Learning to balance on a slackline: Development of coordinated multi-joint synergies.

Previous research has investigated synergies involved in locomotion and balance reactions; however, there is limited insight into the emergence of skilled balance control with practice of challenging tasks. We explored motor learning of tandem and single leg stance on an unstable surface-a slackline. Balance was tested in 10 naïve healthy adults at four time points: baseline, after one slackline practice session, after 1 week of practice, and 1 week following the final practice session. We recorded kinematics of the upper and lower arms bilaterally, trunk, and thigh and foot unilaterally while participants balanced in tandem and single leg stance on a slackline and narrow rigid beam (transfer task). When participants first attempted to stand on the slackline, they exhibited fast and frequent movements across all joints with actions along the frontal plane (particularly the hip) and fell after a short period (~3 seconds). Performance improved rapidly (fewer falls), and this was accompanied by dampened trunk and foot oscillations and the development of coordinated movement patterns with a progressive emphasis on more distal upper body segments. Continuous relative phase angles between joint pairs began to cluster around either 0° (indicating in-phase movement) or 180° (indicating anti-phase movement). Participants also began to demonstrate coordinated upper body synergies and performance improvements (fewer falls) on the transfer task, while a control group (n = 10) did not exhibit similar synergies or performance improvements. Our findings describe the emergence of coordinated movement synergies involving the upper body as healthy adults learn a challenging balance task.

Ageing reduces light touch and vibrotactile sensitivity on the anterior lower leg and foot dorsum.

Cutaneous mechanoreceptors in the anterior lower leg and foot dorsum provide important information about contact with objects and movement at the knee and ankle. This cutaneous feedback contributes to static and dynamic balance control. We conducted experiment 1 to determine the effects of ageing on anterior lower leg cutaneous feedback. We measured light touch (monofilament) perceptual thresholds (MPT) at seven skin sites across the anterior lower leg and foot dorsum in 12 young (5 male, aged 21-28) and 13 older adults (8 male, aged 73-92). Results showed that older adults had ~5.5× higher MPTs across these skin sites. We conducted experiment 2 to probe how different cutaneous mechanoreceptor subtypes are affected by ageing through measures of vibrotactile perceptual threshold (VPT) at 3, 15, and 40Hz at six skin sites across the anterior lower leg and foot dorsum in 10 young (5 male, aged 21-26) and 10 older adults (3 male, aged 75-85). In this group, we also assessed functional balance using the timed-up-and-go (TUG) and functional reach test (FRT). Older adults demonstrated significantly higher VPTs overall, and this effect was largest at 40Hz - a frequency primarily transmitted by fast adapting cutaneous afferents. Furthermore, higher thresholds at each frequency tended to correlate with poorer performance on the TUG within the older adult group (3Hz: r=0.550; 15Hz: r=0.689; 40Hz: r=0.663). These results suggest ageing influences cutaneous feedback from regions of the lower leg that provide important information about movement and contact.

The vestibular system does not modulate fusimotor drive to muscle spindles in contracting leg muscles of seated subjects.

We previously showed that sinusoidal galvanic vestibular stimulation (GVS) does not modulate the firing of spontaneously active muscle spindles in relaxed human leg muscles. However, given that there is little, if any, fusimotor drive to relaxed human muscles, we tested the hypothesis that vestibular modulation of muscle spindles becomes apparent during volitional contractions at levels that engage the fusimotor system. Unitary recordings were made from 28 muscle spindle afferents via tungsten microelectrodes inserted percutaneously into the common peroneal nerve of seated awake human subjects. Twenty-one of the spindle afferents were spontaneously active at rest and each increased its firing rate during a weak static contraction; seven were silent at rest and were recruited during the contraction. Sinusoidal bipolar binaural galvanic vestibular stimulation (±2 mA, 100 cycles) was applied to the mastoid processes at 0.8 Hz. This continuous stimulation produced a sustained illusion of "rocking in a boat" or "swinging in a hammock" but no entrainment of EMG. Despite these robust vestibular illusions, none of the fusimotor-driven muscle spindles exhibited phase-locked modulation of firing during sinusoidal GVS. We conclude that this dynamic vestibular input was not sufficient to modulate the firing of fusimotor neurones recruited during a voluntary steady-state contraction, arguing against a significant role of the vestibular system in adjusting the sensitivity of muscle spindles via fusimotor neurones.

Medio-lateral balance adjustments preceding reflexive limb withdrawal are modified by postural demands.

We have recently observed medio-lateral balance adjustments (BA) preceding reflexive stepping elicited by noxious stimulation. While task specific modulation is evident for BA prior to voluntary leg movement, it is unclear whether rapid BA reactions (prior to 'reflexive' stepping) represent a generic response to evoked limb withdrawal or can be modified to suit task-conditions. This study was designed to establish whether the CNS is able to modify rapid onset latency BAs to match task conditions. Reflexive stepping was evoked by applying a noxious stimulus (50 ms stimulus train, 1 ms pulses, 300 Hz, 4 x perceptual threshold) to the plantar surface of the either the left or right foot. Task conditions were varied prior to stimulation by having subjects maintain one of three different static positions: (1) lean left (70% body weight (BW) on left), (2) neutral (50% BW both sides), (3) lean right (70% BW on right). BAs were denoted by centre-of-pressure (CoP) excursions towards the swing foot after the onset of noxious stimulation (average onset latency of 128 ms). There was a significant increase in frequency of occurrence and a significant increase in magnitude of CoP shift when the stimulation was applied to a loaded limb (leaning with 70% BW on the stimulated foot) as compared to an unloaded limb (30% BW). In addition, 78% of loaded trials featured steps taken with the unstimulated foot, which delayed removal of the stimulated foot. Collectively, the results indicate modifiability of the very rapid onset balance adjustments that precede the onset of limb withdrawal revealing complex control of balance exists even over very brief latencies.

