Author(s): MONTE, A., TRINCHI, M., NUNES, J.P., BIZET, B., DAINI, M., LATINI, D., PAVEI, G., BLAZEVICH, A.J., ZAMPARO, P., Institution: UNIVERSITY OF VERONA, Country: ITALY, Abstract-ID: 860

INTRODUCTION:
Skeletal muscle efficiency – the ability to convert chemical energy into mechanical work – peaks at around 25-30%, but the ratio between mechanical output and metabolic input exceeds these values (e.g. up to 60% in hopping and 80% in running) during human movements involving the stretch-shortening cycle (SSC); this ratio is thus called “apparent” efficiency (AE). The reasons AE is so elevated in SSC remain a matter of debate, but may be attributed either to tendon mechanical capacity (stiffness or elastic recoil) or to the active (muscle) MTU component behaviours (i.e. fascicle behaviours). In this study we combined metabolic, kinematic, kinetic, and ultrasound data to explore the determinants of AE during a SSC task.
METHODS:
Fifteen healthy subjects (8M/7F) (age: 27.6 ± 4.6 y; body mass: 63.4 ± 11.6 kg; height: 1.69 ± 0.08 m) performed bilateral hops both unloaded and with added mass of 15% or 30% of body mass (BM), to manipulate this task’s mechanical and metabolic demands (and hence AE). During each trial, whole-body kinematics, plantarflexor EMG activity (soleus, gastrocnemius medialis and lateralis), ground reaction forces, and ultrasound images of the medial gastrocnemius fascicles and the Achilles tendon (AT) were collected using a 3D motion-capture, electromyography (EMG), force platform, and ultrasonography. Oxygen uptake was measured breath-by-breath using a metabolimeter. Total mechanical power at the whole-body level (Ptot) was calculated from kinetic data. Net metabolic power (Pmet) was computed from net oxygen uptake, and AE was calculated as Ptot / Pmet. Fascicle shortening velocity (Vfas) was determined during the stance phase by tracking ultrasound images. AT mechanical power (Pten) and stiffness were derived from the tendon force‒elongation relationship. Plantarflexor cumulative muscle activation (ACT) was also calculated from EMG signals. Changes in AE across loading conditions were analysed using multivariate regression combined with dominance analysis to quantify the relative contribution of each parameter to the observed changes in AE.
RESULTS:
Ptot and AE decreased with increasing load, whereas Pmet increased. Vfas and Pten decreased as load increased, while AT stiffness and ACT increased. The model explained 77% of the variance in AE across loading conditions. Reductions in Pten accounted for 35% and 38% of the AE changes at 15% and 30% BM, respectively, whereas increases in AT stiffness accounted for 18% and 20%. Vfas contributed 10% and 12% of the AE variance, while ACT accounted for the remaining variance (14% and 7%).
CONCLUSION:
Changes in the AT mechanical capacity explain about 55% of the changes in AE with increasing load, while the activity of the contractile components accounts for 20%. These data suggest that the decrease in AE observed in obese individuals results from an impairment of tendon elasticity rather than of the active (muscle) components of the MTU.