Do individual differences in the distribution of activation between synergist muscles reflect individual strategies?

Abstract

Individual differences in the distribution of activation between synergist muscles have been reported during a wide variety of tasks. Whether these differences represent actual individual strategies is unknown. The aims of this study were to: (i) test the between-day reliability of the distribution of activation between synergist muscles, (ii) to determine the robustness of these strategies between tasks, and to (iii) describe the inter-individual variability of activation strategies in a large sample size. Eighty-five volunteers performed a series of single-joint isometric tasks with their dominant leg [knee extension and plantarflexion at 25% of maximal voluntary contraction (MVC)] and locomotor tasks (pedalling and walking). Of these participants, 62 performed a second experimental session that included the isometric tasks. Myoelectrical activity of six lower limb muscles (the three superficial heads of the quadriceps and the three heads of the triceps surae) was measured using surface electromyography (EMG) and normalized to that measured during MVC. When considering isometric contractions, distribution of normalized EMG amplitude among synergist muscles, considered here as activation strategies, was highly variable between individuals (15.8% < CV < 42.7%) and robust across days (0.57 < ICC < 0.82). In addition, individual strategies observed during simple single-joint tasks were correlated with those observed during locomotor tasks [0.37 < r < 0.76 for quadriceps (n = 83); 0.30 < r < 0.66 for triceps surae (n = 82); all P < 0.001]. Our results provide evidence that people who bias their activation to a particular muscle do so during multiple tasks. Even though inter-individual variability of EMG signals has been well described, it is often considered noise which complicates the interpretation of data. This study provides evidence that variability results from actual differences in activation strategies.

Force-sharing within the Triceps Surae: An Achilles Heel in Achilles Tendinopathy

“…The between-day reliability of the distribution of normalized EMG amplitude among the heads of the triceps surae has been demonstrated in a previous study (20) suggesting that it represents a robust individual strategy. Previous studies have estimated the activation of the triceps surae muscles in people with Achilles tendinopathy during walking (26), running (27), isometric plantarflexion tasks (24), and dynamic plantarflexion tasks (28,29).…”

Synergic control of a single muscle: The example of flexor digitorum superficialis

“…2003; Cuadra et al . 2019). This set‐up uses four unidirectional piezoelectric sensors (model 208C02; PCB Piezotronics, Depew, NY, USA) to measure the forces produced by individual fingers.…”

“…2003; Cuadra et al . 2019), which allows setting tasks that require the subjects to apply accurate time‐varying forces by pressing with the intermediate phalanges.…”

Mind-Muscle Connection: Verbal Instructions Alter Electromyographic Activity for Elbow Flexors and Extensors During Co-Contraction Training

“…Specifically, the task of contracting antagonist muscles and maintaining a zero motion at the joint level can be performed with varied muscle activation patterns – which supports the observed individual divergence. Such individual differences were demonstrated before (Crouzier et al., 2019; Ting et al., 2015) and encompass the fact that individuals approach the task and instructions differently.…”