Three components of postural control associated with pushing in symmetrical and asymmetrical stance

A number of occupational and leisure activities that involve pushing are performed in symmetrical or asymmetrical stance. The goal of this study was to investigate early postural adjustments (EPAs), anticipatory postural adjustments (APAs), and compensatory postural adjustments (CPAs) during pushing performed while standing. Ten healthy volunteers stood in symmetrical stance (with feet parallel) or in asymmetrical stance (staggered stance with one foot forward) and were instructed to use both hands to push forward the handle of a pendulum attached to the ceiling. Bilateral EMG activity of the trunk and leg muscles and the center of pressure (COP) displacements in the anterior–posterior (AP) and medial–lateral (ML) directions were recorded and analyzed during the EPAs, APAs, and CPAs. The EMG activity and the COP displacement were different between the symmetrical and asymmetrical stance conditions. The COP displacements in the ML direction were significantly larger in staggered stance than in symmetrical stance. In staggered stance, the EPAs and APAs in the thigh muscles of the backward leg were significantly larger, and the CPAs were smaller than in the forward leg. There was no difference in the EMG activity of the trunk muscles between the stance conditions. The study outcome confirmed the existence of the three components of postural control (EPAs, APAs, and CPAs) in pushing. Moreover, standing asymmetrically was associated with asymmetrical patterns of EMG activity in the lower extremities reflecting the stance-related postural control during pushing. The study outcome provides a basis for studying postural control during other daily activities involving pushing.     

长期太极拳和慢跑锻炼对侧向突发干扰下老年男性神经肌肉反应时和肌电达峰值时间影响

目的比较长期太极拳和慢跑锻炼对老年男性突发侧向姿势干扰下神经肌肉反应时和肌电达峰值时间的差异,探索提高老年男性侧向姿势挑战下神经肌肉反应和肌肉收缩效率的有效锻炼方式。方法利用足底水平干扰触发平台对年轻男性、无规律锻炼的老年男性、长期慢跑锻炼的老年男性、长期太极拳锻炼的老年男性进行突发侧向姿势干扰。表面肌电测试和分析系统用于收集腓骨长肌、胫骨前肌、臀中肌和竖脊肌的肌电信号。结果突发侧向干扰下,无规律锻炼老年男性腓骨长肌、胫骨前肌和臀中肌的神经肌肉反应时明显地慢于年轻男性,长期太极拳锻炼老年男性胫骨前肌和竖脊肌的神经肌肉反应明显地快于老年对照组;年轻男性腓骨长肌、胫骨前肌和臀中肌的收缩速度明显地快于3组老年人。结论长期太极拳锻炼可以使老年男性踝关节和躯干肌的神经肌肉反应更加迅速以应对侧向的姿势挑战,而对于提高老年男性肌肉收缩效率的效果不明显。     

The distal hindlimb musculature of the cat

Summary Chronic recordings were made of electromyographic (EMG) activity, tension, and length of distal hindlimb muscles in six cats performing a variety of normal motor tasks. Muscles studied thoroughly or in part were medial gastrocnemius, lateral gastrocnemius, plantaris, soleus, flexor digitorum brevis, flexor digitorum longus, flexor hallucis longus, tibialis posterior, tibialis anterior, extensor digitorum longus, peroneus longus, and peroneus brevis. Postural and locomotor activities were examined, as well as jumping, landing, scratching, and paw shaking. In general, muscles could be assigned to traditional groupings (e.g. extensor, flexor) related to the demands of the motor task. Patterns of muscle activity were most often consistent with current understanding of muscle mechanics and neural coordination. However, purely functional distinctions between flexor digitorum longus and flexor hallucis longus (anatomical synergists) were made on the basis of activity patterns. Likewise, the activity of plantaris and flexor digitorum brevis, which are attached in series, was differentiated in certain tasks. The rhythmical oscillatory patterns of scratching and paw shaking were found to differ temporally in a manner consistent with the limb mechanics. In several cases, mechanical explanations of specific muscle activity required length and force records, as well as EMG patterns. Future efforts to study motor patterns should incorporate information about the relationships between muscle activation, tension, length and velocity.Abbreviations EDL extensor digitorum longus - FDB flexor digitorum brevis - FDL flexor digitorum longus - FHL flexor hallucis longus - LG lateral gastrocnemius - MG medial gastrocnemius - PB peroneus brevis - PL peroneus longus - PLT plantaris - SOL soleus - TA tibialis anterior - TP tibialis posterior Limbs A ankle - K knee - LF left forelimb - LH left hindlimb - RF right forelimb - RH right hindlimb Step Cycle Phases E₁ first extension, late swing phase prior to footfall - E₂ second extension, early stance phase - E₃ third extension, late stance phase - F flexion, early swing phase     

