Ankle and hip postural strategies defined by joint torques

Area of Science:

• Biomechanics
• Human motor control
• Neuroscience

Background:

• Previous research identified distinct ankle and hip strategies for sagittal plane posture control.
• Ankle strategy dominates on stable surfaces, while hip strategy is seen on unstable surfaces.
• Optimization models suggest hip strategy may occur during fast translations on flat surfaces.

Purpose of the Study:

• To investigate experimental evidence for hip strategy during backward translations on a flat surface.
• To determine if joint torque analysis can differentiate postural strategies.
• To explore the continuum of postural responses.

Main Methods:

• Normal subjects stood on a flat surface and experienced backward translations at varying velocities (5–55 cm/s).
• Electromyography (EMG) and body kinematics were recorded.
• Joint torques were analyzed to assess postural control mechanisms.

Main Results:

• EMG and kinematics showed a shift towards a mixed hip and ankle strategy as platform velocity increased.
• Joint torque analysis revealed the addition of hip flexor torque during fast translations, augmenting ankle plantarflexor torque.
• Pure hip strategy (hip torque without ankle torque) was not observed.

Conclusions:

• Postural responses represent a continuum, integrating ankle and hip strategies rather than discrete control patterns.
• Joint torques provide a valuable metric for defining and understanding postural control strategies.
• The biomechanics of the body adapt to create a spectrum of postural corrections based on movement demands.