Voluntary control of forward leaning posture relates to low-frequency neural inputs to the medial gastrocnemius muscle.

BACKGROUND: Variability is an inherent feature of the motor output. Although low-frequency oscillations (<0.5 Hz) are the most important contributor to the variability of force during single-joint isolated force tasks, it remains unclear whether they contribute to the variability of a more complex task, such as a voluntary postural task.

RESEARCH QUESTION: Do low-frequency oscillations contribute to postural sway (center of pressure (COP) variability) when participants attempt to voluntarily maintain posture in a forward leaning position?

METHODS: Fourteen healthy young adults performed two tasks: 1) stand quietly (control condition); 2) leaned their body forward to 60% of their maximum lean distance by dorsiflexing the ankle joint. We recorded the COP and electromyographic (EMG) activity from the medial gastrocnemius (MG) and soleus (SL) muscles. We quantified the following: 1) COP variability as the standard deviation (SD) of anteroposterior COP displacements; 2) modulation of COP as the power in COP displacements from 0 to 2 Hz; 3) modulation of EMG bursting as the power in the rectified and smoothed EMG from 0 to 2 Hz; 4) modulation of the interference EMG as the power in the EMG from 10 to 35 and 35-60 Hz.

RESULTS: The SD of COP displacements related to the COP oscillations <0.5 Hz in both quiet standing and lean tasks. However, only for the lean task, the <0.5 Hz COP oscillations related to the EMG burst oscillations <0.5 Hz of the MG muscle. The EMG burst oscillations <0.5 Hz of the MG muscle further related to the interference EMG oscillations from 35 to 60 Hz for the lean task.

SIGNIFICANCE: Voluntary control of forward leaning posture relates to low-frequency neural inputs to the MG muscle.