衰老对侧向姿势干扰下神经肌肉反应的影响

目的应用表面肌电比较老年人和年轻人在侧向姿势干扰下的神经肌肉反应,为深入研究老年人侧向姿势控制能力下降的机制提供理论依据。方法利用足底水平干扰触发平台对14名老年人和14名年轻人进行突发侧向姿势干扰。表面肌电测试系统收集受试者左侧腓骨长肌、胫骨前肌、臀中肌和竖脊肌的肌电反应信号。结果对于突发侧向姿势干扰,老年人胫骨前肌、臀中肌和竖脊肌的神经肌肉反应时明显延迟,并且腓骨长肌、胫骨前肌和臀中肌肌电振幅达峰值的时间明显延长。结论应对突发姿势变化时,老年人踝关节、髋关节和躯干肌肉反应延迟以及踝关节和髋关节肌肉收缩效率不高,可能是老年人侧向姿势控制能力下降的主要原因。     

An unique variation of the peroneus tertius muscle

Peroneus tertius (fibularis tertius) is a muscle unique to humans. It often appears to be a part of extensor digitorum longus, and might be described as its "fifth tendon". Although its insertion variation has been reported by many authors, variations of its origin points are not common. A variation of the peroneus tertius muscle was found during routine dissection of a 75-year-old male cadaver. The muscle originated from the extensor hallucis longus. The muscle belly of the extensor hallucis longus arose from the middle two-fourths of the medial surface of the fibula, medial to the extensor digitorum longus, and anterior surface of the interosseous membrane. It lay under the extensor digitorum longus, and lateral to the tibialis anterior muscle. The muscle belly of the extensor hallucis longus divided into medial and lateral parts 17 cm below its origin point. The lateral part, named as peroneus tertius, continued downward to reach the medial part of the dorsal surface of the base of the fifth metatarsal bone. The medial part ran also downward and divided into two tendons reaching the dorsal surface of the base of the distal phalanx of the great toe. This kind of variation may be important during foot or leg surgery.     

The consistent presence of the human accessory deep peroneal nerve

Twenty-four human legs were dissected macroscopically to study the morphological details of the accessory deep peroneal nerve. This nerve arose from the superficial peroneal nerve and descended in the lateral compartment of the leg, deep to peroneus longus along the posterior border of peroneus brevis. Approaching the ankle joint, this nerve passed through the peroneal tunnels to wind around the lateral malleolus; it then crossed beneath the peroneus brevis tendon anteriorly to reach the dorsum of the foot. The accessory deep peroneal nerve was found in every case examined and constantly gave off muscular branches to peroneus brevis and sensory branches to the ankle region. In addition, this nerve occasionally had muscular branches to peroneus longus and extensor digitorum brevis, and sensory branches to the fibula and the foot. The anomalous muscles around the lateral malleolus were also innervated by this nerve. Neither cutaneous branches nor communicating branches with other nerves were found. The present study reveals that the accessory deep peroneal nerve is consistently present and possesses a proper motor and sensory distribution in the lateral region of the leg and ankle. It is not an anomalous nerve as has previously been suggested.     

