A motor programme for the initiation of forward-oriented movements in humans

Area of Science:

• Neuroscience
• Biomechanics
• Motor Control

Background:

• Voluntary forward movements are preceded by specific electromyographic (EMG) sequences.
• Understanding the neural control and mechanical consequences of these preparatory muscle activations is crucial for explaining motor initiation.

Purpose of the Study:

• To quantitatively analyze the EMG sequence preceding fast, forward-oriented voluntary movements.
• To investigate the relationship between this EMG sequence and the resulting changes in the center of foot pressure (CoP).
• To determine how postural conditions and movement speed modulate the spatio-temporal parameters of the EMG sequence.

Main Methods:

• Quantitative analysis of EMG activity in normal subjects during various voluntary forward movements.
• Measurement of muscle activation latencies and co-variation with movement onset.
• Assessment of the mechanical effects on the center of foot pressure (CoP) in the sagittal plane.

Main Results:

• A consistent EMG sequence was identified: initial soleus inhibition followed by tibialis anterior (TA) activation.
• This sequence linearly co-varied with movement speed, demonstrating a tight temporal correlation.
• The sequence modulated the CoP, with soleus inhibition and TA activation individually capable of producing backward CoP displacement, but with different time courses.

Conclusions:

• The described EMG sequence functions as a pre-set motor program for forward movements.
• This program precisely adjusts external forces by controlling the CoP, enabling appropriate interaction with prime movers.
• Movement parameters, including amplitude and timing, are pre-programmed based on postural context and intended velocity.