Task-dependent compensatory responses to perturbations applied during rhythmic movements in humans

Modulation of the responses to perturbation applied during different phases of three rhythmic movements in humans-running, cycling, and hopping-was studied. The perturbation was an electrical stimulus. The results showed gating and modulation of the responses in both ipsi- and contralateral limb muscles. The responses during running and cycling were only excitatory in nature, while during hopping an inhibitory response was observed. These responses were not correlated with the normal activity during the movement. The latency of the response in general was not altered for different stimulation phases. The alterations in the step cycle demonstrated overt behavioral changes due to the responses. There were differences between the responses observed during these movements and walking. In running, the major adaptation to perturbations appears to be in the contralateral side as seen in the changes in the step cycle. During cycling (except for one phase) and hopping, the same set of muscles was activated in response to perturbation. This represents a simplifying strategy in response organization. The dependency of the response on the task characteristics, postural stability requirement, and external constraints imposed on the subject is discussed. These studies provide insights into task-dependent strategies adopted by the nervous system to meet unexpected perturbation during rhythmic movements in humans.     

Phase-dependent reflex modulation in tibialis anterior during passive viewing of walking

It is well established that reflexes are highly adaptive, as they depend both on our intention and on the active state of the muscles. Reflex gains change dynamically during actions such as walking and running, with the gain of cutaneous reflexes being increased at the end of the stance phase but decreased at the end of the swing phase in the tibialis anterior (TA) muscle. Reflex gains can even change during the mere observation of an action. The mechanisms and functions of such modulations are unclear. It has been suggested that the changed reflex gains prevent the actual performance of actions that we see. However, the modulation of reflexes in response to seeing an action has never been reproduced for the active execution of such actions. In the present study, medium-latency cutaneous reflexes from the TA muscle, of which the activity and reflexes during walking are well known, were measured in human subjects. The results show that the gain changes of the medium-latency responses of the TA are the same as during active walking. We conclude that reflexes do not represent an inhibitory mechanism that prevents motor output during action observation. Instead, our findings provide evidence that even the peripheral spinal motor system is actively involved in the motor resonance processes, without evoking any measurable motor responses.     

Speed and accuracy of compensatory responses to limb disturbances

This study examines the speed and accuracy of compensatory responses to flexion-extension perturbations of the wrist in the horizontal plane. In Experiments 1 and 2 the subjects were required to establish an initial flexion or extension force of approximately 15% maximum at a prescribed initial muscle length. The perturbations changed the load force by +/-5% in both simple and choice reaction protocols. The results showed that the latencies to compensate for the perturbation were longer when the direction of disturbance was unknown (i.e., choice effect) and when the perturbation unloaded the muscle (i.e., directional effect). Accuracy constraints on the compensatory response increased movement time and reduced the variability of latency without affecting mean latency. In Experiment 3, a visual stimulus generated a comparable choice effect on latency to that produced by the perturbations, but no directional effect in relation to the preload was apparent. Our behavioral analysis of compensatory responses triggered by wrist perturbations confirms that these responses are susceptible to variables that influence the initiation of voluntary movements. Our analysis also demonstrates a directional preload effect that is stimulus specific.     

Anticipatory responses to perturbation of co-ordination in one-handed catching.

Anticipatory responses to perturbation have rarely been studied in the co-ordination of dynamic interceptive actions. In this study, the kinematics of ball catching were examined in skilled catchers when mechanical perturbation of the catching arm was expected and unexpected. During trials where the perturbation was anticipated, participants initiated movements earlier (207 +/- 32 ms) than in randomly perturbed trials (223 +/- 34 ms). Furthermore, several individuals also tended to move their hand faster when perturbations were expected compared to baseline trials. Individual analyses revealed that three out of eight participants exhibited changes in the relative timing of the grasp phase to adapt to the specific manipulation of task constraints. Anticipatory responses were revealed in changes not only at movement initiation but also in the resulting adaptations to the co-ordination of reach and grasp phases of ball catching. When the catchers could not anticipate perturbations, movement strategies suggested the use of a continuous tracking-based mode of control rather than a prediction-based mode of control.     