The role of plantar cutaneous sensation in unperturbed stance

Considerable evidence shows that sensation from the feet and ankles is important for standing balance control. It remains unclear, however, to what extent specific foot and ankle sensory systems are involved. This study focused on the role of plantar cutaneous sensation in quasi-static balance control. Iontophoretic delivery of anesthesia was used to reduce the sensitivity of the forefoot soles. In a follow-up experiment, subjects received intradermal injections of local anesthetic into the entire weight-bearing surface of the foot soles. Properties of the center-of-foot-pressure (COP) trajectories and ground reaction shear forces were analyzed using stabilogram-diffusion analysis and summary statistics. Effects of foot-sole anesthesia were generally small and mostly manifested as increases in COP velocity. Magnitude of COP displacement was unaffected by foot-sole anesthesia. Forefoot anesthesia mainly influenced mediolateral posture control, whereas complete foot-sole anesthesia had an impact on anteroposterior control. During bipedal stance, statistically significant effects of foot-sole anesthesia on COP were present only under eyes-closed conditions and included increases in COP velocity (11-12%) and shear force root-mean-square (13%), the latter indicating increases in body center-of-mass accelerations due to the foot-sole anesthesia. Similar effects were seen for unipedal stance in addition to an increase in anteroposterior COP median frequency (36%). Changes in stabilogram-diffusion parameters were confined to the short-term region suggesting that sensory information from the foot soles is mainly used to set a relevant background muscle activity for a given posture and support surface characteristic, and consequently is of little importance for feedback control during unperturbed stance. In general, this study demonstrates that plantar sensation is of moderate importance for the maintenance of normal standing balance when the postural control system is challenged by unipedal stance or by closing of the eyes. The impact of reduced plantar sensitivity on postural control is expected to increase with the loss of additional sensory modalities such as the concomitant proprioceptive deficits commonly associated with peripheral neuropathies.     

Accessory peroneal nerves in the human

The Nervus peroneus profundus accessorius was described by Ruge (1878) in the lower mammals and for the first time identified by Bryce (1897) in man. It is an accessory terminal branch of the superficial peroneal (musculocutaneous) nerve which winds round the lateral malleolus beneath the tendons of the peronei muscles and reaches the dorsum of the foot; there it often supplies the lateral portion of the extensor digitorum brevis muscle. In further investigations this nerve has been traced in 21.2% of subjects resp. in 13.5% of legs. This nerve, however, is not the only accessory branch of the common peroneal nerve: In 14 out of 140 subjects (10%) resp. in 22 out of 280 legs (7.9%) a Nervus peroneus superficialis accessorius has been found. This nerve pierces the anterior crural intermuscular septum either in common with deep peroneal (anterior tibial) nerve or at a lower point. Then it descends in front of the septum rarely giving off muscular branches to the extensor digitorum longus and peroneus tertius muscles; in the lower half of the leg it pierces the crural fascia, passes in front of the ankle joint and becomes the medial cutaneous nerve of the dorsum of the foot. This accessory superficial peroneal nerve may be of importance in surgery of the leg and foot.     

Electromechanical assessment of ankle stability

The goal of this study was to approach ankle instability by measuring the electromechanical delay of the peroneal muscles (foot pronators). For that purpose, supramaximal electrical stimulation of common peroneal nerve was applied when the subject was standing on a force plate in bipedal stance or monopedal stance, postures requiring greater ankle stabilization. The electromechanical delay (EMD) was defined as the time interval between the onset of the peroneus longus (PL) electromyogram detected by surface electrodes and the onset of the lateral ground reaction force (Fy) measured on a force plate. Ten healthy subjects (control group, C-G) and ten subjects with functional ankle instability ("FAI" group, FAI-G) performed the tests. In C-G, the mean EMD values decreased significantly (P<0.001) from the bipedal [10.54 (0.71) ms; mean (SD)] to the monopedal stance [8.67 (0.63 ms)]. Since a monopedal stance is known to require higher leg muscle tone resulting in a higher ankle stiffness, these results indicate that PL EMDs are sensitive to musculo-tendinous stiffness. They validate the choice of a PL EMD measurement as an indirect index of musculo-tendinous stiffness at the ankle. In both bipedal and monopedal stance conditions, EMD values were significantly higher in FAI subjects. They ranged from 12.64 (1.14) ms in the bipedal stance to 10.85 (1.07) ms in the monopedal stance. This suggests a lower musculo-tendinous stiffness at the ankle, which may contribute to the ankle instability. Electronic Publication     

The distal hindlimb musculature of the cat. Patterns of normal use

Chronic recordings were made of electromyographic (EMG) activity, tension, and length of distal hindlimb muscles in six cats performing a variety of normal motor tasks. Muscles studied thoroughly or in part were medial gastrocnemius, lateral gastrocnemius, plantaris, soleus, flexor digitorum brevis, flexor digitorum longus, flexor hallucis longus, tibialis posterior, tibialis anterior, extensor digitorum longus, peroneus longus, and peroneus brevis. Postural and locomotor activities were examined, as well as jumping, landing, scratching, and paw shaking. In general, muscles could be assigned to traditional groupings (e.g. extensor, flexor) related to the demands of the motor task. Patterns of muscle activity were most often consistent with current understanding of muscle mechanics and neural coordination. However, purely functional distinctions between flexor digitorum longus and flexor hallucis longus ("anatomical synergists") were made on the basis of activity patterns. Likewise, the activity of plantaris and flexor digitorum brevis, which are attached in series, was differentiated in certain tasks. The rhythmical oscillatory patterns of scratching and paw shaking were found to differ temporally in a manner consistent with the limb mechanics. In several cases, mechanical explanations of specific muscle activity required length and force records, as well as EMG patterns. Future efforts to study motor patterns should incorporate information about the relationships between muscle activation, tension, length and velocity.     

