Muscle gearing during isotonic and isokinetic movements in the ankle plantarflexors

Muscle–tendon gearing is the ratio of the muscle–tendon unit velocity to the fascicle velocity and can be expressed as the product of the gearing within the muscle belly and the gearing due to tendon stretch. Previous studies have shown that gearing is variable and increases at higher velocities. Changes in the muscle activation levels and force development have been suggested to affect tendon gearing and thus muscle–tendon unit gearing. However, the role of belly gearing as a part of muscle–tendon gearing and its associations with structural aspects of muscle and thus movement performance are important facets that need to be studied. The two gastrocnemii of twenty young adults were tested during isokinetic and isotonic contractions on an ankle dynamometer. Ultrasound images of both muscles were collected during contractions and were later digitised. Gearing was also predicted using a 2-dimensional panel model of these muscles. The results from experimental and models tests showed increases in gearing with greater torque levels at slower contraction velocities. However, in the isotonic models there was a substantial increase in gearing at faster contraction velocities. The level of muscle–tendon unit gearing is largely determined by the belly gearing, but its variability is driven by changes in tendon gearing that in turn is a factor of the muscle activation and coordination. The belly thickness of the medial gastrocnemius decreased during contractions, but increased for the lateral gastrocnemius. It is likely that changes to the belly shape and 3-dimensional structure are important to the gearing of the muscle.