The relationship between passive ankle joint stiffness and the stiffnesses of muscles, nerve and tendon

To clarify whether passive ankle joint stiffness (slope of angle-passive torque curve) is related to stiffnesses in the triceps surae muscles, tibial nerve, and Achilles tendon.

Thirty-eight healthy adults participated in the study. The passive ankle joint stiffness and shear wave velocities (an index of soft tissue stiffness) of the medial and lateral gastrocnemius and soleus muscles and tibial nerve were measured at 5° of ankle plantarflexion and 5°, 15°, and 25° of ankle dorsiflexion. The shear wave velocity of the Achilles tendon was measured only at 5° of plantarflexion. A multiple regression model was constructed at each angle, specifying the shear wave velocities as the independent variables and passive joint stiffness as the dependent variable.

At 5° of plantarflexion, no shear wave velocities were significantly related to passive joint stiffness (all p ≥ 0.05). At 5° and 15° of dorsiflexion, only the shear wave velocities of the tibial nerve were significantly positively related to passive joint stiffness (p = 0.024 and 0.008, β = 0.385 and 0.489, respectively). At 25° of dorsiflexion, the shear wave velocities of the lateral gastrocnemius muscle and tibial nerve were significantly positively related to passive joint stiffness (p = 0.002 and 0.001, β = 0.484 and 0.470, respectively).

Passive ankle joint stiffness may moderately reflect tibial nerve stiffness and, at a higher length, lateral gastrocnemius muscle stiffness.