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

• Published in: Frontiers in sports and active living Vol. 5; p. 1125095
• Main Authors: Bennett, Eric C, Machado, Esthevan, Fletcher, Jared R
• Format: Journal Article
• Language: English
• Published: Switzerland Frontiers Media S.A 17-04-2023
• Subjects: energy cost; metabolic cost of force production; running economy; Sports and Active Living; strain energy storage; ultrasound; strain energy storage; energy cost; metabolic cost of force production; running economy; ultrasound
• ISSN: 2624-9367; 2624-9367
• DOI: 10.3389/fspor.2023.1125095

The relationship between the Achilles tendon moment arm length (AT
) and the energy cost of running (E
) has been disputed. Some studies suggest a short AT
reduces E
while others claim a long AT
reduces E
. For a given ankle joint moment, a short AT
permits a higher tendon strain energy storage, whereas a long AT
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
, since AT energy storage comes at a metabolic cost. Neither of these proposed mechanisms have been examined together.
We measured AT
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
while E
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
 = 11, AT
 = 29.5 ± 2.0 mm) and long (LONG,
 = 9, AT
 = 36.6 ± 2.5 mm) AT
group was considered based on a bimodal distribution of measured AT
Mean E
was 4.9 ± 0.4 J · kg
 · m
. The relationship between AT
and E
was not significant (
 = 0.13,
 = 0.12). Maximum AT force during stance was significantly lower in LONG (5,819 ± 1,202 N) compared to SHORT (6,990 ± 920 N,
 = 0.028). Neither AT stretch nor AT strain energy storage was different between groups (mean difference: 0.3 ± 1 J · step
,
 = 0.84). Fascicle force was significantly higher in SHORT (508 ± 93 N) compared to LONG (468 ± 84 N.
 = 0.02). Fascicle lengths and velocities were similar between groups (
 > 0.72). Muscle energy cost was significantly lower in LONG (0.028 ± 0.08 J · kg · step
) compared to SHORT (0.045 ± 0.14 J · kg · step
 = 0.004). There was a significant negative relationship between AT
and total muscle energy cost relative to body mass across the stance phase (
 = -0.699,
 < 0.001).
Together these results suggest that a LONG AT
serves to potentially reduce E
by reducing the muscle energy cost of the plantarflexors during stance. The relative importance of AT energy storage and return in reducing E
should be re-considered.