Relationship between ankle plantar flexor force steadiness and postural stability on stable and unstable platforms

Immediate effects of electronic stimulation to the plantar foot on foot function and postural stability during landing

BACKGROUND

Sports injuries often occur during landing, necessitating postural stability for injury prevention. Electrical stimulation of the plantar foot induces activities of the intrinsic foot muscles and improves somatosensory and postural stability during landing. However, this effect remains unclear. Therefore, the aim of this study was to investigate the immediate effects of electrical stimulation on the activities of the intrinsic foot muscles, plantar somatosensory system, and postural stability during landing.

METHODS

Twenty-two college athletes were divided into an electrical stimulation group and a control group. Electrical stimulation was applied to the plantar foot using a commercial device. The toe function and plantar tactile sensations were evaluated. The activities of the intrinsic muscles and the parameters of the ground reaction force were measured and calculated. We compared pre- and postintervention outcomes.

RESULTS

In the electrical stimulation group, two subjects showed improvement in toe function, and plantar tactile sensation improved significantly postintervention. The control group exhibited no significant change in plantar tactile sensation. A significant interaction was observed in anteroposterior postural stability during landing, notably improving in the electrical stimulation group.

CONCLUSIONS

Electrical stimulation of the plantar foot immediately improved toe function, plantar tactile sensation, and postural stability during landing. Such stimulation would be beneficial in preventing sports injuries.

Influence of complexity of the task and the level of physical activity in the association between the muscular function and posture of older adults

INTRODUCTION

Postural control involves the coordination of muscle synergies of lower limb for the performance of postural strategies. However, the relationship between muscle function and postural control of older adults with different levels of physical activity has not yet been fully elucidated. The aim of this study was to analyze the influence of muscle function of lower limb on postural control in tasks with disturbance in the visual and somatosensory systems of older adults with different levels of physical activity.

METHODS

61 older adults participated in this study. The assessment of postural control was performed using posturography in tasks with visual and somatosensory systems disturbance. To assess peak torque and mean power of the lower limb, an isokinetic dynamometer was used. A regression test was performed to investigate the association between muscle function and posture. The regression model was adjusted for the level of physical activity.

RESULTS

The complexity of the task and level of physical activity influenced the association between muscle function and postural control. The conditions that involved simultaneous disturbance in the visual and somatosensory systems were the ones that showed the greatest combination of associations, regardless of the stance type.

CONCLUSION

The ability to produce ankle and hip muscle power seems to be crucial for the postural control of the older adults in tasks that involve reducing the base of support associated with disturbances in the visual and somatosensory system. Physical activity programs for older adults should include strength and power training for the ankle, knee, and hip muscles, with emphasis on hip abductors.

A NEW POSTURAL MOTOR RESPONSE TO SPINAL CORD STIMULATION: POST-STIMULATION REBOUND EXTENSION

Spinal cord stimulation (SCS) has emerged as a therapeutic tool for improving motor function following spinal cord injury. While many studies focus on restoring locomotion, little attention is paid to enabling standing which is a prerequisite of walking. In this study, we fully characterize a new type of response to SCS, a long extension activated post-stimulation (LEAP). LEAP is primarily directed to ankle extensors and hence has great clinical potential to assist postural movements. To characterize this new response, we used the decerebrate cat model to avoid the suppressive effects of anesthesia, and combined EMG and force measurement in the hindlimb with intracellular recordings in the lumbar spinal cord. Stimulation was delivered as five-second trains via bipolar electrodes placed on the cord surface, and multiple combinations of stimulation locations (L4 to S2), amplitudes (50-600 uA), and frequencies (10-40 Hz) were tested. While the optimum stimulation location and frequency differed slightly among animals, the stimulation amplitude was key for controlling LEAP duration and amplitude. To study the mechanism of LEAP, we performed in vivo intracellular recordings of motoneurons. In 70% of motoneurons, LEAP increased at hyperpolarized membrane potentials indicating a synaptic origin. Furthermore, spinal interneurons exhibited changes in firing during LEAP, confirming the circuit origin of this behavior. Finally, to identify the type of afferents involved in generating LEAP, we used shorter stimulation pulses (more selective for proprioceptive afferents), as well as peripheral stimulation of the sural nerve (cutaneous afferents). The data indicates that LEAP primarily relies on proprioceptive afferents and has major differences from pain or withdrawal reflexes mediated by cutaneous afferents. Our study has thus identified and characterized a novel postural motor response to SCS which has the potential to expand the applications of SCS for patients with motor disorders.

