Paul J. Stapley

Neurons in the pontomedullary reticular formation signal posture and movement both as an integrated behavior and independently

We have previously suggested that the discharge characteristics of some neurons in the pontomedullary reticular formation (PMRF) are contingent on the simultaneous requirement for activity in both ipsilateral flexor muscles and contralateral extensors. To test this hypothesis we trained cats to stand on four force platforms and to perform a task in which they were required to reach forward with one forelimb or the other and depress a lever. As such the task required the cat to make a flexion movement followed by an extension in the reaching limb while maintaining postural support by increasing extensor muscle tonus in the supporting limbs. We recorded the activity of 131 neurons from the PMRF of three cats during left, ipsilateral reach. Of these, 86/131 (66%) showed a change in discharge frequency prior to the onset of activity in one of the prime flexor muscles and 43/86 (50%) showed a bimodal pattern of discharge in which activity decreased during the lever press. Among the remaining cells, 28/86 (33%) showed maintained activity throughout the reach and the lever press. Most cells showed a broadly similar pattern of discharge during reaches with the right, contralateral limb. We suggest these results support the view that a population of neurons within the PMRF contributes to the control of movement in one forelimb and the control of posture in the other forelimb as a coordinated unit. Another population of neurons contributes to the control of postural support independently of the nature of the activity in the reaching limb.

The Pontomedullary Reticular Formation Contributes to the Compensatory Postural Responses Observed Following Removal of the Support Surface in the Standing Cat

This study was designed to determine the contribution of reticular neurons in the pontomedullary reticular formation (PMRF) to the postural responses produced to compensate for an unexpected perturbation. We recorded the activity of 48 neurons in the PMRF, including 41 reticulospinal neurons, to removal of the support surface under each of the four limbs in four cats. The perturbations produced robust postural responses that were divided into three periods: an initial postural response (P1) that displaced the center of vertical pressure over the two diagonal supporting limbs; a secondary response (P2) during which the cat restored a tripedal support pattern; and a prolonged tertiary response (P3) that maintained a stable posture over all three supporting limbs. Most (44/48) reticular neurons showed modified activity to perturbation of at least one limb and a majority (39/48) showed changes in activity to perturbations of more than one limb. A few (7/48) discharged to perturbations of all four limbs. Discharge frequency in neurons showing increased activity during P1 was relatively high (>100 Hz in 57% of the neurons responding to perturbations of either the left or right forelimbs, lFl and rFL) and of short latency (17 ms for the lFL and 14 ms for the rFL). Discharge activity in most neurons was sustained throughout P2 and P3 but at a reduced level. These data show that neurons in the PMRF discharge strongly in response to unexpected perturbations and in a manner consistent with a contribution to the compensatory responses that restore equilibrium.

The role of anticipatory postural adjustments during whole body forward reaching movements.

THE purpose of this study was to examine the role of anticipatory postural adjustments (APAs) in the execution of forward oriented whole body reaching movements. From the standing position, eight healthy subjects were asked to reach an object placed at 45 cm from the feet, at both naturally paced and fast speeds. Electromyographic signals of six antagonistic muscles were analysed in conjunction with centre of mass (CM) displacements, centre of foot pressure displacements and resultant ground reaction forces. Results revealed that APAs created necessary angular momentum of body segments for effective task execution. These results suggest that APAs can initiate movements conducted from a fixed base of support, and in this context do not act solely to stabilize the CM.

Neck muscle fatigue and postural control in patients with whiplash injury

OBJECTIVES To examine if patients with whiplash injury show identifiable increases in neck muscle fatigability and associated increase in postural body sway after contractions of dorsal neck muscles, and if physiotherapy treatment reduces these effects. METHODS Sway was measured during stance in 13 patients before and after 5 min of isometric dorsal neck muscle contractions and after recovery, pre- and post-physiotherapy, using a force platform. Amplitude and median frequency of neck muscle EMG were calculated during the contracting period. After each stance trial, patients gave a subjective score of sway. RESULTS Pre-treatment, seven patients showed EMG signs of fatigue (increases in amplitude, decreases in median frequency) and increases in sway (eyes closed) after contractions. The other patients showed neither fatigue nor increased sway. Post-treatment, no signs of fatigue or imbalance were recorded in all patients, for the same levels of muscle contraction. CONCLUSIONS As in normal human subjects, increases in sway are associated with signs of neck muscle fatigue in some whiplash injury patients. Physiotherapy decreases the susceptibility to fatigue of neck muscles and is an effective choice of treatment of subjective instability and sway. SIGNIFICANCE This study demonstrates a pathophysiological link between neck muscle fatigue and impaired postural control, and also that physiotherapy can relieve symptoms and signs of impaired neck muscle function by reducing muscle fatigability.

