Mireille Bonnard

Stride variability in human gait: the effect of stride frequency and stride length

This study focused on spatial and temporal variability of the stride in human gait. We determined the role of stride frequency (F) and stride length (L) on those parameters. Eight healthy subjects walked on a treadmill using 25 different FL combinations (0.95<L<1.5 m, and 0.8<F<1.26 Hz). The results showed that spatial and temporal variabilities tend to increase in concert with respect to change in stride parameters. In addition, stride variability was found (1) to be minimal at F=1 Hz; and (2) to increase with smaller L. During additional trials, subjects walked freely at various speeds. Although it is generally hypothesized that freely chosen behaviors are optimal in terms of variability, our data show that this is not always the case in human gait.

Steady-state fluctuations of human walking

In steady-state walking, fluctuations in space-time behavior are observed for normal adult subjects. In the present study, the intrinsic fluctuations of gait have been analyzed when walking on a subject-driven treadmill (with adjustable inertial forces). Furthermore, these intrinsic fluctuations have been compared with those observed in natural overground locomotion which involves a real subjects displacement and thus an optical flow. Four adult subjects participated in both experimental sessions. It was found that the frequency and amplitude of the instantaneous fluctuations of leg movement were weak and of equal magnitude with or without optical flow. This was also the case for instantaneous fluctuations in displacement speed. Secondly, a low-frequency fluctuation in walking speed was observed when no optical flow information was available to the subject. This fluctuation results from the addition of a series of leg-movement fluctuations, whose values are all either positive or negative. As the optical flow provides information about the displacement speed, it allows the subject to avoid such addition, and thus plays a role in maintaining steady leg movement. Theoretical models linking space-time behavior of rhythmic movement with stiffness strongly suggest that the observed low-frequency fluctuations in speed result from fluctuations in stiffness.

Interaction between different sensory cues in the control of human gait

Abstract. This experiment investigates the interaction of different sensory cues in the control of propulsive forces in human gait which in turn allow the bodys forward progression to be regulated. The aim of this work was to determine how optic flow and leg-somatosensory feedback interact in this control. We therefore determined whether the responses to sinusoidal perturbations of optic flow were accentuated when leg-somatosensory feedback was modified by varying the support resistance. Subjects walked on a treadmill which was driven by their own locomotor activity (1) with a sinusoidal variation of optic flow velocity, (2) with a sinusoidal variation of support resistance which modified leg-somatosensory information and (3) with both visual and leg-somatosensory modification at different frequencies. The response of the subject was measured as changes in speed and propulsive power. The response to sinusoidal perturbations of optic flow was found to be increased and time delayed when visual perturbations are coupled with support perturbations in comparison with the response observed with visual perturbations only. This result shows the influence of leg-somatosensory feedback on the weighting of optic flow. Inversely, it was also found that the motor response to support perturbation was different when the flow was congruent (i.e., corresponding to the subjects virtual speed) and when it was not. This latter result shows the influence of optic flow on the weighting of leg-somatosensory feedback. The interaction between optic flow and leg-somatosensory feedback argues in favor of a multimodal sensory control of propulsive forces. This multimodal sensory control would be based on all the sensory feedback and all their mutual sensorial interaction. Therefore, the modification of one sensory input modifies not only this input but also the integration of the other inputs.

Prior intention can locally tune inhibitory processes in the primary motor cortex: direct evidence from combined TMS-EEG

Human subjects are able to prepare cognitively to resist an involuntary movement evoked by a suprathreshold transcranial magnetic stimulation (TMS) applied over the primary motor cortex (M1) by anticipatory selective modulation of corticospinal excitability. Uncovering how the sensorimotor cortical network is involved in this process could reveal directly how a prior intention can tune the intrinsic dynamics of M1 before any peripheral intervention. Here, we used combined TMS‐EEG to study the cortical integrative processes that are engaged both in the preparation to react to TMS (Resist vs. Assist) and in the subsequent response to it. During the preparatory period, the contingent negative variation (CNV) amplitude was found to be smaller over central electrodes (FC1, C1, Cz) when preparing to resist compared with preparing to assist the evoked movement whereas α‐oscillation power was similar in the two conditions. Following TMS, the amplitude of the TMS evoked‐N100 component was higher in the Resist than in the Assist condition for some central electrodes (FCz, C1, Cz, CP1, CP3). Moreover, for six out of eight subjects, a single‐trial‐based analysis revealed a negative correlation between CNV amplitude and N100 amplitude. In conclusion, prior intention can tune the excitability of M1. When subjects prepare to resist a TMS‐evoked movement, the anticipatory processes cause a decreased cortical excitability by locally increasing the inhibitory processes.