Canine responses to familiar and unfamiliar humans

Dogs were observed during controlled approaches by their owners and by strangers. Significant differences between the dogs' responses to their owners and their responses to strangers were found. These results supported the popular belief that dogs respond differently to different persons, and not merely to different situations in which persons are usually encountered.     

Differential effects of stimulus characteristics during knee joint perturbation on hamstring and quadriceps reflex responses

When stretching muscles of the ankle joint, stretch velocity and amplitude were shown to selectively influence specific parts of the stretch reflex. The present study investigated whether similar effects can be observed at the knee joint. Seventeen subjects were exposed to sudden anterior tibial translations. The influence of stimulus amplitude was analyzed by applying a low (LIMP) or high impulse (HIMP). To investigate the influence of velocity, rate of force development of the perturbation was chosen either low (LRFD) or high (HRFD). Activation of biceps femoris (BF), semitendinosus (ST), vastus medialis (VM) and vastus lateralis (VL) was calculated in four consecutive timeframes (P0, P1, P2, P3). During P1, RFD (ST: p < .05; BF: p < .01; VM: p < .05; VL: p > .05) and during P2, impulse (ST: p < .05; BF: p < .01; VM: p < .01; VL: p < .01) did significantly influence reflex activation. The present study showed that stimulus characteristics influenced specific reflex components of knee joint muscles. As only hamstring muscles were stretched, whereas quadriceps was unloaded, it is concluded that different mechanisms like homonymous and heteronymous muscle afferents as well as joint and skin afferents might contribute to the reflex responses.     

Age and physiological, perceptual, and affective responses during walking at a self-selected pace

The aim of this study was to examine physiological, perceptual, and affective responses during self-paced walking for three age groups. 66 adult women were assigned into three groups by age: 20-25 yr. (n=22), 30-35 yr. (n=22), and 40-45 yr. (n=22). Each participant completed a maximal exercise test and a 20-min. bout of walking at a self-selected pace. The preferred walking speed was similar for all age groups, whereas physiological responses relative to maximal and ventilatory threshold values were greater in the 40-45 yr. group than the other two groups. Nevertheless, perceptual and affective responses were similar for all age groups. These findings suggest that physiological responses, but not perceptual and affective responses, of sedentary women are associated with age during walking at a self-selected pace.     

Effects of passive interpersonal light touch during walking on postural control responses: An exploratory study

The effects of passive interpersonal light touch (PILT) on postural stability can be observed through improved postural coordination through haptic feedback from the contact provider to the contact receiver while walking. It is unclear, however, whether PILT affects the contact receiver's detailed physical responses, such as muscle activity, body sway, and joint movements. In this study, surface electromyography and an inertial measurement unit were used simultaneously to explore changes in walking speed and control responses induced by PILT. We evaluated fourteen healthy participants for their walking speed and physical responses under two walking conditions: no-touch (NT) and PILT. As a physical response during walking, we measured muscle activity (rectus femoris, semitendinosus, tibialis anterior, and soleus muscles), body sway (pelvis and neck), and joint angles (direction of hip, knee, and ankle joint movements). In PILT condition, fingertip contact force was measured while the contact provider touched the third level of the recipient's lumbar spine. In comparison with the NT condition, PILT condition increased walking speed and decreased body sway on neck position. There were significant correlations between walking speed and neck sway regarding NT and PILT change values. Passive haptic information to the contact receiver may assist in the smooth shift of the center of gravity position during gait through interpersonal postural coordination. These findings suggest that PILT may provide an efficient and stable gait.     

Responses of human ankle muscles to mediolateral balance perturbations during walking

During walking our balance is maintained by muscle action. In part these muscle actions automatically respond to the imbalance. This paper considers responses to balance perturbations in muscles around the ankle, peroneus longus (PL), tibialis anterior (TA) and soleus (SO). It is investigated if their action is related to previously observed balance mechanisms: the ‘braking reaction’ and the mediolateral ankle strategy.Subjects walked on a treadmill and received pushes to the left and pulls to the right in various phases of the gait cycle. Muscle actions were divided into medium latency R1 (100–150 ms), long latency R2 (170–250 ms), and late action R3 (270–350 ms). Short latency responses, before 100 ms, were not observed but later responses were prominent. With inward perturbations (e.g. pushes to the left shortly before or during stance of the right foot) responses in RPL were seen. The forward roll-over of the CoP was briefly stalled in mid stance, so that the heel was not lifted. Stance was shortened. With outward perturbations, pushes to the left shortly before or during stance of the left foot, responses in all three muscles, LTA, LSO, and LPL were seen. Our interpretation is that these muscle activations induce a ‘braking reaction’ but could also contribute to the ‘mediolateral ankle strategy’. The resultant balance correction is small but fast, and so diminishes the need for later corrections by the stepping strategy.     