A reduction in the knee adduction moment with medial thrust gait is associated with a medial shift in center of plantar pressure

The knee adduction moment (KAM) is an established marker of compartmental load distribution across the tibiofemoral joint. Research suggests a link between the magnitude of the KAM and center of plantar pressure (COP) thus alterations in the two may be related. The objective of this study was to investigate whether the COP predictably shifts when the KAM is reduced through a gait adaptation. Twenty healthy adults underwent gait analysis walking with their normal gait pattern and with medial thrust gait, a gait adaptation known to significantly reduce the KAM. Simultaneous COP and 3-D kinetics were acquired to allow for a comparison of the change in COP to the change in the KAM. The COP was quantified by determining a customized medial-lateral pressure index (MLPI) which compares the COP tracing line during the first and second halves of stance to the longitudinal axis of the foot. Linear regressions assessing the association between the changes in KAM and MLPI indicated that 48.3% (p = 0.001) of the variation in MLPI during the first half of stance can be explained by the KAM during the same period. A trend was observed between the association between the KAM and MLPI during the second half of stance (R² = 0.16, p = 0.080). Backwards elimination regression analysis was used to explore whether simultaneous consideration of the KAM and other potential confounding factors such as sagittal plane knee moments and speed explained variance in the MLPI during the first half of stance. Only the KAM exhibited explanatory power (β = 0.695, p = 0.001). During medial thrust gait, a reduction in the KAM was associated with a medial shift in the MLPI, and an increase in the KAM was associated with a lateral shift in the MLPI, especially in the first half of the stance phase. Together, these results demonstrate an inherent link between foot pressure and the KAM during medial thrust gait, and suggest that manipulating foot pressure may be a biomechanical mechanism for an intervention designed to improve loading conditions at the knee.     

Exercise Sandals Increase Lower Extremity Electromyographic Activity During Functional Activities

OBJECTIVE: Anecdotal evidence suggests that use of Exercise Sandals results in a number of positive clinical outcomes. However, little research has been conducted to determine their efficacy objectively. Our purposes were to determine the effect of Exercise Sandals on lower leg electromyography (EMG) during activities in the Exercise Sandals and to compare EMG associated with Exercise Sandals with traditional lower extremity rehabilitation exercises. DESIGN AND SETTING: Two within-subjects, repeated-measures designs were used to identify differences in lower extremity EMG: (1) between activities with and without Exercise Sandals and (2) between Exercise Sandals activities and traditional rehabilitation activities. All data were collected in the Sports Medicine Research Laboratory. SUBJECTS: Eighteen subjects involved in rehabilitation using Exercise Sandals for at least 2 weeks within the year before data collection. MEASUREMENTS: Mean EMG amplitudes from the tibialis anterior, peroneus longus, soleus, and lateral gastrocnemius muscles were measured during single-leg stance, side stepping, and "high knees," all performed with and without the Exercise Sandals, as well as single-leg stance on a foam surface and T-band kicks in the sagittal and frontal planes. RESULTS: Exercise Sandals increased lower leg EMG activity, particularly in the ankle invertors and evertors. Also, activities involving the Exercise Sandals resulted in EMG activity similar to or exceeding that associated with traditional ankle-rehabilitation exercises. CONCLUSIONS: These results, coupled with the fact that Exercise Sandals are used in a functional closed kinetic chain manner, suggest that they are an effective means of increasing lower extremity muscle activity.     

