Age Effects on Multi-muscle Modes during Voluntary Body Sway

Postural control in preparation to a step during support surface perturbation

We investigated changes in the anticipatory activity of the leg and trunk muscles in preparation for a step during support surface perturbation. Eight healthy subjects performed stepping tasks under three conditions: normal, forward perturbation, and backward perturbation. R and C indices were calculated for the reciprocal and co-activation patterns of muscle pairs within the time intervals typical of anticipatory postural adjustments. When the support surface perturbation occurred, anticipatory muscle activations were predominantly in the C indices in the leg muscles. Significant differences in the maximum displacement of the centre of pressure were seen between conditions (FS vs NS; BS vs FS). The results suggest that activation of the leg muscles rather than the trunk muscles was modified to ensure equilibrium for taking a step in response to support surface perturbation.

Aging effect on muscle synergies in stepping forth during a forward perturbation

PURPOSE

We explored changes in muscle interactions during healthy aging as a window into neural control strategies of postural preparation to action/perturbation. In particular, we quantified the strength of multi-muscle synergies stabilizing the center of pressure (COP) displacement during the preparation for making a step associated with support surface translations.

METHODS

Young and elderly subjects were required to make a step in response to support surface perturbations. Surface muscle activity of 11 leg and trunk muscles was analyzed to identify sets of 4 muscle modes (M-modes). Linear combinations of M-modes and their relationship to changes in the COP displacement in the anterior-posterior direction were then determined. Uncontrolled manifold analysis was performed to determine variance components in the M-mode space and indices of M-mode synergy stabilizing COP displacement.

RESULTS

Prior to the step initiation, the older subjects showed strong synergies that stabilized COP displacement to forward perturbation of the support surface. However, the synergy indices were significantly lower than those of the young subjects during preparation for making a step. The timings of early postural adjustment (EPA) and anticipatory postural adjustment (APA) were consistently earlier in the young subjects as compared to the older subjects. For both groups, the timing of EPA did not change across tasks, while APA showed delayed timing in response to the support surface translations.

CONCLUSIONS

We infer that changes in the indices of synergies with age may present challenges for the control of postural preparation to external perturbation in older adults. They may lead to excessive muscle co-contractions and low stability of COP displacement. The results reported here could have clinical relevance when identifying the risk of making a step, which has been linked to an increased risk of falls among the elderly.

Electromyographic normalization of vastus lateralis and biceps femoris co-contraction during gait of elderly females

Abstract Introduction: Analyze muscle co-contraction using electromyographic signals, which are normalized to compare individuals, muscles and studies. Maximum voluntary isometric contraction (MVIC) and peak electrical activity (PEA) during movement are the most widely used forms of normalization. Objective: Compare inter-subject variability and investigate the association between the co-contraction indices of the vastus lateralis and biceps femoris during gait, normalized by MVIC and PEA. Methods: Thirty elderly women, aged 70.33 ± 3.69 years took part. Electrical muscle activity during MVIC and gait was recorded using a Biopac MP100 electromyograph. MVIC was performed in a Biodex isokinetic dynamometer. For normalization, the signals were divided by the Root Mean Square values of MVIC and PEA of gait. Results: The coefficient of variation of non-normalized data was 69.3%, and those normalized by PEA and MVIC were 30.4% and 48.9% respectively. Linear regression analysis resulted in a prediction model: PEA = 0.04 + 0.16 x MVIC. The goodness of fit of the regression model was statistically significant (p=0.02). The confidence interval (95% CI) for the intercept was between 0.02 and 0.29 and for MVIC between 0.03 and 0.06. Conclusions: The data normalized by PEA showed less variation than those normalized by MVIC. A 100% variation in data normalized by MVIC resulted in a 16% variation in data normalized by PEA, while variation in normalization by MVIC accounts for 17% of the variation in normalization by PEA and vice versa.

Muscle synergies underlying control of taking a step during support surface translation

PURPOSE

We investigated the muscle activation patterns and the center of pressure (COP) displacement in stepping behavior to determine the relations between anticipatory synergy adjustments (ASAs) and anticipatory postural adjustments (APAs) during support surface translation.

METHODS

Surface muscle activity of eleven leg and trunk muscles was analyzed to identify sets of four muscle modes (M-modes). Linear combination of M-modes and their relationship to changes in the COP shift in the anterior–posterior (AP) direction were then determined. Uncontrolled manifold (UCM) analysis was performed to determine variance components in the M-mode space and indices of M-mode synergy stabilizing the COP shift.

RESULTS

Prior to the step initiation, synergies stabilizing COP were seen in both conditions. The synergy index started to drop before a change in the averaged activation levels across trials in postural muscles. The magnitude of synergy index was significantly larger under the perturbation condition.

CONCLUSIONS

Results of the study have revealed that the central nervous system is able to prepare multi-muscle synergies when a step is performed during support surface translation. Prior to APAs, ASAs reduce stability of COPAP coordinate that is to be adjusted during the APAs. These findings may help get closer to understanding of physiological mechanism of postural preparation to external perturbation.

Aging and the complexity of center of pressure in static and dynamic postural tasks

The experiment was set-up to investigate the hypothesis that there is an age-related bi-directional change in complexity of the center of pressure (COP) depending on postural task demands. Healthy young (19-28 year) and old (65-74 year) adults were instructed to match, in two 25s trials for each condition, the COP with constant and sine-wave targets at 2 levels (5 and 40%) of the maximal COP distance (MCD). The root mean square error of COP increased with aging in both MCD levels and tasks. Multi-scale entropy (MSE) and detrended fluctuation analysis showed that the complexity of COP in the old adults was lower compared to the young in the constant target, whereas it was higher in the sine-wave target. The task dependent age-related bi-directional change of COP complexity is counter to the hypothesis of a universal loss of complexity with aging but shows that there is loss of adaptive change in complexity driven by the COP dynamics.