Association between physical fitness tests and neuromuscular properties

PURPOSE

While various fitness tests have been developed to assess physical performances, it is unclear how these tests are affected by differences, such as, in morphological and neural factors. This study was aimed to investigate associations between individual differences in physical fitness tests and neuromuscular properties.

METHODS

One hundred and thirty-three young adults participated in various general physical fitness tests and neuromuscular measurements. The appendicular skeletal muscle mass (ASM) was estimated by bioelectrical impedance analysis. Echo intensity (EI) was evaluated from the vastus lateralis. During submaximal knee extension force, high-density surface electromyography of the vastus lateralis was recorded and individual motor unit firings were detected. Y-intercept (i-MU) and slope (s-MU) from the regression line between the recruitment threshold and motor unit firing rate were calculated.

RESULTS

Stepwise multiple regression analyses revealed that knee extension strength could be explained (adjusted R2 = 0.712) by ASM (β = 0.723), i-MU (0.317), EI (- 0.177), and s-MU (0.210). Five-sec stepping could be explained by ASM (adjusted R2 = 0.212). Grip strength, side-stepping, and standing broad jump could be explained by ASM and echo intensity (adjusted R2 = 0.686, 0.354, and 0.627, respectively). Squat jump could be explained by EI (adjusted R2 = 0.640). Counter-movement jump could be explained by EI and s-MU (adjusted R2 = 0.631). On the other hand, i-MU and s-MU could be explained by five-sec stepping and counter-movement jump, respectively, but the coefficients of determination were low (adjusted R2 = 0.100 and 0.045).

CONCLUSION

Generally developed physical fitness tests were mainly explained by morphological factors, but were weakly affected by neural factors involved in performance.

Concentrating to avoid falling: interaction between peripheral sensory and central attentional demands during a postural stability limit task in sedentary seniors

Evidence suggests falls and postural instabilities among seniors are attributed to a decline in both the processing of afferent signals (e.g., proprioceptive, vestibular) and attentional resources. We investigated the interaction between the non-visual and attentional demands of postural control in sedentary seniors. Old and young adults performed a postural stability limit task involving a maximal voluntary leaning movement with and without vision as well as a cognitive-attentional subtraction task. These tasks were performed alone (single-task) or simultaneously (dual-task) to vary the sensory-attentional demands. The functional limits of stability were quantified as the maximum center of pressure excursion during voluntary leaning. Seniors showed significantly smaller limits of postural stability compared to young adults in all sensory-attentional conditions. However, surprisingly, both groups of subjects reduced their stability limits by a similar amount when vision was removed. Furthermore, they similarly decreased their anterior-posterior stability limits when concurrently performing the postural and the cognitive-attentional tasks with vision. The overall average cognitive performance of young adults was higher than seniors and was only slightly affected during dual-tasking. In contrast, older adults markedly degraded their cognitive performance from the single- to the dual-task situations, especially when vision was unavailable. Thus, their dual-task costs were higher than those of young adults and increased in the eyes-closed condition, when postural control relied more heavily on non-visual sensory signals. Our findings provide the first evidence that as posture approaches its stability limits, sedentary seniors allot increasingly large cognitive attentional resources to process critical sensory inputs.

Motor Unit Firing Properties During Force Control Task and Associations With Neurological Tests in Children

The present study aimed to clarify the development of motor unit (MU) firing properties and the association between those neural properties and force steadiness (FS)/neurological tests in 6- to 12-year-old children. Fifty-eight school-aged children performed maximal voluntary knee extension contraction, a submaximal FS test at 10% of maximal voluntary knee extension contraction, knee extension reaction time to light stimulus test, and single-leg standing test, and data from 38 children who passed the criteria were subject to analysis. During the FS test, high-density surface electromyography was recorded from the vastus lateralis muscle to identify individual MU firing activity. FS was improved with an increase in age (r = -.540, P < .001). The MU firing rate (MUFR) was significantly decreased with an increase in age (r = -.343, P = .035). MUFR variability was not associated with age. Although there was no significant correlation between FS and MUFR, FS was significantly correlated with MUFR variability even after adjustment for the effect of age (r = .551, P = .002). Neither the reaction time nor the single-leg standing test was correlated with any MU firing properties. These findings suggest that MUFR variability makes an important contribution to precise force control in children but does not naturally develop with age.

Knee extensor force control as a predictor of dynamic balance in healthy adults

BACKGROUND

Previous research has demonstrated that force control in various muscles of the lower limb (measured according to the magnitude of force fluctuations) explains significant variance in static balance. Given the dynamic nature of many functional activities and sports, assessment of balance and its determinants under dynamic conditions is of importance.