EMG analysis of peroneal and tibialis anterior muscle activity prior to foot contact during functional activities

The purpose of this investigation was to determine the pre-activity of the tibialis anterior (TA), peroneus longus (PL), and peroneus brevis (PB) prior to foot contact during three conditions. Twenty-six subjects (age 22 +/- 2 yrs; 15 male, 11 female) with no lower extremity injuries reported for data collection. Data were collected from each subject's dominant leg using surface electromyography (EMG). EMG electrodes were applied over the test muscles using a standard protocol. A heel-toe strike transducer was affixed to the bottom of the subject's shoe. The subject completed two randomized trials of walking on a treadmill (5.6 kph), jogging on a treadmill (9.3 kph) and drop landing from a 38 cm box. Isometric reference positions (IRPs) were recorded for the TA, PL, and PB. Muscle data were normalized to IRPs and the average processed EMG for the 200 ms prior to heel strike during walking and jogging and prior to toe strike when dropping from the box was used for analysis. A one-way repeated measures MANOVA was used to detect differences in pre-activity of the muscles between the three conditions. Univariate tests were used to determine differences for each muscle and Tukey's was applied post hoc to determine individual effect differences. The MANOVA revealed significant differences among the three conditions (F2.50 = 10.770; P < .0005). Average TA activity was significantly higher during jogging (Tukey's; P < .0005). Significant differences existed between each condition for the TA. Average PL and PB activity was significantly higher when drop landing (Tukey's; P < .0005). There was no significant difference between walking and jogging for the PL and PB. The amount of muscle pre-activity occurring before heel or toe strike provides useful information for the examination of reaction times to unexpected inversion during dynamic activities.     

髋外展肌疲劳对不同性别人群单腿侧跳落地时神经肌肉控制的影响

目的 比较髋外展肌疲劳对不同性别人群单腿侧跳落地期间的姿势稳定性及其神经肌肉控制的影响。方法 比较20名男性和20名女性在髋外展肌疲劳干预前后进行单腿侧跳落地期间的压力中心(center of pressure, COP)、地面反作用力(ground reaction force, GRF)、下肢运动学、关节力矩、肌肉活动度等。结果 疲劳后,男性和女性COP在冠状面的最大位移和平均速度增加,髋关节外展峰值角度和踝关节外翻峰值角度增加,踝关节内翻峰值力矩增加。触地前200 ms,男性股直肌、股二头肌、胫前肌、腓骨长肌的激活小于女性;触地后200 ms,男性股二头肌激活小于女性。结论 髋外展肌疲劳导致冠状面姿势稳定性下降,髋、踝关节冠状面稳定性下降,可能增加关节损伤风险。不同性别人群的姿势调控策略存在差异,提示下肢关节损伤机制的性别差异值得进一步探究。     

Reflex responses in active muscles elicited by stimulation of low-threshold afferents from the human foot.

1. Reflex responses were elicited in muscles that act at the ankle by electrical stimulation of low-threshold afferents from the foot in human subjects who were reclining supine. During steady voluntary contractions, stimulus trains (5 pulses at 300 Hz) were delivered at two intensities to the sural nerve (1.2-4.0 times sensory threshold) or to the posterior tibial nerve (1.1-3.0 times motor threshold for the intrinsic muscles of the foot). Electromyographic (EMG) recordings were made from tibialis anterior (TA), peroneus longus (PL), soleus (SOL), medial gastrocnemius (MG), and lateral gastrocnemius (LG) muscles by the use of intramuscular wire electrodes. 2. As assessed by averages of rectified EMG, stimulation of the sural or posterior tibial nerves at nonpainful levels evoked a complex oscillation with onset latencies as early as 40 ms and lasting up to 200 ms in each muscle. The most common initial responses in TA were a decrease in EMG activity at an onset latency of 54 ms for sural stimuli, and an increase at an onset latency of 49 ms for posterior tibial stimuli. The response of PL to stimulation of the two nerves began with a strong facilitation of 44 ms (sural) and 49 ms (posterior tibial). With SOL, stimulation of both nerves produced early inhibition beginning at 45 and 50 ms, respectively. With both LG and MG, sural stimuli produced an early facilitation at 52-53 ms. However, posterior tibial stimuli produced different initial responses in these two muscles: facilitation in LG at 50 ms and inhibition in MG at 51 ms. 3. Perstimulus time histograms of the discharge of 61 single motor units revealed generally similar reflex responses as in multiunit EMG. However, different reflex components were not equally apparent in the responses of different single motor units: an individual motor unit could respond slightly differently with a change in stimulus intensity or background contraction level. The multiunit EMG record represents a global average that does not necessarily depict the precise pattern of all motor units contributing to the average. 4. When subjects stood erect without support and with eyes closed, reflex patterns were seen only in active muscles, and the patterns were similar to those in the reclining posture. 5. It is concluded that afferents from mechanoreceptors in the sole of the foot have multisynaptic reflex connections with the motoneuron pools innervating the muscles that act at the ankle. When the muscles are active in standing or walking, cutaneous feedback may play a role in modulating motoneuron output and thereby contribute to stabilization of stance and gait.     