Magnitude effects of galvanic vestibular stimulation on the trajectory of human gait.

This study examines the contribution of the vestibular system during different magnitudes of galvanic vestibular stimulation (GVS) during human walking. Anodal threshold levels of GVS were determined for right and left sides for each subject. Seven conditions were tested (no stimulation, left and right anode stimulation) at one, two and three times threshold. GVS was delivered to the mastoid processes at first heel contact and continued for the duration of the trial. All subjects responded by deviating towards the anode while walking. In addition, the magnitude of deviation increased as the stimulus intensity increased. Our results demonstrate that the vestibular system is sensitive to GVS intensity changes and responds by altering the magnitude of the response accordingly. These data provide a strong argument in support of a significant role for vestibular information during dynamic tasks.

Preparatory balance adjustments precede withdrawal response to noxious stimulation in standing humans.

Self-initiated leg movement in standing humans is preceded by a medio-lateral preparatory balance adjustment (PBA); however, such preparatory balance control is often absent in reflex-like stepping responses evoked by whole-body instability. The presence or absence of the PBA may reflect a task-dependent modulation of the response serving to preserve lateral stability (PBA present) or avoid delay in the lifting of the foot (PBA absent). To examine whether such task-dependent modulation can occur during more stereotypical limb movements, we examined spinally-mediated withdrawal responses evoked by noxious stimulation of the foot. Results showed that rapid limb withdrawal was preceded by a large PBA when subjects were standing but not when they were supine. The PBA caused limb withdrawal to the noxious stimulation to be delayed. However, the onset of the PBA in the standing trials was equivalent in timing to the onset latency of the classic withdrawal responses recorded during the supine trials. Evidence of a preparatory balance adjustment evoked, in advance of a delayed withdrawal response, at very rapid latencies (underlying muscle activation at 70-120 ms) may raise new questions about the neural mechanisms underlying the co-ordination of balance and movement.

NIOSH equation horizontal distances associated with the Liberty Mutual (Snook) lifting table box widths.

A study was conducted to determine the NIOSH equation horizontal distances (dH) associated with the three different box widths (34, 49 and 75 cm) and lift starting heights (floor, knuckle and shoulder) used in the psychophysically based Liberty Mutual lifting tables (Snook 1978, Snook and Ciriello 1991). Data were collected with 12 male and 12 female subjects and three repetitions were performed for each of the nine lifting conditions. No gender effects were observed so male and female data were pooled. The value of dH was positively related to box width but there was also a significant interaction between box width and starting height. When pooled across lift heights the average values of dH were 44, 49 and 57 cm for the 34, 49 and 75 cm box widths, respectively. When pooled across box widths the average values of dH were 52, 45 and 52 cm for the floor, knuckle and shoulder height lifts, respectively. A knowledge of the dH associated with each box width will allow for direct comparisons to be made between the NIOSH and Liberty Mutual outputs. This will facilitate further validation of the NIOSH equations. A variable (GAP) was calculated to indicate the horizontal distance from the ankles to the edge of the box. Previously, this GAP has been assumed to remain constant and values of 15, 20 and 25 cm have been proposed. The GAP was observed to have an overall mean of 23.6 cm with individual condition means ranging from 14.6 cm to 31.2 cm. When pooled across conditions the mean GAP values were equal to dH minus half the box width.

A validation of techniques using surface EMG signals from dynamic contractions to quantify muscle fatigue during repetitive tasks.

The purpose of the current study was to determine the validity of quantifying biceps brachii fatigue with dynamic measures of surface electromyo-graphic (EMG) mean power frequency (MPF) through comparisons with the well-established isometric methodology. Subjects performed repetitive elbow flexion-extension movements with a hand held load of 7 kg until volitional exhaustion. Elbow joint angle and biceps brachii EMG signals were recorded continuously during the fatiguing movement (in 250-ms segments) and during isometric, isotonic contractions (in 1000-ms segments) performed at a 90 degrees flexion angle before and after the trial. The MPF and average EMG amplitude (AEMG) were also calculated with each sample, and a polynomial regression analysis was used to characterize the time history of changes and to determine the rested and fatigued values for the dynamic EMG with: (a) all dynamic samples above 5% MVC and (b) only samples where the elbow joint was between 80 degrees and 100 degrees of flexion. There was a significant increase in AEMG and a decrease in MPF for the isometric contractions and both dynamic methods. When compared to dynamic values at rest and fatigue, the isometric AEMG and MPF were substantially lower and slightly higher, respectively. No significant differences were observed between the AEMG or MPF results from the two methods of processing the dynamic EMG. The decreases in MPF ranged from 25% to 29% and did not differ between methods. The absolute and relative increases in isometric AEMG were substantially lower than with both dynamic methods. The current results support the use of MPF values from surface EMG signals recorded during dynamic contractions to quantify fatigue of the biceps brachii muscle. The proposed methodology can be used to monitor fatigue continuously throughout a dynamic movement with minimal disturbance to the task being performed and without the need to monitor joint angles.