How humans react to changing rewards during visual foraging

Much is known about the speed and accuracy of search in single-target search tasks, but less attention has been devoted to understanding search in multiple-target foraging tasks. These tasks raise and answer important questions about how individuals decide to terminate searches in cases in which the number of targets in each display is unknown. Even when asked to find every target, individuals quit before exhaustively searching a display. Because a failure to notice targets can have profound effects (e.g., missing a malignant tumor in an X-ray), it is important to develop strategies that could limit such errors. Here, we explored the impact of different reward patterns on these failures. In the Neutral condition, reward for finding a target was constant over time. In the Increasing condition, reward increased for each successive target in a display, penalizing early departure from a display. In the Decreasing condition, reward decreased for each successive target in a display. The experimental results demonstrate that observers will forage for longer (and find more targets) when the value of successive targets increases (and the opposite when value decreases). The data indicate that observers were learning to utilize knowledge of the reward pattern and to forage optimally over the course of the experiment. Simulation results further revealed that human behavior could be modeled with a variant of Charnov’s Marginal Value Theorem (MVT) (Charnov, 1976) that includes roles for reward and learning.     

Psychophysiological response to cognitive workload during symmetrical,asymmetrical and dual-task walking

Walking with a lower limb prosthesis comes at a high cognitive workload for amputees, possibly affecting their mobility, safety and independency. A biocooperative prosthesis which is able to reduce the cognitive workload of walking could offer a solution. Therefore, we wanted to investigate whether different levels of cognitive workload can be assessed during symmetrical, asymmetrical and dual-task walking and to identify which parameters are the most sensitive. Twenty-four healthy subjects participated in this study. Cognitive workload was assessed through psychophysiological responses, physical and cognitive performance and subjective ratings. The results showed that breathing frequency and heart rate significantly increased, and heart rate variability significantly decreased with increasing cognitive workload during walking (p < .05). Performance measures (e.g., cadence) only changed under high cognitive workload. As a result, psychophysiological measures are the most sensitive to identify changes in cognitive workload during walking. These parameters reflect the cognitive effort necessary to maintain performance during complex walking and can easily be assessed regardless of the task. This makes them excellent candidates to feed to the control loop of a biocooperative prosthesis in order to detect the cognitive workload. This information can then be used to adapt the robotic assistance to the patient’s cognitive abilities.     

Age-related differences in head and trunk coordination during walking

The purpose of this study was to examine the effect of ageing on the pattern and structure of head and trunk accelerations during walking. Head and trunk accelerations of young (n=8; mean=23 years, SD=4 years) and elderly (n=8; mean=74 years, SD=3 years) individuals were measured using triaxial accelerometers while performing preferred speed walking. Accelerations were examined using power-spectral analysis and measures of signal smoothness, regularity and coupling. No differences in walking speed or signal regularity were detected between age groups. Compared to the young participants, the elderly had (1) a greater proportion of signal power above 6 Hz for the trunk, (2) a smaller difference in signal smoothness between the trunk and head, (3) less signal smoothness in the mediolateral direction, and (4) a greater degree of directional coupling for the head compared to the trunk. Overall these results suggest that the pattern of head accelerations was relatively unaffected by age, and that both age groups achieved similar levels of head stability despite differences in trunk acceleration characteristics. The manner in which head stability was achieved differed between age groups, with the elderly employing an upper body coordination strategy that enhanced coupling between acceleration directions of the head compared to the trunk. The findings of this study also suggest that an absence of age-related differences in signal complexity at one level of postural system, combined with differences at another level, may provide information about the way in which the motor system prioritises postural control during gait.     

