Effects of muscle fatigue on multi-muscle synergies

Abstract

We studied the effects of fatigue of ankle dorsiflexors on multi-muscle synergies defined as co-varied adjustments of elemental variables (M-modes) that stabilize a task related performance variable (trajectory of the center of pressure, COP). M-modes were defined as muscle groups with parallel changes in activation levels. Healthy participants performed voluntary body sway in the anterior-posterior direction while trying to minimize sway in the medio-lateral direction at 0.25, 0.5 and 0.75 Hz. The trials were repeated before and during fatigue induced with a timed voluntary contraction against a constant load. Factor extraction using the principal component method was used to identify four M-modes within the space of integrated indices of muscle activity. Variance in the M-mode space at different phases across sway cycles was partitioned into two components, one that did not affect the average value of COP shift and the other that did. There were no significant effects of fatigue on variability of performance of the explicit task and on the amplitude of the COP shift. Variance of muscle activation indices and M-mode magnitudes increased during fatigue for muscles (and M-modes) both involved and not involved in the fatiguing exercise. Most of the M-mode variance increase was within the sub-space compatible with the unchanged COP trajectory resulting in an increase of the index of the multi-M-mode synergy. We conclude that one of the adaptive mechanisms to fatigue within a redundant multi-muscle system involves an increase in the variance of activation of non-fatigued muscles with a simultaneous increase in co-variation among muscle activations. The findings can be interpreted within the referent configuration hypothesis on the control of whole-body actions.

Effects of fatigue on synergies in a hierarchical system

“…As in earlier studies (Singh et al, 2010a; Singh & Latash, 2011), we observed disproportionate changes in the two variance components, “good “ (V UCM ) and “bad” (V ORT ) with a larger increase in V UCM . Hence, the adaptive increase in the synergy index to fatigue has been confirmed.…”

“…We suggest that this result was a consequence of selecting frequency of force production close to preferred frequency by the subjects. Preferred frequency has been shown to be close to the natural frequency in several studies of movements of body segments (Latash, 1992; Singh & Latash, 2011; Treffner & Turvey, 1993). In isometric force production tasks, the notion of natural frequency is poorly defined.…”

“…Overall, fatigue led to a predictable increase in the force variance indices (Singh et al, 2010a; Singh & Latash, 2011). As in earlier studies (Singh et al, 2010a; Singh & Latash, 2011), we observed disproportionate changes in the two variance components, “good “ (V UCM ) and “bad” (V ORT ) with a larger increase in V UCM .…”

“…The relative difference (ΔV) between V UCM and V ORT has been used as a quantitative index of synergy. Our previous studies of exercise-induced fatigue have shown an increase in the index of synergy for accurate multi-digit force production tasks as well as in multi-muscle postural tasks (Park, Singh, Zatsiorsky, & Latash, 2012; Singh & Latash, 2011; Singh, SKM, Zatsiorsky, & Latash, 2010a, 2010b). …”

Prehension Synergies During Fatigue of a Single Digit: Adaptations in Control With Referent Configurations

“…As a result, a large increase in ΔV at the upper level of the hierarchy could have a strong negative effect on the synergy stabilizing the VF force at the lower level of the hierarchy. The current results show that the controller in response to fatigue of either the thumb or index finger did not adopt the strategy of increasing the synergy index at the highest level (as could be expected from earlier studies, (Singh & Latash, 2011; Singh et al, 2010b; Singh et al, 2012). …”