Figure 1.
Initial/final posture and experimental set-up.
Upper panels: front views; Lower panels: profile views. The subject of the photograph has given written informed consent, as outlined in the PLOS consent form, to publication of the photograph.
Figure 2.
Example of biomechanical traces during paired leg flexion with index-finger extension in the reaction-time and self-initiated conditions (one trial in one representative subject in the young and elderly adults group).
yP, y”G, y’G, yG, z’G: mediolateral (ML) centre of pressure (CoP) displacement, ML centre of gravity (CoG) acceleration, ML CoG velocity, ML CoG displacement and vertical CoG velocity, respectively. t0, t1, t2, t3: onset variation of yP trace from baseline, swing heel-off, index-finger extension (hand response) and swing foot-off time, respectively. Sw, St, Up, Down: swing leg side, stance leg side, upward and downward, respectively. APAd, yPAPA, y’’GMAX, y’GFO, yGFO, z’GMAX: APA duration, peak of ML CoP displacement during APA, peak of ML CoG acceleration, ML CoG velocity and displacement at the foot-off time and peak of vertical CoG velocity, respectively.
Figure 3.
Comparison of simultaneous index-finger extension and swing heel-off inter-onset latency (IOL) in the pilot (A) and main (B) experiments.
RT, SI: reaction-time and self-initiated condition, respectively. Positive IOL indicates a precession of index-finger extension over swing heel-off. Negative IOL indicates a precession of swing-heel off over index-finger extension. Reported values are mean ± 1 standard deviation (all participants combined). RT: reaction-time condition; SI: self-initiated condition. ###: significant main effect of posture with p < 0.001; **, ***: significant difference with p < 0.01 and p < 0.001, respectively, as revealed with the planned comparisons. Note the inversion of the IOL values between the RT and SI conditions in the Figures A and B.
Figure 4.
Comparison of selected spatio-temporal features of APA and focal movement performance across groups (young vs. elderly adults) and conditions (self-initiated vs. reaction-time).
APAd, yPAPA, z”GMAX: duration of APA, peak of mediolateral centre of pressure displacement towards swing leg side, peak of vertical centre of gravity velocity, respectively. Reported values are mean ± 1 standard deviation (all participants combined). RT: reaction-time condition; SI: self-initiated condition. ###: significant main effect of time pressure with p < 0.001; ***: significant difference with p < 0.001 as revealed with the post hoc test; +++: significant main age effect with p < 0.001.
Figure 5.
Comparison of postural dynamics at swing foot-off across groups (young vs. elderly adults).
Only statistics related to the age effect are reported in this figure since there was no significant effect of temporal pressure on the reported variables. STAB1, STAB2, y’GFO: ML “gap” between the centre of gravity and centre of pressure position at swing foot-off, ML “gap” between the extrapolated centre of mass and centre of pressure position at swing foot-off, ML centre of gravity velocity at swing foot-off, respectively. STAB1=0 indicates that the ML coordinate of the centre of gravity and the centre of pressure are aligned vertically at swing foot-off (only the ML direction is considered in this study). Positive STAB1 value indicates that the centre of gravity has not yet passed above the centre of pressure. STAB2=0 indicates that the extrapolated centre of mass and centre of pressure are aligned vertically. Negative STAB2 value indicates that the extrapolated centre of mass is located beyond the centre of pressure (towards the external side of the stance leg). Reported values are mean ± 1 standard deviation (all participants combined). +, ++, +++: significant main age effect with p < 0.05, p < 0.01 and p < 0.001, respectively.