Figure 1.
A – schematic illustration of the horizontal body weight support system. The subject lay on the right side with each leg suspended in an exoskeleton allowing low-friction segment movements. The arms were unrestricted and unsupported. The head rested on a small pillow placed on a 10-cm wide horizontal belt (not shown) in such a way that the lower arm moved unimpededly. B – an example of foot (MP marker) anterior–posterior displacement evoked by hand-walking (represented by DP marker displacement). FW – forward, BW – backward. L indicates foot excursion. Vertical dashed lines indicate the onset of arm and leg movements. Note the difference between the foot and hand cycle periods (T1 and T2, respectively). C – pie chart showing the percentage of subjects (n = 33) in which hand-walking evoked significant (L>10 cm) foot displacements and of subjects with small or no effect (L<10 cm).
Table 1.
Experimental conditions.
Figure 2.
Examples of the delay of evoked air stepping from the onset of arm movements (at speeds indicated on the top of the figure) in three different subjects. From top to bottom: anterior-posterior (a-p) displacement (disp.) of the left hand, EMG activity of 6 muscles of the left arm, left hip and foot a-p displacements, left hip and knee joint angles and EMG activity of 7 muscles of the left leg. Vertical dashed lines indicate the onset of arm and leg movements. FCU, flexor carpi ulnaris; ECU, extensor carpi ulnaris; TRIC, long head of triceps brachii; BIC, long head of biceps brachii; DELTa, anterior deltoid; DELTp, posterior deltoid; ST, semitendinosus; BF, biceps femoris; Vmed, vastus medialis; RF, rectus femoris; TA, tibialis anterior; MG gastrocnemius medialis; SOL soleus. Note variability in the onset of leg stepping across the subjects.
Figure 3.
A – schematic representation of the biomechanical model used to estimate the mechanical effect of periodic a-p hip displacements on leg motion. B – examples of passive lower limb motion evoked by relatively large (10 and 5 cm) hip oscillations in two subjects. The upper curves represent passive hip displacements manually induced by the experimenter while the lower curves represent the resulting leg movements (foot a-p displacements and joint angles) and leg muscle EMGs. Dotted lines represent the prediction made using the biomechanical model. Note the absence of EMG activity during lower limb movements evoked by passive hip motion. C – mean (+SD) horizontal foot and joint angular excursion (peak-to-peak) evoked by passive hip motion and by hand-walking (mean for all treadmill speeds) as well as predicted by the model. Note significantly smaller foot displacements evoked by passive hip motion or estimated from the model (L<10 cm) relative to those during hand-walking (∼60 cm). Asterisks denote significant differences (p<0.05) with Tukey HSD multiple comparison tests.
Figure 4.
A – stick diagrams of left arm and leg movements in three different subjects during hand-walking at 3 km/h in the a-p direction (left) and voluntary (vol.) air stepping (right). B – mean (+SD, n = 9) arm and leg joint angular amplitudes across different hand-walking speeds in the a-p direction (left) and different conditions of the main protocol: hand-walking at 2 km/h in the p-a direction, arm air-stepping (arm a-s) and voluntary leg air-stepping. C – cycle duration and a-p foot excursion. D – frequency relation between arm and leg movements. In scatter plots each point illustrates the arm or leg movement frequency during different conditions for each single participant. The dotted lines indicate integer arm:leg frequency ratios (1∶1 and 2∶1). Bars in the right panel represent mean (+SD) arm/leg frequency ratios over all conditions.
Figure 5.
EMG patterns in two representative subjects for different hand-walking speeds.
From top to bottom: anterior-posterior displacement of the left hand, EMG activity of 6 muscles of the left arm, a-p displacement of the left foot and EMG activity of 7 muscles of the left leg. Note non-linear changes in the EMG activity with speed: subject 1 (A) showed an increment in hamstring muscle activity while subject 2 (B) showed a decrease in hamstring and an increase in quadriceps activity. Note also the absence of noticeable activity in distal leg muscles.
Figure 6.
Ensemble averaged (across subjects) kinematic and EMG patterns during hand-walking.
From left to right: hand walking at 2 km/h in a-p direction, p-a direction, arm air-stepping and voluntary leg air-stepping. From top to bottom: joint angles (mean±SD, n = 9) (A), non-normalized (B) and normalized (C) EMG envelopes (black lines indicate the mean and dashed lines indicate mean+SD). Patterns are plotted versus normalized leg cycle. Note similar timing of hamstring muscle activity (around the onset of the leg cycle) across all conditions.
Figure 7.
Coordination of arm and leg movements.
A – arm and leg kinematics and EMGs in one representative participants performing hand-walking at 2 km/h and voluntary air-stepping. The left portion of each panel shows the oscillations of left upper and lower limbs accompanied by respective EMG activity of some representative muscles. Grey areas demarcate single limb cycles defined separately for arms and legs as the time between two consecutive maxima in the a-p displacement of the respective endpoint. The right portion of each panel displays the Fourier spectra derived from the signals on their left. Dashed lines show the position of the first five multiples of the fundamental frequency (f1) whose percentage of variance explained is displayed (PVf1). B – mean (±SD, n = 9) peak frequencies of EMGs and corresponding arm (upper plot) and leg (middle plot) kinematic patterns for hand-walking at different speeds and for voluntary leg air-stepping (vol.). Lower plot represent the phase (relative to the onset of leg cycle) of the first harmonic for leg EMGs and thigh and shank elevation angles (zero corresponds to the cosine function with zero time shift). Only EMGs with peak activity greater than 2 µV and PVf1>20% were included in this analysis.
Figure 8.
Hand-walking direction specificity of evoked leg movements.
An example of leg movements during hand walking in anterior-posterior (a-p, A) and medio-lateral (m-l, B) directions in one representative subject (s9). Similar format as in Fig. 5. R – right, L – left. Note minute if any leg movements during hand walking in the m-l direction (B).
Figure 9.
Effect of transient leg block and termination of hand-walking.
A – Upper and lower limb kinematics and EMG activity in two subjects during the transient block of the legs performed manually by an experimenter. Dashed lines delimit the period of transient leg block: the cessation and onset of foot displacements. One subject (s8) exhibited a tonic response in leg muscles during the block while the other (s2) showed a phasic response. All subjects restored non-voluntary air stepping movements after the release of the legs. B – continuation of leg movements after terminating hand-walking. Note EMG activity of ST and BF muscles associated with a few post-cycles of foot movements.
Table 2.
Number of trials in each subject with tonic and phasic responses in the hamstring muscle to the transient block of the legs.