Center Of Pressure Excursion Research Articles

Age-related changes in postural responses to externally- and self-triggered continuous perturbations

This is a cross sectional study of eight healthy young (age 22 ± 2 years) and eight healthy older (age 70 ± 5 years) adults who participated in an experiment in which they had to keep their balance while standing on a platform moving in the anterior/posterior direction. The 20 cm peak-to-peak platform movements were sinusoidal at increasing frequencies up to 0.61 Hz. The frequency increases were either sudden and unpredictable (externally-triggered perturbations) or controlled by the subjects themselves (self-triggered perturbations). To maintain balance, all subjects used anticipatory adjustments such as early postural muscle activation, leading to fewer steps being taken, tighter coupling of the center of pressure (COP) and the platform movements, and small COP excursions. These adjustments were found to be lacking for externally-triggered perturbations in older subjects; in these subjects the COP range was larger and more often in less safe regions at the boundaries of the base of support.

Reliability Of Single Leg Landing On A Portable Force Platform

Impact forces at landing are related to valgus moments at the knee and potential ACL injury risk. Reliable tests that are portable and applicable to large scale screening are important to identify high risk individuals in order to target them for appropriate interventions. PURPOSE The purpose of this study was to investigate within session reliability of measures of single leg landing in a large group of female athletes. The hypothesis tested was that within session measures of force and center of pressure (COP) would be reliable during single leg landing. METHODS 45 high school and collegiate soccer players volunteered participation. Each performed a 50cm single leg hop and were instructed to balance for 10 seconds after the landing. A portable force platform (AccuPower, AMTI) was used with force and center of pressure (COP) data. Balance during the trial was assessed by calculating the standard deviation of each variable (COPML, COPAP, FML, FAP, FV). The range of COP excursion (ML and AP) was also calculated during the balance portion of each trial. Stability algorithms utilized sequential estimation during the initial 3 seconds after impact (Colby, 1999). The time of stability (medial/lateral (ML), anterior/posterior (AP) and vertical force (V)) derived from sequential estimation has been previously shown to be reliable when collecting 10 trials. Intraclass correlation coefficients (ICC[3, k]) were performed in SPSS for each variable. RESULTS Maximum vertical ground reaction force had high reliability on both the right (r = 0.823) and left side (r = 0.877). The maximum vertical ground reaction force for the three trials on the right side was 2.40BW, 2.41BW and 2.40BW, respectively. The average (right and left side) reliability for the standard deviation was: COPML (r = 0.831), COPAP(r = 0.651), FML(r = 0.824), FAP(r = 0.714) and FV(r = 0.741). Average COP medial/lateral excursion had a reliability of r = 0.736 and anterior/posterior excursion was r = 0.607. The stability time measurement that had an ICC over 0.6 was calculating the vertical force with sequential estimation (right, r = 0.787 and left r = 0.767). CONCLUSIONS Maximum vertical force and the standard deviation and excursion measures had high reliability during three trials in female athletes. Calculation of stability time from medial/lateral and anterior/posterior force was less reliable. Calculation of a stability time from sequential estimation based on the vertical component was improved compared to the use of M/L and A/P.

Effects of Treatment With a Cetylated Fatty Acid Topical Cream on Static Postural Stability and Plantar Pressure Distribution in Patients With Knee Osteoarthritis

The purpose of the present investigation was to examine the effects of 30 days of treatment with a topical cream consisting of cetylated fatty acids on static postural stability and plantar pressures in patients with osteoarthritis (OA) of one or both knees. Forty patients diagnosed with knee OA were randomly assigned to 1 of 2 topical treatment groups: (a) cetylated fatty acid (CFA; N = 20; age = 62.7 +/- 11.7 years); or (b) placebo (P; N = 20; age = 64.6 +/- 10.5 years). Patients were tested on 2 occasions: (a) baseline (T1), and (b) following a 30-day treatment period consisting of cream application twice per day (T2). Assessments included 20- and 40-second quiet standing protocols on a force plate to measure center of pressure (COP) total excursion length, COP velocity, and rearfoot and forefoot plantar pressure distribution. In the CFA group, a significant reduction in the COP excursion length and velocity were observed at T2, whereas no significant differences were observed in the P group. No significant differences in mean forefoot, rearfoot, or rearfoot-to-forefoot plantar pressure ratios were observed in either group at T2. However, in a subgroup of participants designated to be right- or left-side dominant, improvements in the right-to-left forefoot plantar pressure ratios were observed in both groups. These data indicate that 30 days of treatment with a topical cream consisting of cetylated fatty acids improves static postural stability in patients with knee OA presumably due to pain relief during quiet standing. Such over-the-counter treatment may help improve the exercise trainability of people with OA.