RESEARCH QUESTION

Does muscle force control explain significant variance in dynamic balance, as measured using the Y balance test (YBT)?

METHODS

YBT performance and knee extensor muscle force control were measured in 28 healthy participants. The YBT involved stance on the right leg and attempting maximal reach with the left leg in the anterior, posteromedial, and posterolateral directions. Force control was assessed during isometric knee extension contractions of the right leg at 10%, 20% and 40% maximal voluntary contraction (MVC) and was quantified according to the magnitude (using the coefficient of variation [CV]), and the temporal structure (using sample entropy, SampEn; and detrended fluctuation analysis α), of force fluctuations.

RESULTS

Significant correlations were observed for YBT anterior reach and muscle force CV (r = -0.44, P = 0.02) and SampEn (r = 0.47, P = 0.012) during contractions at 40% MVC. A subsequent regression model demonstrated that muscle force CV and SampEn at 40% MVC significantly explained 54% of variance in YBT anterior reach. Significant correlations were also observed for YBT posteromedial reach and MVC (r = 0.39, P = 0.043) and muscle force CV during contractions at 40% MVC (r = -0.51, P = 0.006). The regression model demonstrated that MVC and muscle force CV at 40% MVC significantly explained 53.9% of variance in YBT posteromedial reach.

SIGNIFICANCE

These results are the first to indicate that a moderate amount of variance in dynamic balance can be explained by measures of isometric force control.

Age and sex differences in force steadiness and intermuscular coherence of lower leg muscles during isometric plantar flexion

Age- and sex-related alterations in the control of multiple muscles during contractions are not well understood. The purpose of the present study was to examine the age and sex differences in force steadiness and intermuscular coherence (IMC), and thereby to clarify the functional role of IMC during plantar flexion. Twenty-six young (YNG, 23-34 years), thirty middle-aged (MID, 35-64 years) and twenty-four older adults (OLD, 65-82 years) performed submaximal isometric contractions of plantar flexion, while electromyography was recorded from the soleus (SOL), gastrocnemius lateralis/medialis (GL/GM) and tibialis anterior (TA) muscles. Coefficient of variation (CV) of torque and IMC in the alpha, beta and gamma bands was calculated. We found that OLD demonstrated significantly higher torque CV than YNG and MID, and males demonstrated significantly higher torque CV than females (both p < 0.05). The IMC in the gamma band (five out of the six pairs) was significantly higher in YNG than MID and/or OLD (p < 0.05), while the gamma band IMC between GL and SOL was significantly higher in females. However, age or sex differences were not detected in the alpha or beta band. Moreover, the gamma band IMC between SOL and TA had a weak (r =  - 0.229) but significant (p < 0.05) negative correlation with torque CV. These results suggest that force steadiness differs with age and sex, and that the higher gamma band IMC may contribute to more stable force control during plantar flexion.

The Time-Course Changes in Knee Flexion Range of Motion, Muscle Strength, and Rate of Force Development After Static Stretching

Previous studies have shown that longer-duration static stretching (SS) interventions can cause a decrease in muscle strength, especially explosive muscle strength. Furthermore, force steadiness is an important aspect of muscle force control, which should also be considered. However, the time course of the changes in these variables after an SS intervention remains unclear. Nevertheless, this information is essential for athletes and coaches to establish optimal warm-up routines. The aim of this study was to investigate the time course of changes in knee flexion range of motion (ROM), maximal voluntary isometric contraction (MVIC), rate of force development (RFD), and force steadiness (at 5 and 20% of MVIC) after three 60-s SS interventions. Study participants were sedentary healthy adult volunteers (n = 20) who performed three 60-s SS interventions of the knee extensors, where these variables were measured before and after SS intervention at three different periods, i.e., immediately after, 10 min, and 20 min the SS intervention (crossover design). The results showed an increase in ROM at all time points (d = 0.86-1.01). MVIC was decreased immediately after the SS intervention (d = -0.30), but MVIC showed a recovery trend for both 10 min (d = -0.17) and 20 min (d = -0.20) after the SS intervention. However, there were significant impairments in RFD at 100 m (p = 0.014, F = 6.37, η_p ² = 0.101) and 200 m (p < 0.01, F = 28.0, η_p ² = 0.33) up to 20 min after the SS intervention. Similarly, there were significant impairments in force steadiness of 5% (p < 0.01, F = 16.2, η_p ² = 0.221) and 20% MVIC (p < 0.01, F = 16.0, η_p ² = 0.219) at 20 min after the SS intervention. Therefore, it is concluded that three 60-s SS interventions could increase knee flexion ROM but impair explosive muscle strength and muscle control function until 20 min after the SS intervention.

