Muscle synergy differences between voluntary and reactive backward stepping

Reactive stepping responses are essential to prevent falls after a loss of balance. It has previously been well described that both voluntary and reactive step training could improve the efficacy of reactive stepping in different populations. However, the effect of aging on neuromuscular control during voluntary and reactive stepping remains unclear. Electromyography (EMG) signals during both backward voluntary stepping in response to an auditory cue and backward reactive stepping elicited by a forward slip-like treadmill perturbation during stance were recorded in ten healthy young adults and ten healthy older adults. Using muscle synergy analysis, we extracted the muscle synergies for both voluntary and reactive stepping. Our results showed that fewer muscle synergies were used during reactive stepping than during voluntary stepping in both young and older adults. Minor differences in the synergy structure were observed for both voluntary and reactive stepping between age groups. Our results indicate that there is a low similarity of muscle synergies between voluntary stepping and reactive stepping and that aging had a limited effect on the structure of muscle synergies. This study enhances our understanding of the neuromuscular basis of both voluntary and reactive stepping as well as the potential effect of aging on neuromuscular control during balance tasks.

The Effect of Perturbation-Based Balance Training and Conventional Intensive Balance Training on Reactive Stepping Ability in Individuals With Incomplete Spinal Cord Injury or Disease: A Randomized Clinical Trial

Introduction: Impaired balance leads to falls in individuals with motor incomplete spinal cord injury or disease (iSCI/D). Reactive stepping is a strategy used to prevent falls and Perturbation-based Balance Training (PBT) can improve this ability.Objective: The objective of this study was to determine if PBT results in greater improvements in reactive stepping ability than frequency-matched Conventional Intensive Balance Training (CIBT) in adults with iSCI/D.Design: Randomized clinical trial.Setting: Tertiary SCI/D rehabilitation center.Participants: Twenty-one adults with chronic (>1 year) iSCI/D were randomized. Due to one drop out 20 participants completed the study.Methods: Participants were randomly allocated to complete either PBT or CIBT three times per week for 8 weeks. Both programs included challenging static and dynamic balance tasks, but the PBT group also experienced manual external balance perturbations.Main Outcome Measures: Assessments of reactive stepping ability using the Lean-and-Release test were completed at baseline, and after 4 and 8 weeks of training, and 3 and 6 months after training completion. A blinded assessor evaluated secondary outcomes.Results: Twenty-five participants were screened and 21 consented; one withdrew. Ten PBT and 10 CIBT participants were included in analyses. Across all participants there were improvements in reactive stepping ability (p = 0.049), with retention of improvements at follow up assessments. There were no differences in reactive stepping ability between groups [median (interquartile range): PBT 0.08 (0.68); CIBT 0.00 (0.22)]. One participant in the PBT group experienced a non-injurious fall during training.Conclusions: Balance training is beneficial for individuals with iSCI/D, but the addition of manual perturbations (i.e., PBT) did not prove advantageous for performance on a measure of reactive stepping ability.Clinical Trial Registration:www.ClinicalTrials.gov, identifier: NCT02960178.

Associations between lower limb isometric torque, isokinetic torque, and explosive force with phases of reactive stepping in young, healthy adults

Reactive stepping is one of the only strategies that can lead to successful stabilization following a large challenge to balance. Improving function of specific muscles associated with reactive stepping may improve features of reactive balance control. Accordingly, this study aimed to determine the relationship between lower limb muscle strength and explosive force with force plate-derived timing measures of reactive stepping. Nineteen young, healthy adults (27.6 ± 3.0 years of age; 10 women: 9 men) responded to 6 perturbations (~13-15% of body weight) using an anterior lean-and-release system (causing a forward fall), where they were instructed to recover balance in as few steps as possible. Foot-off, swing, and restabilization times were estimated from force plates. Peak isokinetic torque, isometric torque, and explosive force of the knee extensors/flexors and plantar/dorsiflexors were measured using isokinetic dynamometry. Correlations were run based on a priori hypotheses and corrected for the number of comparisons (Bonferroni) for each variable. Knee extensor explosive force was negatively correlated with swing time (r = −0.582, p = 0.009). Knee flexor peak isometric torque also showed a negative association with restabilization time (r = −0.459, p = 0.048), however this was not statistically significant after correcting for multiple comparisons. There was no significant relationship between foot-off time and knee or plantar flexor explosive force (p > 0.025). These findings suggest that there may be utility to identifying specific aspects of reactive step timing when studying the relationship between muscle strength and reactive balance control. Exercise training aimed at improving falls risk should consider targeting specific aspects of muscle strength depending on specific deficits in reactive stepping.

The effect of perturbation magnitude on lower limb muscle activity during reactive stepping using functional data analysis

This study aimed to determine the effect of perturbation magnitude on stance and stepping limb muscle activation during reactive stepping using functional data analysis. Nineteen healthy, young adults responded to 6 small and 6 large perturbations using an anterior lean-and-release system, evoking a single reactive step. Muscle activity from surface electromyography was compared between the two conditions for medial gastrocnemius, biceps femoris, tibialis anterior, and vastus lateralis of the stance and stepping limb using functional data analysis. Stance limb medial gastrocnemius and biceps femoris activation increased in the large compared to small perturbation condition immediately prior to foot-off and at foot contact. In the stepping limb, significant increases in medial gastrocnemius, biceps femoris, and tibialis anterior activity occurred immediately prior to foot-off during the large perturbations. Similar to the stance limb, medial gastrocnemius and biceps femoris activity significantly increased during and following foot contact in the large, compared to small, perturbation condition. Lastly, vastus lateralis activity significantly increased for large, compared to small, perturbations during foot-off and immediately following foot contact. These findings highlight lower limb muscle activity modulation associated with perturbation magnitude throughout reactive stepping and the additional benefit of implementing functional data analysis to study reactive balance control.

Balance and Reactive Steps in Older Adults With and Without Self-Reported Musculoskeletal Conditions

Older adults with musculoskeletal conditions (MSC), including arthritis and osteoporosis, may have a higher risk of falls and falls-related injuries. Differences in balance between individuals with and without self-reported MSC are not well understood. Therefore, this study compared measures of balance (static and dynamic) and reactive stepping between older adults (N=99) with (75.79±5.38 years, n=38, 82% female) and without (75.93±6.36 years, n=61, 67% female) MSC. A cross-sectional design was used. Static balance was assessed via postural sway area (PSA) and PSA root mean square (PSARMS) during quiet stance. Dynamic balance was assessed with the Timed Up & Go (TUG), a dual-task cognitive TUG (TUG-COG), and the Four Square Step Test (FSST). Reactive stepping was measured as the first step latency, length, width, time, total number of recovery steps, and time until balance recovery after a backward lean and release. Linear regression was used to assess group differences. After adjusting for age, sex, body mass index (BMI), and grip strength, there were no significant differences between groups in static balance (PSA (p=0.884); PSARMS (p=0.246)) and reactive stepping outcomes (first step latency (p=0.184); total number of steps (p=0.423); step width (p=0.964)). The other reactive step outcomes are not reported since explained variance was not statistically significant (p>0.05). With dynamic balance, significant group differences showed individuals with MSC took more time to complete TUG (p=0.011) and TUG-COG (p=0.005), but not the FSST (p=0.493). Our findings suggest improving dynamic balance, especially with a walking component, in older adults with self-reported MSC is needed.