Postural muscle activity during bilateral and unilateral arm movements at different speeds.

The muscle activation patterns in anterior and posterior leg muscles were investigated with two types of perturbations to standing balance. Subjects stood with each foot on adjacent force platforms and performed arm flexion movements to shoulder height. Nine subjects performed ten repetitions unilaterally and bilaterally at 100, 75, 50, 25, and 12.5% of maximal acceleration as measured by an accelerometer placed on the dominant hand. Four subjects also performed the fastest movements while leaning forwards and backwards. The area and latency of the EMG activity from the quadriceps (QUAD), hamstrings (BF), soleus (SOL), and tibialis anterior (TA) were measured bilaterally, along with the excursions of the center of pressure (COP) during each movement. In both unilateral and bilateral tasks, subjects showed a scaling of EMG area and COP excursion with the acceleration of the arm movement. Prior to movement onset, significant scaling of EMG area with movement speed occurred in both unilateral and bilateral tasks in most muscles. Following movement onset, EMG areas scaled significantly to movement speed in only the anterior musculature, with the exception of the left BF. The latency of BF was consistent for the four fastest movements. Only the slowest movements resulted in a significant rightward shift of the BF EMG latency. During the unilateral task, the ipsilateral hamstrings were activated significantly earlier than in the bilateral task and the contralateral hamstrings were activated significantly later. It was also observed that subjects utilized one of two different strategies to maintain balance. Five individuals displayed simultaneous anterior/posterior muscle activation while the other four displayed a reciprocal pattern of activation. Regardless of the initial standing position (leaning forwards or backwards), subjects used the same simultaneous or reciprocal activation strategy. The results indicate that muscle activation patterns change with different tasks, but remain the same during variations of the same task.

Effects of environmental and job-task factors on workers' gait characteristics on slippery surfaces

The objective of this study was to investigate the kinetic and kinematic aspects of slips associated with gait on a slippery surface under various environmental and job-task risk factors. Forty healthy industrial workers (age: 40.3 ± 14.9 years) participated in the study. Using a strain gauge type force platform and a video-based motion analysis system, kinetic and kinematic measurements of the subjects' foot movements were obtained. Of all gait trials, there were 1558 slips (60.9%). Slips were likely to occur when subjects were negotiating a turning path and an oily surface. Greater anterior-posterior center of pressure (CP) excursion and maximum required coefficient of friction (RCOF) were found for oily surfaces compared to dry surfaces. Subjects changed their gait patterns by shortening their stride length, slowing walking speed, and decreasing heel contact angle in the poorly lit and slippery environment. Significant correlations were found between slip occurrence and anterior-posterior CP excursion, mean RCOF, sliding distance and sliding velocity, but not the coefficient of friction (COF) of shoes. In addition to good housekeeping and proper selection of floor materials and safety shoes, slip and fall prevention should include proper workers' training in dealing with risk factors of slips and falls in the workplace.

Age-induced modifications of static postural control in humans

We examined how young and older adults adapt their posture to static balance tasks of increasing difficulty. Participants stood barefoot on a force platform in normal quiet, Romberg-sharpened and one-legged stance. Center of pressure (CoP) variations, electromyographic (EMG) activity of ankle and hip muscles and kinematic data were recorded. Both groups increased postural sway as a result of narrowing the base of support. Greater CoP excursions, EMG activity and joint displacements were noted in old compared to younger adults. Older adults displayed increased hip movement accompanied by higher hip EMG activity, whereas no similar increase was noted in the younger group. It is concluded that older adults rely more on their hip muscles when responding to self induced perturbations introduced by increased task constraints during quiet standing.

