Variability of trunk muscle synergies underlying the multidirectional movements and stability trunk motor tasks in healthy individuals

Abstract

Muscle synergy analysis is useful for investigating trunk coordination patterns based on the assumption that the central nervous system reduces the dimensionality of muscle activation to simplify movement. This study aimed to quantify the variability in trunk muscle synergy during various trunk motor tasks in healthy participants to provide reference data for evaluating trunk control strategies in patients and athletes. Sixteen healthy individuals performed 11 trunk movement and stability tasks with electromyography (EMG) recording of their spinal and abdominal muscles (6 bilaterally). Non-negative matrix factorization applied to the concatenated EMG of all tasks identified the five trunk muscle synergies (W) with their corresponding temporal patterns (C). The medians of within-cluster similarity defined by scalar products in W and r_max coefficient using the cross-correlation function in C were 0.73-0.86 and 0.64-0.75, respectively, while the inter-session similarities were 0.81-0.96 and 0.74-0.84, respectively. However, the lowest and highest values of both similarity indices were broad, reflecting the musculoskeletal system's redundancy within and between participants. Furthermore, the significant differences in the degree of variability between the trunk synergies may represent the different neural features of synergy organization and strategies to overcome the various mechanical demands of a motor task.

Figure 1

Eleven trunk movement and stability motor tasks. (1) Rocking backward (RB), (2) rocking forward (RF), (3) cross extension right (Cert), (4) cross extension left (Celt), (5) cat and dog (CAD), (6) forward bend (FB), (7) side bend right (SBrt), (8) side bend left (SBlt), (9) backward bend (BB), (10) rotation right (ROTrt), (11) rotation left (ROTlt).

Figure 2

Flow-chart of the analysis trunk muscle synergies (W) and their temporal patterns (C). W and C were extracted using the NMF algorithm. To determine if the extracted synergies in the current study represented the same underlying a variety of locomotion and stability tasks found in our previous study, the similarities found using the scalar product of the cluster centroids of W in the current study, and those extracted in the previous study (excluding muscle weighting components in the lower limbs) were computed. The analysis of variability was focused on within-cluster similarities and similarities between session in both W and C. The similarity values for W and C were computed using scalar product and the r_max coefficient using the cross-correlation function, respectively.

Figure 3

Muscle activation patterns of 1 l trunk movement tasks. The concatenated EMG envelope of 1 l trunk stability and movement tasks is shown. The mean of the EMG envelopes for 16 participants is plotted as a line and the standard deviation as a shading around it. The amplitude is normalized to the maximum value for each muscle over all tasks and standardized to have a unit variance to equally weight the EMG activity across all muscles before each synergy extraction procedure.

Figure 4

Individual (black thin lines) and participant (blue thick line) means of the percentage of variability accounted for (VAF). When 5 synergies were extracted, the mean value of VAF is 91.3% (± 0.02).

Figure 5

Representative (a) five-trunk muscle synergies (W1–W5) and (b) their temporal patterns (C1–C5). Non-negative matrix factorization on the concatenated electromyography (EMG) envelopes of 11 trunk motor tasks extracted five trunk muscle synergies for each participant. We sorted the trunk muscle synergies of all participants into five groups using hierarchical cluster analysis.

Figure 6

Intra-cluster similarity of (a) muscle synergies (W1–W5) and (b) temporal patterns (C2–C5). Kruskal–Wallis test showed p < 0.05 for both W and C. Multiple comparisons show statistically significant differences between W and C as follows: W1 and W3, W1 and W4, W1 and W5, W2 and W3, W2 and W4, and W2 and W5 (p < 0.05). C1 and C3, C1 and C5, C2 and C3, C2 and C4, and C2 and C5 (p < 0.05).

Figure 7

Inter-session similarity of (a) muscle synergies (W1–W5) and (b) temporal patterns (C2–C5). Kruskal–Wallis test showed p = 0.2084 for W and p < 0.05 for C. Multiple comparisons show statistically significant differences in C as follows: C1 and C3 and C1 and C4 (p < 0.05).