Reaching to multiple targets when standing: the spatial organization of feedforward postural adjustments.

We examined the spatial organization of feedforward postural adjustments produced prior to and during voluntary arm reaching movements executed while standing. We sought to investigate whether the activity of postural muscles before and during reaching was directionally tuned and whether a strategy of horizontal force constraint could be observed. To this end, eight human subjects executed self-paced reach-to-point movements on the random illumination of one of 13 light targets placed within a 180 degrees array centered along the midline of the body. Analysis was divided into two periods: a first corresponding to the 250 ms preceding the onset of the reaching movements (termed pPA period) and a second 250-ms period immediately preceding target attainment (the aPA period). For both periods, electromyographic activity of the lower limb muscles revealed a clear directional tuning, with groups of muscles being activated for similar directions of reach. Analysis of horizontal ground reaction forces supported the existence of a force constraint strategy only for the pPA period, however, with those in the aPA period being more widely dispersed. We suggest that the strategy adopted for feedforward pPAs is one where the tuned muscle synergies constrain the forces diagonally away from the center of mass (CoM) to move it within the support base. However, the need to control for final finger and body position for each target during the aPA phase resulted in a distribution of vectors across reaching directions. Overall, our results would support the idea that endpoint limb force during postural tasks depends on the use of functional muscle synergies, which are used to displace the CoM or decelerate the body at the end of the reach.

Postural adjustments for online corrections of arm movements in standing humans

The aim of this study was to investigate how humans correct ongoing arm movements while standing. Specifically, we sought to understand whether the postural adjustments in the legs required for online corrections of arm movements are predictive or rely on feedback from the moving limb. To answer this question we measured online corrections in arm and leg muscles during pointing movements while standing. Nine healthy right-handed subjects reached with their dominant arm to a visual target in front of them and aligned with their midline. In some trials, the position of the target would switch from the central target to one of the other targets located 15°, 30°, or 45° to the right of the central (midline) target. For each target correction, we measured the time at which arm kinematics, ground reaction forces, and arm and leg muscle electromyogram significantly changed in response to the target displacement. Results show that postural adjustments in the left leg preceded kinematic corrections in the limb. The corrective postural muscle activity in the left leg consistently preceded the corrective reaching muscle activity in the right arm. Our results demonstrate that corrections of arm movements in response to target displacement during stance are preceded by postural adjustments in the leg contralateral to the direction of target shift. Furthermore, postural adjustments preceded both the hand trajectory correction and the arm-muscle activity responsible for it, which suggests that the central nervous system does not depend on feedback from the moving arm to modify body posture during voluntary movement. Instead, postural adjustments lead the online correction in the arm the same way they lead the initiation of voluntary arm movements. This suggests that forward models for voluntary movements executed during stance incorporate commands for posture that are produced on the basis of the required task demands.

Differences in gender and performance in off-road triathlon

Abstract The aims of this study were: (1) to examine performance trends and compare elite male and female athletes at the off-road triathlon (1.5-km swim, 30-km mountain biking, and 11-km trail running) world championships since its inception in 1996, and (2) to compare gender-related differences between off-road triathlon and conventional road-based triathlon. Linear regression analyses and ANOVA were used to examine performance trends and differences between the sexes. Elite male performance times stabilized over the 2005–2009 period, whereas elite female performance times continued to improve, especially for the run leg. Differences in performance times between the sexes were less marked in swimming than in mountain biking and running, whereas differences in power output were more marked for mountain biking than for swimming and running. In addition, differences in cycling between the sexes were greater for off-road than conventional on-road triathlon. The specific aspects of mountain biking (e.g. level and terrain) may partly explain the significant differences between the sexes recorded in cycling for off-road triathlon. Future studies will need to focus on the physiological bases of off-road triathlon and how they differ from conventional triathlon.

