Fig 1.
Testing protocol, Experimental set-up and typical trace at 100BW.
Testing protocol (A) performed on the LBPP treadmill (B) which allows running at either normal bodyweight (100BW) or in unweighting conditions. The protocol included two randomized running series performed at 80 and 60% bodyweight (80BW and 60BW, respectively). Each series included three conditions, referred as INITIAL, unweighted (UNW) and reloaded (RLD). The unweighting and reloading transitions (UNWtr and RLDtr, respectively) were induced by a progressive increase and decrease in pressure of the LBPP chamber (C). Frame D shows an individual example of the vertical ground reaction force (Fz) (solid line) and vertical center of mass displacement (ΔH) (dashed line) during the preactivation, contact phase (composed of braking and push-off phases) and flight phase. IPF initial peak force, APF active peak force, and ΔHflight vertical center of mass displacement during the flight.
Fig 2.
(A): Unweighting-induced changes (Δ% INITstab) in the temporal, kinetic and kinematic stride characteristics at the end of the transition phase (UNWtr) and once stabilized (UNWstab) at 60BW. (B): Reloading-induced changes (Δ% UNWstab) in the same parameters. For each variable, the median and interquartile range represents the individual changes. *p < 0.05 and **p < 0.01 when statistically different from their reference values (INITstab and UNWstab, respectively). The stride number corresponding to the onset of significant change is indicated in between parentheses. Significant differences between the last stride of the transition and the mean values once stabilized are indicated by § with p < 0.05.
Fig 3.
The upper panel shows changes in (A) bodyweight (BW), (B) active peak force (APF) and (C) impact peak force (IPF) along the 13 strides of the UNWtr in the 60BW running series. Group mean (+/- standard error) differences from the initial condition values at 100BW (INITstab) are presented as closed points (in black) when significant at p < 0.05. The lower panels (D and E) show the correlations between changes in BW and those of either APF (D) or IPF (E) in the 60BW running series.
Table 1.
Pearson product-moment correlation coefficients (r) between LBPP-induced relative changes of bodyweight and of stride mechanical parameters.
Fig 4.
Unweighting-induced changes (% INITstab) in the EMG activity of the thigh and shank muscles at the end of the transition phase (UNWtr) and once stabilized (UNWstab) at 80BW (upper graph) and at 60BW (lower graph).
Significant group mean (± standard error) differences are presented with * p < 0.05 and ** p < 0.01 when statistically different from the reference INITstab values. GAM value is indicated numerically as it goes well beyond the chosen scale. Significant differences between the last stride of the transition and the mean values once stabilized are indicated by § with p < 0.05.
Table 2.
Unweighting-induced changes (% INITstab) in mean EMG activity of the recorded left limb muscles during the transition (UNWtr) and once stabilized (UNWstab) in the 80BW and 60BW series.
Fig 5.
Significant relationships between the changes in VM, VL and SOL mean muscle activities in the braking phase and (A) the bodyweight changes or (B) the mean braking force changes during the UNWtr in the 60BW running series. r: Pearson coefficient, p: statistical level.
Table 3.
Reloading-induced changes (% UNWstab) in mean EMG activity of the recorded left limb muscles during the transition (RLDtr) and once stabilized (RLDstab) in the 80BW and 60BW series.