Effects of a task failure exercise on the peroneus longus and brevis during perturbed gait

Ankle inversion injuries represent the most common trauma sustained by athletes. Muscle fatigue from activity may contribute to a delay in the response of the ankle proprioceptors and dynamic restraints during unexpected inversion. The purpose of this investigation was to determine changes in peroneal average EMG, peak EMG, and time to peak EMG following a task failure exercise. Thirty-two subjects (age 20+/-1.43 yrs; 21 male, 11 female) with no lower extremity injuries reported for data collection. Data were collected from each subject's dominant leg using surface electromyography (EMG). EMG electrodes were applied over the peroneus longus (PL) and brevis (PB) using a standard protocol Subjects walked at a fixed pace on a 6.1 m runway with one section that could be unexpectedly dropped into 30 degrees of inversion upon foot contact. Trials with perturbed and unperturbed gait were randomized to reduce prediction of the unexpected inversion. Once 3 trials of perturbed gait were recorded, subjects completed an isotonic activity that isolated the peroneals. The task was completed to failure. Immediately following the task failure exercise, subjects walked on the perturbation runway once again until 3 trials of perturbed gait were recorded. Analysis revealed no significant differences with regard to average muscle activity between pre- and post-task failure exercise for the PL (F1,31 = 0.133; p = 0.718) or for the PB (F1,31 = 0.795; p = 0.380). There was also no significant difference in peak muscle activity pre- to post-task failure for the PL (F1,31 = 0.032; p = 0.859) or the PB (F1,31 = 0.156; p = 0.695). Finally, there was no significant difference in time-to-peak muscle activity pre- to post-task failure for the PL (F1,31 = 0.830; p = 0.369) or the PB (F1,31 = 1.037; p = 0.316). We concluded that the task failure exercise did not contribute to changes in peroneal activity during perturbed gait. These results indicate that peroneal fatigue does not play a significant role in the incidence of inversion ankle sprains.     

Inspiratory muscles experience fatigue faster than the calf muscles during treadmill marching

The possibility that respiratory muscles may fatigue during extreme physical activity and thereby become a limiting factor leading to exhaustion is debated in the literature. The aim of this study was to determine whether treadmill marching exercise induces respiratory muscle fatigue, and to compare the extent and rate of respiratory muscle fatigue to those of the calf musculature. To identify muscle fatigue, surface electromyographic (EMG) signals of the inspiratory (sternomastoid, external intercostals), expiratory (rectus abdominis and external oblique) and calf (gastrocnemius lateralis) muscles were measured during a treadmill march of 2 km at a constant velocity of 8 km/h. The extent of fatigue was assessed by determining the increase in root-mean-square (RMS) of EMG over time, and the rate of fatigue was assessed from the slope of the EMG RMS versus time curve. Results indicated that (i) the inspiratory and calf muscles are the ones experiencing the most dominant fatigue during treadmill marching, (ii) the rate of fatigue of each muscle group was monotonic between the initial and terminal phases of exercise, and (iii) the inspiratory muscles fatigue significantly faster than the calf at the terminal phase of exercise, and are likely to fatigue faster during the initial exercise as well. Accordingly, this study supports the hypothesis that fatigue of the inspiratory muscles may be a limiting factor during exercise.     