A prolonged motor imagery session alter imagined and actual movement durations: Potential implications for neurorehabilitation

Mental practice with motor imagery improves motor performance, for example reducing the duration of goal-directed movements. However, it is not known whether an experimental session involving prolonged sequences of motor imagery induces mental fatigue and alters motor and mental performances. In this study, participants imagined 100 point-to-point arm movements combined with actual pointing movements every 10 or 50 imagined movements. Participants reported a subjective feeling of mental fatigue after imagining 100 pointing movements. When participants performed actual movements every 50 imagined movements, the duration of both actual and imagined movements increased at the end of the protocol. On the contrary, no change in actual and imagined movement duration was observed when participants performed actual movements every 10 imagined movements. These results suggested that the repetition of many imagined movements induced mental fatigue and altered the mental simulation and the actual execution processes of the movement. However, the regular execution of actual movements seemed to counteract the negative effect of mental fatigue as both actual and imagined movement duration remained constant with actual trials inserted between mental rehearsals. We suggest that during training or rehabilitation programs, actual movements should be executed and/or imagined movement duration should be controlled to avoid the negative effects of mental fatigue on motor performance.

Neuromuscular control and exercise-related leg pain in triathletes

UNLABELLED Previous studies have shown that cycling can directly influence neuromuscular control during subsequent running in some highly trained triathletes. A relationship between this altered neuromuscular control of running and musculoskeletal pain and injury has been proposed; however, this link has not been investigated. PURPOSE This study aimed to evaluate the influence of cycling on neuromuscular control during subsequent running in highly trained triathletes with and without exercise-related leg pain (ERLP). METHODS Participants were 34 highly trained triathletes: 10 triathletes with a history of ERLP and 24 training-matched control triathletes with no history of ERLP. Knee and ankle kinematics and leg muscle recruitment were compared between a baseline run (no prior exercise) and a transition run (preceded by cycling; i.e., run vs cycle run). RESULTS Knee and ankle joint kinematics were not different between baseline and transition runs for any triathletes: absolute mean difference (+/-95% confidence interval) was 1.49 degrees +/- 0.17 degrees. However, muscle recruitment was different between baseline and transition runs, defined by absolute mean difference in EMG amplitude > or = 10%, in 5 of 24 control triathletes (11/130 muscles exhibited altered recruitment) and in 5 of 10 triathletes with a history of ERLP (12/50 muscles exhibited altered recruitment). This represents a relative risk of 2.40 (0.89-6.50; P = 0.089) when defined by athletes and 2.62 (1.34-6.01; P < 0.01) when defined by muscles. The magnitude of change in muscle recruitment between baseline and transition runs was not different between control (14.10% +/- 2.34%) and ERLP triathletes (16.31% +/- 3.64%; P = 0.41). CONCLUSIONS This study demonstrates an association between ERLP in triathletes and their neuromuscular control when running off the bike.

Evidence for subjective values guiding posture and movement coordination in a free-endpoint whole-body reaching task.

When moving, humans must overcome intrinsic (body centered) and extrinsic (target-related) redundancy, requiring decisions when selecting one motor solution among several potential ones. During classical reaching studies the position of a salient target determines where the participant should reach, constraining the associated motor decisions. We aimed at investigating implicit variables guiding action selection when faced with the complexity of human-environment interaction. Subjects had to perform whole body reaching movements towards a uniform surface. We observed little variation in the self-chosen motor strategy across repeated trials while movements were variable across subjects being on a continuum from a pure ‘knee flexion’ associated with a downward center of mass (CoM) displacement to an ‘ankle dorsi-flexion’ associated with an upward CoM displacement. Two optimality criteria replicated these two strategies: a mix between mechanical energy expenditure and joint smoothness and a minimization of the amount of torques. Our results illustrate the presence of idiosyncratic values guiding posture and movement coordination that can be combined in a flexible manner as a function of context and subject. A first value accounts for the reach efficiency of the movement at the price of selecting possibly unstable postures. The other predicts stable dynamic equilibrium but requires larger energy expenditure and jerk.