The effects of release height on center of pressure and trunk muscle response following sudden release of stoop lifting tasks

Background and objectives: Sudden release of load during lifting threatens postural stability and is countered by trunk muscle response, which can generate high loads on the spine, and may be a cause of tissue injury. The postural threat following sudden release and the consequent muscular response are likely to depend on the posture at the time of release. This study investigates the effects of sudden release of load at two release heights of one- and three-quarters of the knee to shoulder distance during stoop lifting. Methods: Ten normal southern Chinese male volunteers were subject to sudden release of 20, 40, 60 and 80 N loads during stoop lifting trials. The release was randomly selected to be on the third, fourth or fifth cycle of a trial and was triggered at heights of one- and three-quarters of the total knee to shoulder lifting distance. The subjects stood on a force platform to allow the postural disturbance to be recorded by monitoring the center of pressure (COP), and electromyographic (EMG) data were collected from the rectus abdominus, internal oblique, external oblique, erector spinae and latissimus dorsi muscle groups. Results: The COP excursion moved closer to the posterior limit of stability with increasing release load, and this effect was significantly more marked for release from the lower of the two heights. The minimum posterior COP separation from the posterior limit of stability was significantly less for the lower release height at all loads ( p<0.001 in all cases). EMG data showed that the agonist–antagonist co-contraction durations were higher for the lower release height, and unlike sudden release from the higher level, showed a significant increase with increasing load. Conclusions: Sudden release at lower release height during stoop lifting results in significantly greater postural disturbance and spinal loading. The mean load predicted to result in fall or stumble at the lower release height (133 N) is significantly less than that predicted at the higher of the two release heights (245 N). A more marked effect of release load is also seen in the postural disturbance and trunk muscle co-contraction time for the lower release height, and particular care should therefore be taken when handling potentially unstable loads under these conditions. If the security of the load cannot be guaranteed, storage at a higher level may reduce the risk of injury due to sudden release of the load.

Interaction between Attention Demanding Motor and Cognitive Tasks and Static Postural Stability

Background: Due to the often-reported decrease in postural stability in the elderly, it is important to understand factors that may contribute to reduced postural stability. It is possible that attention-demanding focal tasks performed concurrent with postural regulation influence postural stability. Objective: This study utilized dual-task methodology to determine if motor or cognitive focal tasks interact with center of pressure (COP) excursion during static bipedal stance in healthy young and healthy elderly subjects (n = 18). Methods: The cognitive task involved silently solving an orally-presented multi-step arithmetic problem over a 30-second period. The motor task was a 30-second bilateral static finger-thumb pinch task performed at 10% of maximal voluntary contraction with a pair of pinch-force transducers. Each focal task was performed separately, and in a condition in which both tasks were performed simultaneously. COP excursion was compared in quiet standing (no focal task) and during performance of the focal tasks with full vision and with vision occluded. Results: Performance on the focal tasks was unaffected by increased postural demands during stance as compared to a seated baseline condition. This was the case for both age groups, and for the full vision and occluded vision conditions. Medio-lateral COP excursion was reduced over the quiet standing pretest condition when attentional focus was on the cognitive task, suggesting that COP was influenced centrally during cognition. In contrast, COP excursion increased over the quiet standing pretest condition when performing the motor focal task, suggesting a reduced ability to suppress sway when the motor system was concurrently occupied with a voluntary task that shared the same input-output resources. Conclusion: The ability to share attentional resources among focal and postural tasks was similar in healthy young and elderly subjects.

An investigation of stability limits while holding a load

The objective of this study was to investigate the effects of load height and foot placement on the functional base of support (FBOS) limits and the postures that participants used when they reached their FBOS limits. Twelve young male participants were tested while holding a 12-kg load at reach (above their heads), at shoulder and knuckle height, and unladen under both wide and narrow foot placements. The FBOS limits and the centre of pressure (COP) excursion length were calculated based on data from a force platform. Postural angles when participants reached their FBOS limits were calculated from records of a 2-D motion analysis system. The results showed that the load height had greater effect on the posterior FBOS limit. As the load height decreased, the COP excursion length decreased. Participants were prone to using a hip strategy to maintain postural balance when reaching their FBOS limits. Quantitative data of FBOS limits and postural control while participants hold a load at various heights when reaching their FBOS limits is of value for designing a safe materials handling workplace.

