Age-Related Differences in Muscle Synergy Organization during Step Ascent at Different Heights and Directions

Abstract

The aim of this study was to explore the underlying age-related differences in dynamic motor control during different step ascent conditions using muscle synergy analysis. Eleven older women (67.0 y ± 2.5) and ten young women (22.5 y ± 1.6) performed stepping in forward and lateral directions at step heights of 10, 20 and 30 cm. Surface electromyography was obtained from 10 lower limb and torso muscles. Non-negative matrix factorization was used to identify sets of (n) synergies across age groups and stepping conditions. In addition, variance accounted for (VAF) by the detected number of synergies was compared to assess complexity of motor control. Finally, correlation coefficients of muscle weightings and between-subject variability of the temporal activation patterns were calculated and compared between age groups and stepping conditions. Four synergies accounted for >85% VAF across age groups and stepping conditions. Age and step height showed a significant negative correlation with VAF during forward stepping but not lateral stepping, with lower VAF indicating higher synergy complexity. Muscle weightings showed higher similarity across step heights in older compared to young women. Neuromuscular control of young and community-dwelling older women could not be differentiated based on the number of synergies extracted. Additional analyses of synergy structure and complexity revealed subtle age- and step-height-related differences, indicating that older women rely on more complex neuromuscular control strategies.

The Dynamic Motor Control Index as a Marker of Age-Related Neuromuscular Impairment

“…The number of muscle synergies has been used as a measure of motor ability (Furuya and Altenmüller, 2013;Sawers et al, 2015;Yaserifar et al, 2021) and impairment (Allen and Franz, 2018;Sawers and Bhatt, 2018); however, its suitability for the detection of age-related neuromuscular impairment is questionable (Monaco et al, 2010;Baggen et al, 2020;da Silva Costa et al, 2020;Santuz et al, 2020). Thus, a primary goal for this study was to evaluate if the dynamic motor control index could better quantify age-related differences in neuromuscular complexity.…”

“…Common measures of neuromuscular control evaluate the relationship between the descending inputs from the central nervous system and coordination of muscular outputs (Ivanenko et al, 2004;Clark et al, 2013;Kamp et al, 2013;Palmer et al, 2016;Liu et al, 2019;Rozand et al, 2019;Awad et al, 2020;Opie et al, 2020). A popular measure of neuromuscular control that quantifies the coordinated co-activation of muscles during walking is a muscle synergy analysis, with different muscle synergy metrics, such as the number of synergies and the composition and timing of those synergies, used to express different aspects of control in young adults (Ivanenko et al, 2004;Chvatal and Ting, 2013), older adults (Sawers and Bhatt, 2018;Baggen et al, 2020;Santuz et al, 2020), and individuals with neurological diagnoses (Clark et al, 2010;Ting et al, 2015). The number of muscle synergies that underlie a motor task has consistently been used as a measure of the complexity of neuromuscular control and is associated with functional abilities (Furuya and Altenmüller, 2013;Sawers et al, 2015;Yaserifar et al, 2021) and fall risk (Allen and Franz, 2018;Sawers and Bhatt, 2018).…”

“…Although the number of muscle synergies differentiates between the extremes of skill and impairment, a wide range of clinical presentations are found among individuals with the same number of muscle synergies. Indeed, older adults do not show a reduction in the number of muscle synergies compared to younger adults (Monaco et al, 2010;Baggen et al, 2020;Santuz et al, 2020), despite known walking deficits (Menz et al, 2003;Kim and Kim, 2014;Cruz-Jimenez, 2017). This suggests that the number of muscle synergies may not be suitable for the detection of age-related impairments in the neuromuscular control of walking.…”

Neuromuscular control of gait stability in older adults is adapted to environmental demands but not improved after standing balance training

“…Human gait has been described with four to eight muscle synergies [12–14] and reactive balance control was found to have four shared synergies with walking [14], which could be important for transfer from balance training to gait. Due to aging and changes in sensory and motor organs, adapted synergies are likely required to maintain motor performance [15,16]. Synergy analyses of gait revealed either fewer synergies in older adults than in young adults [17] or no differences [18].…”

Special Issue: Movement Biomechanics and Motor Control

“…This special issue provides a demonstration of this. The papers collected deal with anticipatory postural adjustments [2] and anticipatory locomotor adjustments [3], strategies to tackle obstacles [2][3][4], the effects of weight unloading on gait [5], posture control in special populations: ataxic children [6] and obese subjects [7], surface perturbation during posture [8], effects of sensory information and feedback on postural control [9,10], elderly behavior during a motor-motor double task [11], upper limb control [12,13], different aspects related to running: ankle joint dynamic stiffness [14] and fatigue [15]. There is also a flash on an ecologic condition where a pedestrian has to program its strategy to cross a road in between two moving vehicles [16], and a study on visual-manual control in monkeys [17].…”