Neck muscle fatigue affects postural control in man

We hypothesised that, since anomalous neck proprioceptive input can produce perturbing effects on posture, neck muscle fatigue could alter body balance control through a mechanism connected to fatigue-induced afferent inflow. Eighteen normal subjects underwent fatiguing contractions of head extensor muscles. Sway during quiet stance was recorded by a dynamometric platform, both prior to and after fatigue and recovery, with eyes open and eyes closed. After each trial, subjects were asked to rate their postural control. Fatigue was induced by having subjects stand upright and exert a force corresponding to about 35% of maximal voluntary effort against a device exerting a head-flexor torque. The first fatiguing period lasted 5 min (F1). After a 5-min recovery period (R1), a second period of fatiguing contraction (F2) and a second period of recovery (R2) followed. Surface EMG activity from dorsal neck muscles was recorded during the contractions and quiet stance trials. EMG median frequency progressively decreased and EMG amplitude progressively increased during fatiguing contractions, demonstrating that muscle fatigue occurred. After F1, subjects swayed to a larger extent compared with control conditions, recovering after R1. Similar findings were obtained after F2 and after R2. Although such behaviour was detectable under both visual conditions, the effects of fatigue reached significance only without vision. Subjective scores of postural control diminished when sway increased, but diminished more, for equal body sway, after fatigue and recovery. Contractions of the same duration, but not inducing EMG signs of fatigue, had much less influence on body sway or subjective scoring. We argue that neck muscle fatigue affects mechanisms of postural control by producing abnormal sensory input to the CNS and a lasting sense of instability. Vision is able to overcome the disturbing effects connected with neck muscle fatigue.     

Differences in Postural Control During Single-Leg Stance Among Healthy Individuals With Different Foot Types

OBJECTIVE: To identify differences in postural control among healthy individuals with different architectural foot types. DESIGN AND SETTING: We compared postural control during single-leg stance in healthy individuals with cavus, rectus, and planus foot types in our athletic training research laboratory. SUBJECTS: Thirty healthy, young adults (15 men, 15 women; age, 21.9 +/- 2.0 years; mass, 71.6 +/- 16.7 kg; height, 168.4 +/- 13.6 cm) had their feet categorized based on rearfoot and forefoot alignment measures. The right and left feet of a subject could be classified into different categories, and each foot was treated as a subject. There were 19 cavus, 23 rectus, and 18 planus feet. MEASUREMENTS: Subjects performed three 10-second trials of single-leg stance on each leg with eyes open while standing on a force platform. Dependent measures were center-of-pressure (COP) excursion area and velocity. RESULTS: Subjects with cavus feet used significantly larger COP excursion areas than did subjects with rectus feet. However, COP excursion velocities were not significantly different among foot types. CONCLUSIONS: Clinicians and researchers assessing postural control in single-leg stance with measures of COP excursion area must be cognizant of preexisting differences among foot types. If individuals' foot types are not taken into account, the results of clinical and research investigations assessing COP excursion area after injury may be confounded.     

The effects of human ankle muscle vibration on posture and balance during adaptive locomotion

This study investigated the contribution of ankle muscle proprioception to the control of dynamic stability and lower limb kinematics during adaptive locomotion, by using mechanical vibration to alter the muscle spindle output of individuals' stance limbs. It was hypothesised that muscle length information from the ankle of the stance limb provides information describing location as well as acceleration of the centre of mass (COM) with respect to the support foot during the swing phase of locomotion. Our prediction, based on this hypothesis was that ankle muscle vibration would cause changes to the position and acceleration of the COM and/or compensatory postural responses. Vibrators were attached to both the stance limb ankle plantarflexors (at the Achilles tendon) and the opposing dorsiflexor muscle group (over tibialis anterior). Participants were required to walk along a 9-m travel path and step over any obstacles placed in their way. There were three task conditions: (1) an obstacle (15 cm in height) was positioned at the midpoint of the walkway prior to the start of the trial, (2) the same obstacle was triggered to appear unexpectedly one step in front of the participant at the walkway midpoint and (3) the subjects' walking path remained clear. The participants' starting position was manipulated so that the first step over the obstacle (when present) was always performed with their right leg. For each obstacle condition participants experienced the following vibration conditions: no vibration, vibration of the left leg calf muscles or vibration of the anterior compartment muscles of the lower left leg. Vibration began one step before the obstacle at left leg heel contact and continued for 1 s. Vibrating the ankle muscles of the stance limb during the step over an obstacle resulted in significant changes to COM behaviour [measured as displacement, acceleration and position with respect to the centre of pressure (COP)] in both the medial/lateral (M/L) and anterior/posterior planes. There were also significant task-specific changes in stepping behaviour associated with COM control (measured as peak M/L acceleration, M/L foot displacement and COP position under the stance foot during the step over the obstacle). The results provide strong evidence that the primary endings of ankle muscle spindles play a significant role in the control of posture and balance during the swing phase of locomotion by providing information describing the movement of the body's COM with respect to the support foot. Our results also provide supporting evidence for the proposal that there are context-dependent changes in muscle spindle sensitivity during human locomotion.