Muscle Synergies in Parkinson's Disease

Abstract

Over the last two decades, experimental studies in humans and other vertebrates have increasingly used muscle synergy analysis as a computational tool to examine the physiological basis of motor control. The theoretical background of muscle synergies is based on the potential ability of the motor system to coordinate muscles groups as a single unit, thus reducing high-dimensional data to low-dimensional elements. Muscle synergy analysis may represent a new framework to examine the pathophysiological basis of specific motor symptoms in Parkinson's disease (PD), including balance and gait disorders that are often unresponsive to treatment. The precise mechanisms contributing to these motor symptoms in PD remain largely unknown. A better understanding of the pathophysiology of balance and gait disorders in PD is necessary to develop new therapeutic strategies. This narrative review discusses muscle synergies in the evaluation of motor symptoms in PD. We first discuss the theoretical background and computational methods for muscle synergy extraction from physiological data. We then critically examine studies assessing muscle synergies in PD during different motor tasks including balance, gait and upper limb movements. Finally, we speculate about the prospects and challenges of muscle synergy analysis in order to promote future research protocols in PD.

Keywords: Parkinson’s disease; balance; electromyography; gait; locomotion; motor modules; motor primitives; muscle synergies.

Figure 1

A representative example of the number of synergies, motor primitives and motor modules in animals and humans while walking.

Figure 2

Muscle synergies for human walking. Exemplary motor modules and motor primitives of the four fundamental synergies for human walking extracted via nonnegative matrix factorization (NMF) on a unilateral muscle set. Motor modules are presented on a normalized y-axis base in arbitrary units. For motor primitives, the x-axis full scale represents the averaged gait cycle (with stance and swing normalized to the same amount of points and divided by a vertical dotted line), while the y axis represents the normalized amplitude in arbitrary units. Muscle abbreviations: ME = gluteus medius, MA = gluteus maximus, FL = tensor fasciæ latæ, RF = rectus femoris, VM = vastus medialis, VL = vastus lateralis, ST = semitendinosus, BF = biceps femoris, TA = tibialis anterior, PL = peroneus longus, GM = gastrocnemius medialis, GL = gastrocnemius lateralis, SO = soleus.

Figure 3

Muscles recorded by surface EMG to analyse muscle synergies in Parkinson’s disease patients during postural, walking and reaching tasks.