Alterations in step frequency and muscle activities using body weight support influence the ventilatory response to sinusoidal walking in humans

Abstract

The use of body weight support (BWS) can reveal important insights into the relationship between lower-limb muscle activities and the ventilatory response during sinusoidal walking. Here, healthy participants (n = 15) walked on a treadmill while 0%, 30%, and 50% of their body weight was supported with BWS. The walking speed was varied sinusoidally between 3 and 6 km h^-1, and three different frequencies, and periods ranging from 2 to 10 min were used. Breath-by-breath ventilation ([Formula: see text]) and CO₂ output ([Formula: see text]) were measured. The tibialis anterior (TA) muscle activity was measured by electromyography throughout the walking. The amplitude (Amp), normalized Amp [Amp ratio (%)], and phase shift (PS) of the sinusoidal variations in measurement variables were calculated using a Fourier analysis. The results revealed that the Amp ratio in [Formula: see text] increased with the increase in BWS. A steeper slope of the [Formula: see text]-[Formula: see text] relationship and greater [Formula: see text]/[Formula: see text] values were observed under reduced body weight conditions. The Amp ratio in TA muscle was significantly positively associated with the Amp ratio in the [Formula: see text] (p < 0.001). These findings indicate that the greater amplitude in the TA muscle under BWS may have been a potent stimulus for the greater response of ventilation during sinusoidal walking.

Figure 1

The custom-made body weight support (BWS) apparatus. The participant’s body weight was reduced by 0%, 30%, and 50% without disturbing the normal leg-swing motion during walking.

Figure 2

Sinusoidal walking protocols at a sinusoidal speed between 3 and 6 km min⁻¹ at the periods of 2 and 5 min (left panel) and 10 min (right panel). The interpolated 1-s data of pulmonary ventilation (${\dot{\text{V}}}_{\text{E}}$), the CO₂ output ($\dot{\text{V}}{\text{CO}}_2$), and the muscle activities in the time series at each walking protocol are shown for all three BWS conditions (100%BW, 70%BW, and 50%BW) in a representative participant. GAS gastrocnemius, SOL soleus, TA tibial anterior.

Figure 3

The mean (Mx), amplitude (Amp), and phase shift (PS) values in the step frequency (A–C) and stride length (D–F) under the three BWS conditions. T2, 5, and 10 indicate sinusoidal frequency periods of 2, 5, and 10 min, respectively. ^†p < 0.05 50%BW vs. 100%BW. ^#p < 0.05 70%BW vs. 100%BW. Data are mean ± standard error (SE).

Figure 4

Comparisons of the Amp ratio and the PS in ${\dot{\text{V}}}_{\text{E}}$, tidal volume (VT), breathing frequency (Bf), and the partial pressure of end-tidal CO₂ output (P_ETCO₂) under the three BWS conditions. *p < 0.05 50%BW vs. 70%BW. ^†p < 0.05 50%BW vs. 100%BW. Data are mean ± SE.

Figure 5

The relationship between the Amp ratio in ${\dot{\text{V}}}_{\text{E}}$ and $\dot{\text{V}}{\text{CO}}_2$ under the 100%BW, 70%BW, and 50%BW conditions, respectively when the data of all periods are pooled.

Figure 6

The relationship between the averaged Amp ratio regardless of the period in ${\dot{\text{V}}}_{\text{E}}$ and the lower-limb muscle activities.