The motor cortex drives the muscles during walking in human subjects

Abstract

Indirect evidence that the motor cortex and the corticospinal tract contribute to the control of walking in human subjects has been provided in previous studies. In the present study we used coherence analysis of the coupling between EEG and EMG from active leg muscles during human walking to address if activity arising in the motor cortex contributes to the muscle activity during gait. Nine healthy human subjects walked on a treadmill at a speed of 3.5–4 km h(-1). Seven of the subjects in addition walked at a speed of 1 km h(-1). Significant coupling between EEG recordings over the leg motor area and EMG from the anterior tibial muscle was found in the frequency band 24–40 Hz prior to heel strike during the swing phase of walking. This signifies that rhythmic cortical activity in the 24–40 Hz frequency band is transmitted via the corticospinal tract to the active muscles during walking. These findings demonstrate that the motor cortex and corticospinal tract contribute directly to the muscle activity observed in steady-state treadmill walking.

Figure 1. Data from a single subject during 5 min of slow treadmill walking at 1 km h⁻¹

Cz EEG before (A) and after removal of unwanted components (B) and raw EMG (C) activity from three steps. The black arrows indicate the time of heel strike. The EMG activity had an onset around 600 ms prior to the heel strike and lasted until around 200 ms after the heel strike. Significant estimates of coherence (D) and the imaginary part of coherency (E) between the Cz EEG and TA muscle EMG were observed in the ∼24–40 Hz frequency band for offset values between –600 and –100 ms with a peak at 30 Hz between –500 and –300 ms. Coherence (F) and the imaginary part of coherency (G) between all 28 EEG electrodes and the TA EMG at 30 Hz taken from segments of data from –550 to –50 ms. Significant coupling between the TA muscle EMG and the leg area of the motor cortex was observed. ICA, independent component analysis.

Figure 2. Pooled data from nine subjects during normal walking (3.5–4 km h⁻¹)

Time–frequency analysis of averaged coherence (A) and imaginary part of coherency between the Cz EEG and the TA muscle EMG (B). Significant coupling was observed in the ∼24–40 Hz frequency domain for offsets between –700 ms and –200 ms prior to the heel strike. Average coherence (C) and the imaginary part of coherency (D) at 30 Hz between all 28 EEG electrodes and the TA EMG taken from segments of data from –500 and 0 ms where peak values of coupling were observed.

Figure 3. Pooled data from seven subjects during slow walking at 1 km h⁻¹

Time–frequency analysis of averaged coherence (A) and imaginary part of coherency between the Cz EEG and the TA muscle EMG (B). As for normal walking, significant coupling was observed in the ∼24–40 Hz frequency domain for offsets between –700 ms and –200 ms prior to the heel strike. Average coherence (C) and the imaginary part of coherency (D) at 32 Hz between all 28 EEG electrodes and the TA EMG taken from segments of data from –500 and 0 ms where peak values of coupling were observed. Note the different colour bar scaling compared with Fig. 2.

Figure 4. Pooled data from seven subjects during static muscle contraction

Average coherence between the Cz EEG and the TA EMG (A). A clear peak was detected around 20 Hz. Average coherence (B) and the imaginary part of coherency (C) at 20 Hz between all 28 EEG electrodes and the TA EMG.