Sensor number in simplified insole layouts and the validity of ground reaction forces during locomotion

Research attempted to validate simplified insoles with a reduced number of sensors to facilitate clinical application. However, the ideal sensor number is yet to be determined. The purpose was to investigate the validity of vertical ground reaction forces in various simplified pressure sensor insoles and to identify an optimal compromise between sensor number and measurement performance. A Kistler force plate (1000 Hz) and 99-sensor Pedar-X insole (100 Hz) obtained force data of 15 participants during walking and jogging. Eight simplified insole layouts (3-17 sensors) were simulated. Layout performances were expressed as Pearson's correlation coefficients (r) with force plate as reference and coefficient of variation. Differences were assessed via repeated-measures ANOVA as partial eta square (η p 2 ) at p < .05. All layouts correlated with the force plate (r = .70-.99, p < .01). All layout performances were higher in jogging than in walking by r = +.07 ± .04 (η p 2 =.28-.66, p < .05). The three- and five-sensor layouts yielded the lowest correlation (r = .70-.88) and the highest coefficient of variation (11-22%). Layout performances improved constantly from 7 to 11 sensors. The optimal compromise between simplification and measurement performance, quantified via change in correlation per sensor number, was found in the 11-sensor layout, recommendable for practical settings to improve monitoring and adjusting protocols.

Acute effects of ankle plantar flexor force-matching exercises on postural strategy during single leg standing in healthy adults

BACKGROUND

Ankle plantar flexor force steadiness, assessed by measuring the fluctuation of the force around the submaximal target torque, has been associated with postural stability.

RESEARCH QUESTION

To investigate whether a force-matching exercise, where submaximal steady torque is maintained at the target torque, can modulate postural strategy immediately.

METHODS

Twenty-eight healthy young adults performed ankle plantar flexor force-matching exercises at target torques of 5%, 20%, and 50% of maximum voluntary contraction (MVC), in a randomized crossover trial. Participants with their ankle in a neutral position were instructed to maintain isometric contraction at each target torque, as measured by a dynamometer, for 20 s with 3 sets of 5 contractions. Before and after the force-matching exercises, the anterior-posterior velocities and standard deviation of the center of pressure (COP) on the stable platform and the tilt angle of the unstable platform during 20-seconds single-leg standing were measured. The velocities and standard deviations of the COP and tilt angle before and after the exercises were compared using paired t-tests.

RESULTS

The tilt angle velocity of an unstable platform significantly decreased after the force-matching exercise at a target torque of 5% MVC (p = 0.029), whereas it was unchanged after the exercises at target torques of 20% and 50% MVC. The standard deviations of the tilt angle of unstable platform test did not change significantly after any exercise. Furthermore, no significant differences were observed in the COP velocities or standard deviations on the stable platform test after any exercise.

SIGNIFICANCE

Our findings suggest that repeated exertion training at low-intensity contractions can affect postural stability in an unstable condition. Particularly, force-matching exercise at very low-intensity torque, such as 5% of MVC, may be an effective method to improve postural control in the unstable condition, but not in a stable condition.

Local exercise based on voluntary contractions produces greater warm-up effects on balance control than electro-induced contractions

On the one hand, while general (involving the whole body) warm-up exercises have been extensively used to optimize motor and postural performance, the effect of a local (involving a particular muscular group) warm-up has not yet been addressed. On the other hand, voluntary (VOL) and electro-induced (EI) contractions produce different physiological effects likely to differently affect motor and postural performance. The aim was to analyze and compare the effects of two local warm-up modalities, voluntary and electro-induced (VOL or EI warm-up) on balance control. Balance control was evaluated with a force platform (recording the displacement of the centre of foot pressure - COP) in 27 healthy young subjects before (PRE), immediately after (POST), 5 minutes after (POST5), and 10 minutes after (POST10) either warm-up. Each warm-up included 6 sets of 5 contractions at 10% of maximal voluntary contraction of the quadriceps femoris. The results showed that the VOL warm-up improved balance control at POST, POST5 and POST10 while the EI warm-up improved it only at POST10. In addition, balance control was significantly better after the VOL warm-up than after the EI warm-up at POST5 and POST10. A short and local VOL warm-up improved balance control immediately after its completion and for, at least, ten minutes, while the EI warm-up required some minutes of recovery before producing its improving effects.