Outcomes after trans‐tibial amputation: the relationship between quiet stance ability, strength of hip abductor muscles and gait

Previous research regarding the symmetry of trans-tibial amputees has examined weight distribution and various gait parameters between prosthetic and sound limbs. However, to date, no known research has determined if asymmetry is present in the strength of the hip abductor muscles or if correlations exist between these categories of symmetry. The purpose of the present study was, therefore, to document asymmetry present in stance, strength and gait measures, and to determine the relationship between these variables. Twenty-three elderly, unilateral trans-tibial amputees stood on two adjacent forceplates whilst the weight distribution and standard deviation (SD) of the anterior-posterior and the medio-lateral centre of pressure excursion (COPE) under each limb was recorded during four 40 s trials: quiet stance (QS), with eyes open and eyes closed; and even stance (ES), with eyes open and eyes closed. Gait measures (velocity, cadence, step and stride lengths, stance:swing ratio and period of double support) over 10 m of fast, yet safe walking and measures of the strength of hip abductor muscles were also obtained by use of a stride analyser and a dynamometer, respectively. No significant differences were found between QS and ES measures. However, significantly more weight was taken on the sound limb than on the amputated limb. Notably, more anterior-posterior movement occurred under the sound limb than the amputated limb, with this becoming more apparent with the eyes closed. Movement in the medio-lateral direction was found to be the same between sides. No differences in muscle strength or gait measures between limbs were demonstrated. However, strong hip abductor muscles were correlated with increased weight-bearing on the amputated limb, improved gait parameters and reduced medio-lateral COPE under the amputated limb. This research confirms the asymmetrical nature of amputee stance and demonstrates symmetry of strength and gait measures between limbs. The correlations between hip abductor muscle strength, weight distribution and gait measures illustrates the importance of pre- and post-operative training of these muscles.

Improvement of isokinetic knee extensor strength and reduction of postural sway in the elderly from long-term Tai Chi exercise

Wu G, Zhao F, Zhou X, Wei L. Improvement of isokinetic knee extensor strength and reduction of postural sway in the elderly from long-term Tai Chi exercise. Arch Phys Med Rehabil 2002;83:1364-9. Objectives: To compare isokinetic strength of leg muscles and foot center of pressure (COP) as a measure of sway between long-term Tai Chi practitioners and controls. Design: Cross-sectional study. Setting: Community setting. Participants: Twenty subjects in the Tai Chi group and 19 subjects in the control group (age, >55y). Intervention: Subjects in Tai Chi group had practiced Tai Chi for a minimum of 3 years. Main Outcome Measures: Concentric and eccentric strength of knee extensors and flexors at 60°/s and 120°/s, and foot COP displacement during quiet stance with eyes open or closed. Results: People in the Tai Chi group had significantly higher knee extensor strength at all speeds tested (P<.013), and smaller foot COP excursions for both eyes open and eyes closed conditions (P<.05) than people in control group. No significant difference existed in knee flexors between the 2 groups (P<.713). The COP excursions correlated significantly with the eccentric strength of knee extensors (P<.07) but not with the concentric strength of knee extensors (P<.14) or with the isokinetic strength of knee flexors at most of the speeds (P<.27). Conclusion: These findings support the hypothesis that the maintenance of eccentric strength of postural muscles in the lower extremities, which is beneficial for maintaining good postural stability, is helped through the long-term practice of Tai Chi. © 2002 by the American Congress of Rehabilitation Medicine and the American Academy of Physical Medicine and Rehabilitation

Noise-enhanced balance control in older adults.

Somatosensory information is critical to balance control and fall prevention in older adults. Recently, it has been shown that low-level input noise (electrical or mechanical) can enhance the sensitivity of the human somatosensory system. In this study, we tested the effect of low-level electrical noise, applied at the knee, on balance control in 13 healthy elderly volunteers. Subjects performed multiple single-legged stance trials with imperceptible electrical noise applied at the knee during half of the trials. Balance performance was characterized using a force platform to measure the displacement of the center of pressure (COP) under the subject's stance foot. Seven sway parameters were extracted from the COP time series. Improved balance was defined as a reduction in postural sway as indicated by decreases in the COP measures. Six of the seven sway parameters decreased with electrical noise. Three of these parameters decreased significantly ( < 0.05), and a fourth parameter was borderline significant. Averaged across subjects, the application of electrical noise resulted in a 3.8% reduction in mediolateral COP standard deviation ( = 0.04), a 5.4% decrease in the maximum anteroposterior COP excursion ( = 0.03), a 3.1% reduction in the COP path length ( = 0.04), and a 7.8% decrease in swept area ( = 0.05). The results suggest that imperceptible electrical noise, when applied to the knee, can enhance the balance performance of healthy older adults. These findings suggest that electrical noise-based devices may be effective in improving balance control in elderly people.

