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

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.