Reduced Achilles tendon stiffness in aging associates with higher metabolic cost of walking.

The mechanisms responsible for increased metabolic cost of walking in older adults are poorly understood. We recently proposed a theoretical premise by which age-related reductions in Achilles tendon stiffness (k_AT) can disrupt the neuromechanics of calf muscle force production and contribute to faster rates of oxygen consumption during walking. The purpose of this study was to objectively evaluate this premise. We quantified k_AT at a range of matched relative activations prescribed using electromyographic biofeedback and walking metabolic cost and ankle joint biomechanics in a group of 15 younger (age: 23±4 yrs) and 15 older adults (age: 72±5 yrs). Older adults averaged 44% lower k_AT than younger adults at matched triceps surae activations during isokinetic dorsiflexion tasks on a dynamometer (p=0.046). Older adults also walked with a 17% higher net metabolic power (p=0.017) but indistinguishable peak Achilles tendon forces than younger adults. Thus, data implicate altered tendon length-tension relations with age more than differences in the operating region of those length-tension relations between younger and older adults. In addition, we discovered empirical evidence that lesser k_AT - likely due to the shorter muscle lengths and thus higher relative activations it imposes - was positively correlated with higher net metabolic power during walking (r=-0.365, p=0.048). These results pave the way for interventions focused on restoring ankle muscle-tendon unit structural stiffness to improve walking energetics in aging.