Static and dynamic biomechanics of foot orthoses in people with medial compartment knee osteoarthritis

Objective. Gait biomechanics (knee adduction moment, center of pressure) and static alignment were investigated to determine the mechanical effect of foot orthoses in people with medial compartment knee osteoarthritis. Design. Repeated measures design in which subjects were exposed to three conditions (normal footwear, heel wedge and orthosis) in random order. Background. The knee adduction moment is an indirect measure of medial compartment loading. It was hypothesized that the use of a 5° valgus wedge and 5° valgus modified orthosis would shift the center of pressure laterally during walking, thereby decreasing the adduction moment arm and the adduction moment. Methods. Peak knee adduction moment and center of pressure excursion were obtained in nine subjects with medial compartment knee OA during level walking using an optoelectric system and force plate. Static radiographs were taken in 12 subjects using precision radiographs. Results. There was no difference between conditions in static alignment, the peak adduction moment or excursion of the center of pressure in the medial-lateral direction. No relationship was found between the adduction moment and center of pressure excursion in the medial-lateral plane. The displacement of the center of pressure in the anterior–posterior direction, measured relative to the laboratory coordinate system, was decreased with the orthosis compared to the control condition ( P=0.036) and this measure was correlated with the adduction moment ( r=0.45, P=0.019). Conclusions. The proposed mechanism was not supported by the findings. The reduction in the center of pressure excursion in the anterior–posterior direction suggests that foot positioning was altered, possibly to a toe-out position, while subjects wore the orthoses. Based on the current findings, we hypothesize that toe-out positioning may reduce medial joint load. Relevance Knee Osteoarthritis is the most common cause of chronic disability amongst seniors. Developing inexpensive, non-invasive treatment strategies for this large population has potential to impact health care costs, quality of life and clinical outcomes.

Walking strategy in diabetic patients with peripheral neuropathy.

Diabetic neuropathic patients show a peculiar loading pattern of the foot, which led us to hypothesize that a substantial modification exists in their deambulatory strategy. The aim of the present study was to support this hypothesis by quantifying the changes of the loading patterns and by monitoring the excursion of center of pressure (COP) during gait. -A total of 21 healthy volunteers (C) and 61 diabetic patients were evaluated: 27 diabetic subjects without neuropathy (D), 19 with neuropathy (DN), and 15 with previous neuropathic ulcer (DPU). A piezo-dynamometric platform was used to record the foot-to-floor interaction by measuring loading time and the instantaneous COP position during the stance phase of gait. Loading time was significantly longer in neuropathic patients than in control subjects (DPU: 816.8 +/- 150 ms; DN: 828.6 +/- 152 ms; D: 766.5 +/- 89.9 ms; C: 723.7 +/- 65.7 ms; P < 0.05). COP excursion along the medio-lateral axis of the foot clearly decreased from C to DPU groups (C: 6.41 +/- 0.1 cm; D: 4.88 +/- 0.2 cm; DN: 4.57 +/- 0.1 cm; DPU: 3.36 +/- 0.1 cm; P < 0.05) as well as COP excursion along the longitudinal axis for the DPU group only (C: 26.6 +/- 1 cm; D: 26.9 +/- 1 cm; DN: 27.2 +/- 1 cm; DPU: 24.2 +/- 1 cm; P < 0.05). COP integrals were significantly reduced for all pathological classes (DPU: 14.2 +/- 8 cm(2); DN: 25.8 +/- 6 cm(2); D: 27.7 +/- 3 cm(2); C: 38.6 +/- 6 cm(2); P < 0.05). The accurate quantification of loading patterns and of COP excursions and integrals highlights changes of foot-to-floor interaction in diabetic neuropathic patients. The decreased medio-lateral and longitudinal COP excursions and corresponding changes of loading times and patterns support our hypothesis that a change in the walking strategy of diabetic patients with peripheral neuropathy does occur.

