Surface electromyography testing to reveal synergistic compensation during upper limb testing

Objective This study analyzes muscle synergy patterns during maximal-effort upper limb testing. The aim was to examine neuromuscular coordination and identify hidden compensatory activations that could inform ergonomic and rehabilitative strategies for preventing work-related upper limb disorders (WRULDs).

Material and Methods Surface electromyography (sEMG) was recorded from six pairs of upper limb muscles: the deltoids, biceps brachii, brachioradialis, triceps brachii, wrist flexors, and wrist extensors. Twenty-three healthy adults (median age 24.69±4.13) underwent a standardized series of classic isolated manual strength tests for each muscle group, performed at maximal effort. For each trial, the muscle showing the highest sEMG amplitude was identified as the dominant contributor and compared to the intended target. Statistical analyses were performed using Jamovi, including chi-square tests and Cohen’s Kappa to assess agreement.

Results Across 276 trials, the intended muscle was dominant in 72% of cases. In the remaining 28%, a different muscle produced the highest activation, indicating frequent synergistic compensation. The biceps brachii failed to dominate in 65% of its own tests, with the deltoid or brachioradialis often taking over. The deltoid was the dominant muscle in 26% of all trials. In contrast, triceps and deltoid tests showed high isolation success (98–100%). Concordance between tested and dominant muscles was moderate (Cohen’s κ ≈ 0.66; χ² p < 0.001, statistically significant). No consistent side-dominance patterns were observed.

Conclusions Even in standardized strength testing, healthy individuals frequently recruit alternate muscles. Persistent synergy patterns may contribute to WRULD development over time. Identifying such patterns could enable earlier ergonomic interventions, more targeted rehabilitation, and refinement of muscle testing protocols.