Frigon A, D'Angelo G, Thibaudier Y, Hurteau MF, Telonio A, Kuczynski V, Dambreville C. Speed-dependent modulation of phase variations on a step-by-step basis and its impact on the consistency of interlimb coordination during quadrupedal locomotion in intact adult cats. J Neurophysiol 111: 1885-1902, 2014. First published February 12, 2014 doi:10.1152/jn.00524.2013.-It is well established that stance duration changes more than swing duration for a given change in cycle duration. Small variations in cycle duration are also observed at any given speed on a step-by-step basis. To evaluate the step-bystep effect of speed on phase variations, we measured the slopes of the linear regressions between the phases (i.e., stance, swing) and cycle duration during individual episodes at different treadmill speeds in five adult cats. We also determined the pattern of dominance, defined as the phase that varies most with cycle duration. We found a significant effect of speed on hindlimb phase variations, with significant differences observed between the slowest speed of 0.3 m/s compared with faster speeds. Moreover, although patterns of phase dominance were primarily stance/extensor dominated at the slowest speeds, as speed increased the patterns were increasingly categorized as covarying, whereby both stance/extensor and swing/flexor phases changed in approximately equal proportion with cycle duration. Speed significantly affected the relative duration of support periods as well as interlimb phasing between homolateral and diagonal pairs of limbs but not between homologous pairs of limbs. Speed also significantly affected the consistency of interlimb coordination on a step-by-step basis, being less consistent at the slowest speed of 0.3 m/s compared with faster speeds. We found a strong linear relationship between hindlimb phase variations and the consistency of interlimb coordination. Therefore, results show that phase variations on a step-by-step basis are modulated by speed, which appears to influence the consistency of interlimb coordination. locomotion; speed; phase variations; interlimb coordination DURING OVERGROUND OR TREADMILL locomotion, the cycle can be broadly divided into stance and swing phases. Numerous studies have shown that cycle duration is reduced as speed increases, which is accomplished primarily by a reduction in the duration of the stance phase, while the duration of the swing phase is much less affected (reviewed in Frigon 2012;Gossard et al. 2011). This can be demonstrated by plotting the durations of the stance and swing phases as a function of cycle duration and measuring the slopes of the linear regressions (Halbertsma 1983). In most terrestrial walking species, including insects, birds, rodents, reptiles, dogs, cats, macaques, and humans, the slope of the regression between the stance phase and cycle duration (r STA ) is steeper than the slope of the regression between the swing phase and cycle duration (r SW ) (Arshavskii et al.

In humans, gait adapts to prolonged walking on a split-belt treadmill, where one leg steps faster than the other, by gradually restoring the symmetry of interlimb kinematic variables, such as double support periods and step lengths, and by reducing muscle activity (EMG, electromyography). The adaptation is also characterized by reversing the asymmetry of interlimb variables observed during the early split-belt period when returning to tied-belt locomotion, termed an after-effect. To determine if cats adapt to prolonged split-belt locomotion and to assess if spinal locomotor circuits participate in the adaptation, we measured interlimb variables and EMG in intact and spinal-transected cats before, during and after 10 min of split-belt locomotion. In spinal cats, only the hindlimbs performed stepping with the forelimbs stationary. In intact and spinal cats, step lengths and double support periods were, on average, symmetric, during tied-belt locomotion. They became asymmetric during split-belt locomotion and remained asymmetric throughout the split-belt period. Upon returning to tied-belt locomotion, symmetry was immediately restored. In intact cats, the mean EMG amplitude of hindlimb extensors increased during split-belt locomotion and remained increased throughout the split-belt period, whereas in spinal cats, EMG amplitude did not change. Therefore, the results indicate that the locomotor pattern of cats does not adapt to prolonged split-belt locomotion, suggesting an important physiological difference in the control of locomotion between cats and humans. We propose that restoring left-right symmetry is not required to maintain balance during prolonged asymmetric locomotion in the cat, a quadruped, as opposed to human bipedal locomotion.

Stepping along curvilinear paths produces speed differences between the inner and outer limb(s). This can be reproduced experimentally by independently controlling left and right speeds with split-belt locomotion.Here we provide additional details on the pattern of the four limbs during quadrupedal split-belt locomotion in intact cats. Six cats performed tied-belt locomotion (same speed bilaterally) and split-belt locomotion where one side (constant side) stepped at constant treadmill speed while the other side (varying side) stepped at several speeds. Cycle, stance, and swing durations changed in parallel in homolateral limbs with shorter and longer stance and swing durations on the fast side, respectively, compared with the slow side. Phase variations were quantified in all four limbs by measuring the slopes of the regressions between stance and cycle durations (r STA ) and between swing and cycle durations (r SW ). For a given limb, r STA and r SW were not significantly different from one another on the constant side whereas on the varying side r STA increased relative to tied-belt locomotion while r SW became more negative. Phase variations were similar for homolateral limbs. Increasing left-right speed differences produced a large increase in homolateral double support on the slow side, while triple-support periods decreased. Increasing left-right speed differences altered homologous coupling, homolateral coupling on the fast side, and coupling between the fast hindlimb and slow forelimb. Results indicate that homolateral limbs share similar control strategies, only certain features of the interlimb pattern adjust, and spinal locomotor networks of the left and right sides are organized symmetrically.locomotion; phase variations; interlimb coordination; split-belt DURING TERRESTRIAL LOCOMOTION, stepping in a perfectly straight line for prolonged periods is an infrequent occurrence in everyday life (Reisman et al. 2005), as animals must often turn and step along curvilinear paths. A characteristic of stepping along a curvilinear path is that the left and right sides step at different speeds from one another, as the outer limb(s) must travel a greater distance than the inner one(s) (Courtine and Schieppati 2003;Dietz et al. 1994; Halbertsma 1983;Reisman et al. 2005;Zijlstra and Dietz 1995). Having one side step faster than the other can be achieved experimentally by using a split-belt treadmill composed of two independently controlled surfaces (Dietz et al. 1994;Forssberg et al. 1980;Frigon et al. 2013;Kulagin and Shik 1970;Reisman et al. 2005;Yang et al. 2005;Zijlstra and Dietz 1995). The overall aim of this study was to gain further insight into the control systems regulating adjustments in the locomotor pattern when the left and right sides step at different speeds from one another.Split-belt locomotion produces predictable bilateral changes in phase durations in the hindlimbs of quadrupeds and in the legs of humans. For instance, the limb(s) stepping on the slow belt has relatively longer stance duration w...