PROLONGED ANKLE BRACE APPLICATION DOES NOT AFFECT THE SPECTRAL QUALITIES OF POSTURAL SWAY

In an attempt to prevent recuring ankle sprains, many athletes have resorted to using various external support devices over a course of a season in order to protect the ankle joint. Spectral analysis of center of pressure (COP) data allows for investigation of the relative contribution of the different sensory systems used in controlling posture. Potential long-term effects of ankle bracing on postural sway are not well known. Therefore, the purpose of this study was to determine if prolonged ankle brace application affects selected spectral qualities of medial-lateral (ML) and anterior-posterior (AP) COP during a one-legged stance. The effects of three independent variables (treatment: brace, control; frequency bin: 0–20%, 20–40%, 40–60%, 60–80%, and 80–100% of area; and time: pretest, immediately following ankle brace application, and after 1-, 2-, 3-, and 4-, days brace wearing) on mean frequency amplitude for both AP and ML COP excursions were examined. Twenty-eight healthy volunteers (22.18 ± 1.90 yrs, 173.0 ± 8.02 cm, 72.13 ± 12.13 kg) participated in this study. The first testing session consisted of 10, 20-second, trials (5 without and 5 with an ankle brace). Subjects in the brace group were instructed to wear the brace 8 consecutive hr · day−1. For all subsequent testing sessions, measurements for both groups were taken wearing an ankle brace. A significant difference was found between frequency bins at each time for the mean frequency of ML (F4,108 = 495.8, P < .0001) and AP (F4,108 = 79.751, P < .0001) COP. Mean frequency amplitudes of ML and AP COP were greater at lower frequency ranges than at higher frequency ranges during all 6 measurements (P < .05). No difference was found between the brace and control conditions (P > .05). Application of an ankle brace may not require a modification in the postural control strategy during a one-legged stance in healthy ankles.

Effect of stance width on multidirectional postural responses.

The effect of stance width on postural responses to 12 different directions of surface translations was examined. Postural responses were characterized by recording 11 lower limb and trunk muscles, body kinematics, and forces exerted under each foot of 7 healthy subjects while they were subjected to horizontal surface translations in 12 different, randomly presented directions. A quasi-static approach of force analysis was done, examining force integrals in three different epochs (background, passive, and active periods). The latency and amplitude of muscle responses were quantified for each direction, and muscle tuning curves were used to determine the spatial activation patterns for each muscle. The results demonstrate that the horizontal force constraint exerted at the ground was lessened in the wide, compared with narrow, stance for humans, a similar finding to that reported by Macpherson for cats. Despite more trunk displacement in narrow stance, there were no significant changes in body center of mass (CoM) displacement due to large changes in center of pressure (CoP), especially in response to lateral translations. Electromyographic (EMG) magnitude decreased for all directions in wide stance, particularly for the more proximal muscles, whereas latencies remained the same from narrow to wide stance. Equilibrium control in narrow stance was more of an active postural strategy that included regulating the loading/unloading of the limbs and the direction of horizontal force vectors. In wide stance, equilibrium control relied more on an increase in passive stiffness resulting from changes in limb geometry. The selective latency modulation of the proximal muscles with translation direction suggests that the trunk was being actively controlled in all directions. The similar EMG latencies for both narrow and wide stance, with modulation of only the muscle activation magnitude as stance width changed, suggest that the same postural synergy was only slightly modified for a change in stance width. Nevertheless, the magnitude of the trunk displacement, as well as of CoP displacement, was modified based on the degree of passive stiffness in the musculoskeletal system, which increased with stance width. The change from a more passive to an active horizontal force constraint, to larger EMG magnitudes especially in the trunk muscles and larger trunk and CoP excursions in narrow stance are consistent with a more effortful response for equilibrium control in narrow stance to perturbations in all directions.

Lateral reach: a clinical measure of medio-lateral postural stability.

Medio-lateral postural instability has recently been identified as a risk factor for falls in the elderly community-dwelling population. However, few clinical tests involve challenges to stability limits in the medio-lateral direction. A lateral reach test was thus developed. Preliminary evaluation of the symmetry, accuracy, test-retest repeatability, and construct validity of the lateral reach test was undertaken to ensure its validity and repeatability prior to clinical application. Sixty healthy older female subjects (aged 72.5 +/- 5 years) participated in this study which involved two tasks. The first was the clinical test: a maximal lateral reach to the right and left, recorded by a clinical measure (observation of hand excursion) and a laboratory measure (3D analysis of hand marker excursion). The second was the recording of centre of pressure (COP) stability limits in the lateral direction by use of a dual force platform system. Eighteen subjects returned for repeatability testing. No significant differences in the measures were found between the sides. Lateral reach as measured by the clinical measure was significantly (p < 0.05) correlated with both the laboratory measure of reach (r = 0.650) and with COP stability limits (r = 0.331). The clinical measure of reach was not significantly different (p > 0.01) from the laboratory measure. High test-retest repeatability (r > 0.94) was found for all measures. The clinical lateral reach test was found to be an accurate measure of lateral reach ability. As it was significantly correlated with COP excursion it was deemed a valid indicator of lateral stability limits. In addition, the lateral reach test had high test-retest repeatability and was symmetrical between the sides. These promising results support both it's use where a clinical indicator of medio-lateral postural stability is desired and further investigation of the lateral reach test in clinical populations.

