The Effect of Obesity on Triceps-surae Musculotendinous Stiffness and Damping Coefficient of Postmenopausal Women

Abstract

Excessive or insufficient muscle-tendon unit stiffness was identified as a risk factor for injuries. More specifically, higher and lower values of stiffness can lead to bone and soft tissue injuries, respectively. Additionally, it was suggested that subjects with greater body mass index can have greater ankle stiffness to compensate standing instability. PURPOSE: Investigate the effect of body mass on triceps-surae muscle-tendon unit stiffness and damping coefficient. METHODS: Forty-three postmenopausal women (59.0 ± 4.7 years) participated. Two groups (normal weight, obese) were defined using the cutoff value for body mass index of 25 kg/m2. Muscle-tendon unit stiffness for the right leg was assessed in vivo using a damped oscillation technique with an external pure gravitational mass equivalent to 30% of maximal voluntary isometric contraction previously measured for plantar-flexion. Muscle-tendon unit stiffness was computed from the kinetic data recorded in response to plantar-flexion/dorsi-flexion perturbations caused by the application of a controlled impulse (100 N) that disturbs the foot-ankle angle. RESULTS: T-test revealed that the obese group has higher values (20939 ± 3316 N/m) of muscle-tendon unit stiffness than normal weight group (16022 ± 3377 N/m), with (P <0.001, 95%). Significant differences were also found between the obese group (223.9 ± 31.7 Ns/m) and the normal weight group (194.7 ± 48.3 Ns/m) with (P <0.05, 95%) for damping coefficient. CONCLUSIONS: When greater forces are imparted to the body, greater resistance to movement is needed to produce controlled movements. This suggests that the increased muscle-tendon unit stiffness may contribute to improve the control of movement of the obese subjects, which present higher values of inertia. Damping relates to energy absorption and since obese subjects had increased damping coefficient, this suggests they may have improved the ability of muscle-tendon unit to absorb energy, minimizing the effects of excess weight and reduce the risk of injury.