Intentional on-line adaptation of stride length in human walking.

Abstract. The intentional control of stride length is a fundamental basis for the adaptation of the stride to environmental constraints (obstacle avoidance, for example). Controlling the propulsive forces during the stance and/or controlling the pendular movement of the oscillating leg constitute the two potential and non-exclusive mechanisms underlying intentional stride length modulation. The present experiment was conducted in order to determine if these two mechanisms contribute to voluntary length modulation and, if so, how they cooperate according to whether the subject has to lengthen or shorten a stride and how these mechanisms are implemented at the neuromuscular level. Subjects had to produce a temporarily modulated stride of the same length, but originating from two different initial steady-states: one from shorter stride length and one from longer stride length. We found that the shortening was essentially realized by a swing-duration decrease (an increased activity in the hip extensor – biceps femoris – during the swing of the ipsilaterally shortened stride stopped the pendular leg movement earlier). The lengthening was realized by two mechanisms: (1) an increase in the propulsive forces (via an increased activity of the ankle extensor muscles –soleus – and the hip extensors – biceps femoris – from the stance of the ipsilaterally modulated stride, which was prolonged during the following stance of the contralateral leg), and (2) an increase in swing duration on the ipsilateral leg (an increased activity in hip and ankle flexors –rectus femoris and tibialis anterior – maintained the ipsilateral leg in flexion during the lengthened swing so that the foot landed later). In this experiment, the subjects were faced with a spatial constraint of the same magnitude in the direction of stride lengthening and stride shortening. However, under these conditions, subjects used a different balance between swing control (that directly modifies the foot trajectory without affecting the trajectory of the head-arm-trunk system) and/or the control of propulsive forces (that indirectly influences foot trajectory by modifying the trajectory of the head-arm-trunk system). In the first case, this concerns a voluntary control of gesture produced by the legs and usually implicated in the locomotor pointing; in the second case, this concerns a voluntary control of propulsive forces.

Task-induced modulation of motor evoked potentials in upper-leg muscles during human gait: a TMS study

The aim of this study was to determine the relative involvement of the corticospinal (CS) pathway in voluntarily controlled walking compared to unconstrained walking. In the voluntarily controlled walking condition, subjects had to walk at the same speed as in unconstrained walking with a mechanical constraint, which is known to affect specifically the upper‐leg muscles. The motor cortex was activated transcranially using a focal magnetic stimulation coil in order to elicit motor evoked potentials (MEPs) in the rectus femoris (RF) and the biceps femoris (BF). The magnetic stimulation was delivered at the end of the swing (at 90% of the cycle duration), when the EMG backgrounds were similar in the two experimental conditions. For each subject in each condition, MEPs were measured for several stimulus intensities in order to establish the input/output (I/O) curve (MEPs amplitude plotted against stimulus strength). The results showed a significant increase in the MEPs amplitude of both the RF and BF in voluntarily controlled walking compared to unconstrained walking, which is the first evidence of cofacilitation of MEPs in antagonist upper‐leg muscles during human gait. In conclusion, although a lot of studies have emphasized a privileged input of the corticospinal pathway to the distal lower‐leg muscles, this study shows that, if a locomotory task requires fine control of the proximal upper‐leg muscles, a selective facilitation of MEPs is observed in these muscles.

Corticospinal control of the thumb–index grip depends on precision of force control: a transcranial magnetic stimulation and functional magnetic resonance imagery study in humans

