Loss of muscle performance in seniors: changes to the dynamic muscle structure and muscle gearing

Muscle structure changes with ageing in a manner that can alter its contractile mechanics, resulting in a reduction in strength and mobility. Fascicles within a muscle can shorten at slower velocities than the muscle belly, in a process known as belly gearing. Belly gearing allows the fascicles to produce a greater force when they contract. However, it may be compromised when we age. The gastrocnemii muscles in the calf were imaged in young adults and seniors using ultrasound. Their muscle structures were compared during standing and during ankle extensions. The overall gearing of a muscle-tendon-unit was largely determined by the belly gearing, but its variability was driven by changes in tendon stretch. The decrease in performance and power output in seniors may be associated with reduced gearing due to atrophy of the muscles.