Effect of Integrated Training on Balance and Ankle Reposition Sense in Ballet Dancers

OBJECTIVE

To investigate the effects of a 6-week integrated training program on the ankle joint reposition sense and postural stability in ballet dancers.

METHODS

Sixteen female ballet dancers participated in the study and underwent a 6-week integrated training program consisting of plyometric, proprioception and core stability exercises along with a home program involving additional ankle muscle strengthening and stretching. The ankle joint reposition tests and the parameters of the center of pressure (COP) while performing grand-plie (deep squatting) and releve en demi-pointe (standing on balls of foot) movements were measured before and after training.

RESULTS

After 6 weeks, participants showed significantly smaller absolute ankle joint reposition errors in dorsiflexion (p = 0.031), plantarflexion (p = 0.003) and eversion (p = 0.019) compared to the pre-training measurement. Furthermore, after training, a significantly slower average COP speed at pre-equilibrium during grand-plie movement (p = 0.003) and pre-equilibrium phase of releve en demi-pointe (p = 0.023) were observed. In addition, the maximum COP displacement in the medial-lateral direction was significantly smaller at pre-equilibrium phase during grand-plie (p = 0.044) and releve en demi-pointe movements (p = 0.004) after training.

CONCLUSIONS

The 6-week integrated training program improved the ankle joint reposition sense and postural control in the medial-lateral direction during grand-plie and releve en demi-pointe movements.

Contribution of Achilles tendon mechanical properties to torque steadiness in persons with transfemoral amputation

BACKGROUND

How Achilles tendon mechanics and plantar flexion strength and torque steadiness are altered in the intact leg of persons with trauma-related amputation is unknown. Understanding Achilles tendon mechanics following amputation will further inform rehabilitation approaches to enhance posture, balance, and force control.

OBJECTIVE

Conduct a pilot study to quantify plantar flexion maximal voluntary contraction torque, torque steadiness, and Achilles tendon mechanics in persons with unilateral trauma-related transfemoral amputation and controls without amputation.

STUDY DESIGN

Cross-sectional study.

METHODS

Isometric plantar flexion maximal voluntary contractions were performed with the intact leg of ten males with transfemoral amputation (48 ± 14 years) and the dominant leg of age-matched male controls without amputation. Torque steadiness was calculated as the coefficient of variation in torque over 6 s during submaximal tracking tasks (5%, 10%, 25%, 50%, and 75% maximal voluntary contraction). Achilles tendon elongation and cross-sectional area were recorded with ultrasound to calculate strain, stress, and stiffness.

RESULTS

Maximal voluntary contraction and torque steadiness did not differ between persons with amputation (90.6 ± 31.6 N m, 3.7 ± 2.0%) and controls (95.8 ± 26.8 N m, 2.9 ± 1.2%; p > 0.05). Tendon stiffness (21.1 ± 18.2 N/mm) and strain (5.2 ± 1.3%) did not differ between groups (p > 0.05). Tendon cross-sectional area was 10% greater in persons with amputation leading to 29% lower stress (p = 0.021). Maximal voluntary contraction was a predictor of a lower coefficient of variation in torque (R2 = 0.11, p < 0.05).

CONCLUSION

Persons with trauma-related transfemoral amputation do not differ in plantar flexion maximal voluntary contraction and torque steadiness of the intact leg compared with controls without amputation. Larger tendon cross-sectional area reduces stress and enables distribution of force across a greater area.

Relationship between postural sway on an unstable platform and ankle plantar flexor force steadiness in community-dwelling older women

BACKGROUND

Force steadiness is evaluated as force variability during constant force exertion around a target level. Ankle plantar flexor force steadiness is reported to be related to postural sway on an unstable platform in healthy young adults; however, this relationship in older adults is unclear.

RESEARCH QUESTION

This study aimed to investigate whether ankle plantar flexor force steadiness was related to postural sway on stable and unstable platforms in older adults.

METHODS

Twenty-six community-dwelling older women participated in this study (72 ± 6 years). Maximal isometric strength and force steadiness at 5%, 20 %, and 50 % of the maximal strength of ankle plantar flexion were assessed. Postural sway in the anteroposterior direction during bipedal standing was measured on stable and unstable platforms.

RESULTS

The results showed that force steadiness at any intensity level and maximal isometric strength were not related to postural sway on the stable platform. Force steadiness at 20 % of maximal strength alone was significantly correlated with postural sway on the unstable platform (ρ = 0.441, p < 0.05).

SIGNIFICANCE

These results indicate that the ability to control muscle force could be important for postural stability on an unstable platform in older adults.