Training of instrumental responses in dogs socially reinforced by humans

The motivational bases of the social reinforcement in human-dog relations were examined. In experiment I, performed on seven dogs, it was found that dogs were able to learn and sustain the natural responses of sitting, paw extension, and lying prostrate to conditional stimuli in the form of vocal commands reinforced only by social rewards given by the experimenter, such as petting and vocal encouragement. Overtraining did not produce deterioration of performance but, on the contrary, the continual decrease of latencies. It was evidenced that tactile stimulation plays an important role in social reward. In experiment II, instrumental responses to the auditory conditional stimuli were elaborated in two groups of dogs. The first group (nine dogs) was reinforced by food, and the second group (eight dogs) was reinforced exclusively by petting. A similar course of learning and level of performance during overtraining sessions in both groups indicated that petting serves as a good reinforcement, with rewarding value comparable to that of food reinforcement. It is suggested that a strong rewarding effect of pleasurable sensory stimuli occurs in the formation of the bond between dog and human and in the learning of different tasks.     

Effects of walking on various inclines on EMG patterns of lower limb muscles in humans

Myoelectric signals from several muscles of the lower limb were studied during treadmill locomotion over various inclines. A pattern recognition technique was used to analyse these activity patterns. The analyses revealed the following rules. These are common features among the various muscle activity patterns. The results suggest that the limb is controlled as a unit. Both phasic and average components of the muscle activity patterns are modulated to meet demands imposed by the various inclines. The distal muscles in general are more tightly controlled than the proximal muscles. The changes in average EMG values are muscle-specific, and are not similar for the stance and swing phases of the step cycle. On average, the proximal muscles show greater increases than the distal muscles. These results are compared with those found previously in the different speed and stride-length condition. Studies such as these shed light on the adaptability of the basic locomotor synergy.     

Characterization of statistical persistence in joint angle variation during walking

The objective of this study was to characterize joint angle variation across strides. Specifically, the statistical persistence of variations were quantified using the Hurst exponent. If a time series exhibits statistical persistence, then a parameter which is smaller (or larger) than average will tend to be followed by additional values that are also smaller (or larger) than average. Human walking has statistical persistence between stride durations. Variation in stride duration must arise from variation in the motion of the leg segments during walking. It is unclear, however, if the joint angle variation also exhibits statistical persistence. This study examined kinematic data collected from nine healthy adults walking for 10 min at a self-selected comfortable speed on a treadmill. The joint angle variation in the lower limbs was parameterized using first-order Fourier series which in turn were described by frequency and magnitude coefficients for each stride. To determine if the joint angle variation exhibited statistical persistence, the Hurst exponent was found for each coefficient at each joint. The mean Hurst exponents were 0.54 for the frequency coefficients and 0.61 for the magnitude coefficients. Neither the frequency or magnitude coefficients exhibited statistically significant persistence, although some of the magnitude coefficients were close to reaching statistical significance. This suggests that joint angle variability in healthy adults does not directly produce the statistical persistence observed in stride duration fluctuations.     

Early changes in muscle activation patterns of toddlers during walking

Early locomotor behavior has been the focus of considerable attention by developmentalists over several decades. Few studies have addressed explicitly patterns of muscle activity that underlie this coordination pattern. Our purposes were to illustrate a method to determine objectively the onset and offset of muscle firings during early walking and to investigate the emergence of patterns of activation of the core locomotor muscles. We tested eight toddlers as they walked overground at walking onset (max. of 3-6 independent steps) and after three months of walking experience. Surface electrodes monitored activity of the gastrocnemius, tibialis anterior, quadriceps, and hamstrings. We reduced EMG signals to a frame-by-frame designation of "on-off," followed by muscle state and co-contraction analyses, and probability distributions for each muscle's activity across multiple cycles. Our results clearly show that at walking onset muscle activity was highly variable with few, if any, muscles showing recurring patterns of behavior, within or among toddlers. Variability and co-activation decreased with walking experience but remained inconsistent, in contrast to the significant increase in stability shown for joint coordination and endpoint (foot placement) parameters. We propose this trend emerges because of the high number of options (muscle combinations) available. Toddlers learn first to marshal sufficient force to balance and make forward progress but slowly discover how to optimize these resources.