The corticospinal system (CS) is well known to be of major importance for controlling the thumb–index grip, in particular for force grading. However, for a given force level, the way in which the involvement of this system could vary with increasing demands on precise force control is not well‐known. Using transcranial magnetic stimulation and functional magnetic resonance imagery, the present experiments investigated whether increasing the precision demands while keeping the averaged force level similar during an isometric dynamic low‐force control task, involving the thumb–index grip, does affect the corticospinal excitability to the thumb–index muscles and the activation of the motor cortices, primary and non‐primary (supplementary motor area, dorsal and ventral premotor and in the contralateral area), at the origin of the CS. With transcranial magnetic stimulation, we showed that, when precision demands increased, the CS excitability increased to either the first dorsal interosseus or the opponens pollicis, and never to both, for similar ongoing electromyographic activation patterns of these muscles. With functional magnetic resonance imagery, we demonstrated that, for the same averaged force level, the amplitude of blood oxygen level‐dependent signal increased in relation to the precision demands in the hand area of the contralateral primary motor cortex in the contralateral supplementary motor area, ventral and dorsal premotor area. Together these results show that, during the course of force generation, the CS integrates online top‐down information to precisely fit the motor output to the tasks constraints and that its multiple cortical origins are involved in this process, with the ventral premotor area appearing to have a special role.

Direct Evidence for a Binding between Cognitive and Motor Functions in Humans: A TMS Study

During voluntary motor actions, the cortico-spinal (CS) excitability is known to be modulated, on the one hand by cognitive (intention-related) processes and, on the other hand, by motor (performance-related) processes. Here, we studied the way these processes interact in the tuning of CS excitability during voluntary wrist movement. We used transcranial magnetic stimulation (TMS) both as a reliable tool for quantifying the CS excitability, through the motor-evoked potentials (MEPs), and as a central perturbation evoking a movement (because the stimulation intensity was above threshold) with subjects instructed to prepare (without changing their muscle activation) either to let go or to resist to this evoked movement. We studied the simultaneous evolution of both the motor performance and the MEPs in the wrist flexor and extensor, separately for the successful trials (on average, 66 of the trials whatever the condition) and the unsuccessful trials; this allowed us to dissociate the intentionand performance-related processes. To their great surprise, subjects were found able to cognitively prepare themselves to resist a TMS-induced central perturbation; they all reported an important cognitive effort on the evoked movement. Moreover, because TMS only evoked short-latency MEPs (and no long-latency components), the amplitude of these short-latency MEPs was found to be related in a continuous way to the actual movement whatever the prior intention. These results demonstrate that prior intention allows an anticipatory modulation of the CS excitability, which is not only selective (as already known) but also efficient, giving the intended motor behavior a real chance to be realized. This constitutes a direct evidence of the role of the CS excitability in the binding between cognitive and motor processes in humans.

Intentionality in human gait control: modifying the frequency-to-amplitude relationship.

Tight frequency-to-amplitude relationships are observed in spontaneous human steady gait. If required, however, they can be modified. The following experiments were aimed at the processes underlying this flexibility, which forms the fundamental basis of the intentional adaptive capabilities of locomotion. In Experiment 1, Ss had to intentionally modify the frequency-to-amplitude relationship (leading to preferred or nonpreferred steady states). In Experiment 2, they had to temporarily perturbate the stride-frequency-to-amplitude relationship to intentionally shorten or lengthen 1 stride. Within the important constraints exerted by the head-arm-trunk system on leg movement, the results pointed out 2 main strategies that allow the S to intentionally adapt stride organization on-line: adjustment of the tonic properties of the oscillating leg to achieve nonpreferred steady states and phasic action to ensure temporary movement away from a steady state.

Preparing for a motor perturbation: early implication of primary motor and somatosensory cortices

Although preparation of voluntary movement has been extensively studied, very few human neuroimaging studies have examined preparation of an intentional reaction to a motor perturbation. This latter type of preparation is fundamental for adaptive motor capabilities in everyday life because it allows a desired motor output to be maintained despite changes in external forces. Using fMRI, we studied how the sensorimotor cortical network is implicated in preparing to react to a mechanical motor perturbation. While maintaining a given wrist angle against a small force, subjects were instructed to prepare a reaction to a subsequent wrist angle displacement. This reaction consisted of, either resisting the imposed movement, or remaining passive. During the preparation of both reactions we found an early implication of M1 and S1 but no implication at all of the higher order motor area preSMA. This is clearly different from what has been found for voluntary movement preparation. These results show that the sensorimotor network activation during preparation of voluntary motor acts depends on whether one expects a motor perturbation to occur: when external forces can interfere with ongoing motor acts, the primary sensorimotor areas must be ready to react as quickly as possible to perturbations that could prevent the goal of the ongoing motor act from being achieved. Hum Brain Mapp, 2009.