Number of trials and data structure affect the number and components of muscle synergies in upper-limb reaching movements

Abstract

Objective. Due to the variability of human movements, muscle activations vary among trials and subjects. However, few studies investigated how data organization methods for addressing variability impact the extracted muscle synergies. Approach. Fifteen healthy subjects performed a large set of upper limb multi-directional point-to-point reaching movements. Then, the study extracted muscle synergies under different data settings and investigated how data structure prior to synergy extraction, namely concatenation, averaging, and single trial, the number of considered trials, and the number of reaching directions affected the number and components of muscle synergies. Main results. The results showed that the number and components of synergies were significantly affected by the data structure. The concatenation method identified the highest number of synergies, and the averaging method usually found a smaller number of synergies. When the concatenated trials or reaching directions was lower than a minimum value, the number of synergies increased with the increase of the number of trials or reaching directions; however, when the number of trials or reaching directions reached a threshold, the number of synergies was usually constant or with less variation even when novel directions and trials were added. Similarity analysis also showed a slight increase when the number of trials or reaching directions was lower than a threshold. This study recommends that at least five trials and four reaching directions and the concatenation method are considered in muscle synergies analysis during upper limb tasks. Significance. This study makes the researchers focus on the variability analysis induced by the diseases rather than the techniques applied for synergies analysis and promotes applications of muscle synergies in clinical scenarios.

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“…When recruiting patients, detailed recordings including basic and pathological information, such as sex, age, onset time, degree of impairment, lesion location, and rehabilitation program, should be elaborated, which contribute to the explanation of the outputs and comparisons among studies. In terms of the experimental setups, one recent study [ 162 ] revealed that in order to capture sufficient variability of the task and reduce the influence of random factors, at least five repetitions and four reaching directions are required when performing the reaching tasks; it is in any case needed to find a synthesis between the clinical needs and the requirements of motion variability inherent to the muscle synergy method. As for the number and selection of muscles, there is also no standard recommendation, while some upper limb muscles have been largely used to record muscle activities ( Fig.…”

Anthropomorphic motion planning for multi-degree-of-freedom arms

“…At the same time, the expressions of synergies in different motion patterns vary. Zhao et al (2022) extracted the synergies under different numbers of trials and different arrival directions in point-to-point reaching movement experiments and found that the synergies increased with the number of trials or the number of arrival directions. When the number of experiments or the number of arrival directions reached a threshold, the synergies did not change significantly.…”