long AT_MA reduces muscle fascicle force and muscle energy cost but shortening velocity is increased, elevating the metabolic cost. These are all conflicting mechanisms to reduce E_run, since AT energy storage comes at a metabolic cost. Neither of these proposed mechanisms have been examined together.

Methods: We measured AT_MA using the tendon travel method in 17 males and 3 females (24 ± 3 years, 75 ± 11 kg, 177 ± 7 cm). They ran on a motorized treadmill for 10 min at 2.5 m · s^-1 while E_run was measured. AT strain energy storage, muscle lengths, velocities and muscle energy cost were calculated during time-normalized stance from force and ultrasound data. A short (SHORT n = 11, AT_MA = 29.5 ± 2.0 mm) and long (LONG, n = 9, AT_MA = 36.6 ± 2.5 mm) AT_MA group was considered based on a bimodal distribution of measured AT_MA.

Results: Mean E_run was 4.9 ± 0.4 J · kg^-1 · m^-1. The relationship between AT_MA and E_run was not significant (r ² = 0.13, p = 0.12). Maximum AT force during stance was significantly lower in LONG (5,819 ± 1,202 N) compared to SHORT (6,990 ± 920 N, p = 0.028). Neither AT stretch nor AT strain energy storage was different between groups (mean difference: 0.3 ± 1 J · step^-1, p = 0.84). Fascicle force was significantly higher in SHORT (508 ± 93 N) compared to LONG (468 ± 84 N. p = 0.02). Fascicle lengths and velocities were similar between groups (p > 0.72). Muscle energy cost was significantly lower in LONG (0.028 ± 0.08 J · kg · step^-1) compared to SHORT (0.045 ± 0.14 J · kg · step^-1 p = 0.004). There was a significant negative relationship between AT_MA and total muscle energy cost relative to body mass across the stance phase (r = -0.699, p < 0.001).

Discussion: Together these results suggest that a LONG AT_MA serves to potentially reduce E_run by reducing the muscle energy cost of the plantarflexors during stance. The relative importance of AT energy storage and return in reducing E_run should be re-considered.

Keywords: energy cost; metabolic cost of force production; running economy; strain energy storage; ultrasound.