Rotation amplitude dependency of the intrinsic ankle stiffness during standing

BACKGROUND AND AIM: To ensure upright balance the ankle joint stiffness must be sufficient to resist the gravitational pull. This stiffness arises from both intrinsic and reflexive components. Determining their individual contribution might give insight in neuromuscular and balance related disorders. Ankle joint stiffness is often investigated by fitting a parametric model to a torque response, obtained from an applied joint rotation. Direct, non-parametric estimation is often not applicable because the applied rotations cannot rule out reflex activity. Here the rotational amplitude dependency of the intrinsic ankle stiffness was estimated in standing, using fast ramp-and-hold stretches to circumvent reflexive contributions. METHODS: Eight healthy subjects participated in the study. Subjects stood on the Bilateral Ankle Perturbator (BAP, figure top), with which 0.08-0.005 rad plantar- and dorsiflexion rotations were applied to the individual ankle joints. Rotations consisted of 40 ms ramp-and-hold minimum jerk profiles [1]. The intrinsic stiffness was obtained by dividing the difference in torque exerted on the platform before and after rotation onset by the rotational amplitude. These values were normalized to the critical stiffness [2]. EMG data of the triceps surae and tibialis anterior muscles were recorded to investigate reflex activity. RESULTS: The EMG signals of the streched muscles showed short latency reflex activity starting approximately 5 ms after the rotation ended (figure middle). The EMG of the gastrocnemius medialis is shown. The intrinsic ankle stiffness decreased non-linearly with increasing rotation amplitude. There was no significant difference in stiffness between plantar- and dorsiflexions. A fit to all subjects' pooled data in comparison with other values in literature [2-4] is shown (figure bottom). CONCLUSIONS: The intrinsic ankle stiffness is insufficient to ensure balance, hence changes in muscle activation are required to realize upright stance. Reflex activity is not expected to have influenced the stiffness estimates due to the short latency of the perturbations and the electro-mechanical delay of muscle tissue. The decrease in stiffness is attributed to muscle cross-bridge breakage leading to sliding of filaments, decreasing the overall stiffness. References: [1] Burdet e.a.-2000-J.Biomech. [2] Casadio e.a.-2005-Gait Posture [3] Loram e.a.-2002-J.Physiol. [4] Loram e.a.-2007-J.Physiol.