EMG Biofeedback as a Tool for Simulating the Effects of Specific Leg Muscle Weakness on a Lifting Task

This study investigated the use of EMG biofeedback to simulate weakened rectus femorii and gastrocnemii muscles during the performance of a lifting task. Eight healthy women performed 15 kg free-style lifts from floor level. Three conditions were tested: unconstrained lifting, lifting with rectus femorii activity volitionally limited bilaterally through EMG biofeedback to less than 45% of maximal EMG activity, and lifting with the gastrocnemii limited to a similar level. Limiting leg muscle activity through biofeedback led to an alteration of lifting strategy, with resulting performance variables (joint angles and torques, angular velocities, center of pressure excursion, and segment coordination) comparing favorably with those from lifting trials performed by six women with moderate leg muscle weaknesses. The data indicate that EMG biofeedback can be used to simulate the effects of leg muscle weakness during these lifts, providing a new tool to study the biomechanics of muscle weakness.

The influence of exercise and dehydration on postural stability

The aim of this study was to investigate the effects of exercise-induced and thermal dehydration on postural.balance. Eight male subjects cycled for 2 h at a power output equal to 57 - 63% V O on two different occasions: once without 2 max drinking (NF) and once with intake of 1.9 1 of a carbohydrate-electrolyte solution (FR). Before and after the exercise test, the velocity of the centre of pressure (COP) excursion was measured on a force platform during 30-s bipedal standing in normal position, feet side by side, and tandem position, feet heel to toe. On another occasion, eight subjects underwent seven consecutive sauna sessions (85 C, 50% rh) of 15 min duration with no fluid replacement (S) to induce thermal dehydration. Mean fluid loss was 2.7 (0.4)%, 0.5 (0.5)% and 3.0 (0.6)% of body mass after NF, FR and S, respectively. Mean velocity of COP excursion after the exercise test was significantly higher in the NF than in the FR trial (p < 0.05). Postural stability was not influenced by S. In conclusion, prolonged exercise without fluid ingestion seems to negatively affect postural stability, whereas no effect is observed after exercise with fluid replacement or after thermal dehydration.

Postural responses to unilateral arm perturbation in young, elderly, and hemiplegic subjects

Objective: To compare postural responses during standing associated with self-paced unilateral arm flexion exhibited by young and elderly healthy subjects and subjects with hemiplegia from stroke. Design and Setting: Descriptive cross-sectional study in a laboratory setting. Patients: Ten young, 10 elderly healthy subjects, and 12 volunteers with longstanding hemiparesis following stroke were tested. The hemiplegic group had good functional balance scores on the Berg Balance Scale (BBS). Main Outcome Measures: The peak arm acceleration, center of pressure (CP) excursion speed, and the electromyographic activity in the posterior leg muscles (bilateral hamstrings and triceps surae) were monitored during self-initiated rapid unilateral arm flexion and during quiet stance. Results: During unilateral arm flexion, the young group showed the highest arm accelerations and lowest CP excursion speeds. The variability of postural responses was largest in the elderly and hemiplegic groups with hemiplegic subjects showing the greatest CP excursion speeds for the lowest arm accelerations. The first burst of activity in ipsilateral hamstrings muscle was the same in all subjects. However, the hemiplegic group had less activation (latency and amplitude) of other posterior leg muscles. Conclusion: The elderly subjects had more variable responses to perturbation than the young subjects despite similar BBS scores. This may indicate that the BBS fails to detect differences in balance at the high end of the scale. Although the hemiplegic subjects demonstrated some anticipatory control of standing balance, they consistently performed poorer than elderly and young controls. The failure to coordinate postural leg muscle activity with focal movement may contribute to the instability of subjects with hemiplegia.