EMG responses to maintain stance during multidirectional surface translations

Area of Science:

• Biomechanics and Motor Control
• Neuroscience
• Human Physiology

Background:

• Understanding muscle synergy organization is crucial for characterizing multidirectional control of stance posture.
• Previous models often assume fixed muscle synergies, which may not fully explain complex postural responses.

Purpose of the Study:

• To investigate the organization of muscle synergies during multidirectional stance postural control.
• To analyze the relationship between muscle response latency, amplitude, and perturbation direction.

Main Methods:

• Electromyographic (EMG) activity was recorded from 11 lower limb and trunk muscles in 7 healthy subjects.
• Subjects were subjected to horizontal surface translations in 12 random directions.
• Muscle response latencies, amplitudes, and tuning curves were quantified and analyzed relative to perturbation direction.

Main Results:

• Muscle response latencies were direction-dependent for some trunk and thigh muscles (tensor fasciae latae, rectus abdominis, erector spinae), suggesting varied roles.
• Most muscles exhibited tuning curves within one quadrant, with maximal activity often in diagonal directions.
• Some muscles showed bipolar tuning curves or maximal activity orthogonal to predicted directions, indicating complex recruitment patterns and non-anatomic synergies.

Conclusions:

• Neither simple reflex mechanisms nor fixed muscle synergies adequately explain observed muscle activation patterns during postural control.
• Results support a centrally mediated pattern of muscle latencies combined with peripheral influences on muscle magnitude.
• A flexible, task-dependent continuum of muscle synergies is proposed for effective equilibrium control in stance.