When the intended foot placement changes during a step, either due to an obstacle appearing in our path or the sudden shift of a target, visual input can rapidly alter foot trajectory. However, previous studies suggest that when intended foot placement does not change, the path of the foot is fixed after it leaves the floor and vision has no further influence. Here we ask whether visual feedback can be used to improve the accuracy of foot placement during a normal, unperturbed step. To investigate this we measured foot trajectory when subjects made accurate steps, at fast and slow speeds, to stationary floor-mounted targets. Vision was randomly occluded in 50% of trials at the point of foot-off. This caused an increase in foot placement error, reflecting lower accuracy and higher variability. This effect was greatest for slow steps. Trajectory heading analysis revealed that visually guided corrections occurred as the foot neared the target (on average 64 mm away). They occurred closer to the target for the faster movements thus allowing less time and space to execute corrections. However, allowing for a fixed reaction time of 120 ms, movement errors were detected when the foot was approximately halfway to the target. These results suggest that visual information can be used to adjust foot trajectory during the swing phase of a step when stepping onto a stationary target, even for fast movements. Such fine control would be advantageous when environmental constraints place limitations on foot placement, for example when hiking over rough terrain.

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Key points• Postural physiological hand tremor has a prominent component at ∼8 Hz unlike the associated EMG. Consequently, the gain between EMG and tremor is sharply peaked at ∼8 Hz.• Deduction and a simple model using pre-recorded EMG or random noise as an input show that the ∼8 Hz peak is a consequence of resonance.• During voluntary movement the gain peak enlarges and shifts to a lower frequency but the EMG spectrum shows no corresponding changes. This reflects muscle thixotropy. Adjustment of the muscle properties of the model reproduces the effect.• These findings suggest that the rhythm of hand tremor in posture and movement is related to muscle and limb mechanics rather than a neural oscillator.• The discovery that the gain relating EMG to acceleration is very different when static and moving has implications for the control of movement in health and disease.Abstract Limb resonance imparts a characteristic spectrum to hand tremor. Movement will alter the resonance. We have examined the consequences of this change. Rectified forearm extensor muscle EMG and physiological hand tremor were recorded. In postural conditions the EMG spectrum is relatively flat whereas the acceleration spectrum is sharply peaked. Consequently, the gain between EMG and acceleration is maximal at the frequency where the tremor is largest (∼8 Hz). The shape of the gain curve implies mechanical resonance. Substantial alterations in posture do not significantly change the characteristics of the tremor or the shape or size of the gain curve. By contrast, slow or moderately paced voluntary wrist flexion-extension movements dramatically increase the hand tremor size and lower its peak frequency. These changes in size and frequency of the tremor cannot be attributed to changes in the EMG. Instead they reflect a very large change in the size and shape of the gain curve relating EMG to acceleration. The gain becomes larger and the peak moves to a lower frequency (∼6 Hz). We suggest that a movement-related (thixotropic) alteration in resonant properties of the wrist provides a simple explanation for these changes. The mechanism is illustrated by a model. Our new findings confirm that resonance plays a major role in wrist tremor. We also demonstrate that muscles operate very differently under postural and dynamic conditions. The different coupling between EMG and movement in posture and when moving must pose a considerable challenge for neural predictive control of skeletal muscles.

Background. The ability to make step adjustments while walking is often impaired following a stroke, but the basic sensorimotor control deficits responsible have not been established. Objective. To identify these deficits in Patients who have recovered from stroke leaving only mild lower limb impairment. Methods. Ten stroke and 10 age-matched control patients stepped onto an illuminated rectangle. In 40% of trials it jumped 140 mm either medially or laterally when the stepping foot left the ground, thus provoking a mid-step adjustment. In a separate block, patients performed the same task but with the body supported by a frame to eliminate balance responses. Results. Irrespective of support condition stroke patients produced short-latency foot trajectory adjustments compatible with a fast-acting, possibly subcortical, visuomotor process. However, the latency was slightly but significantly longer for the contralesional leg (148 ms) than the ipsilesional leg (141 ms) and longer than for controls (129 ms). Stroke patients' foot adjustments were executed slower and undershot the target more than controls. These deficits were most pronounced in the medial direction when the body was unsupported. The pattern of undershooting was the same for ipsilesional and contralesional legs. Conclusions. Mildly impaired stroke patients have deficits in initiating and executing visually triggered step adjustments but more profound difficulties with balance control during the adjustment, which caused them to suppress mid-step adjustments of foot placement in the medial direction where balance demands were greatest. Paradoxically, such suppression outside the laboratory may also threaten balance if it leads to unsafe foot placement or obstacle collision.

We have recently described a postural after-effect of walking onto a stationary platform previously experienced as moving, which occurs despite full knowledge that the platform will no longer move. This experiment involves an initial baseline period when the platform is kept stationary (BEFORE condition), followed by a brief adaptation period when subjects learn to walk onto the platform moving at 1.2 m/s (MOVING condition). Subjects are clearly warned that the platform will no longer move and asked to walk onto it again (AFTER condition). Despite the warning, they walk toward the platform with a velocity greater than that observed during the BEFORE condition, and a large forward sway of the trunk is observed once they have landed on the platform. This aftereffect, which disappears within three trials, represents dissociation of knowledge and action. In the current set of experiments, to gain further insight into this phenomenon, we have manipulated three variables, the context, location, and method of the walking task, between the MOVING and AFTER conditions, to determine how far the adaptation will generalize. It was found that when the gait initiation cue was changed from beeps to a flashing light, or vice versa, there was no difference in the magnitude of the aftereffect, either in terms of walking velocity or forward sway of the trunk. Changing the leg with which gait was initiated, however, reduced sway magnitude by approximately 50%. When subjects changed from forward walking to backward walking, the aftereffect was abolished. Similarly, walking in a location other than the mobile platform did not produce any aftereffect. However, in these latter two experiments, the aftereffect reappeared when subjects reverted to the walking pattern used during the MOVING condition. Hence, these results show that a change in abstract context had no influence, whereas any deviation from the way and location in which the moving platform task was originally performed profoundly reduced the size of the aftereffect. Although the moving platform aftereffect is an example of inappropriate generalization by the motor system across time, these results show that this generalization is highly limited to the method and location in which the original adaptation took place.