Effect of elbow joint angles on electromyographic activity versus force relationships of synergistic muscles of the triceps brachii.

Hiroshi Akima,Koji Ishida,Teruhiko Koike,Hisashi Maeda

doi:10.1371/journal.pone.0252644

Abstract

The electromyographic (EMG) activity and force relationship, i.e. EMG-force relationship, is a valuable indicator of the degree of the neuromuscular activation during isometric force production. However, there is minimal information available regarding the EMG-force relationship of individual triceps brachii (TB) muscles at different elbow joint angles. This study aimed to compare the EMG-force relationships of the medial (TB-Med), lateral (TB-Lat), and long heads (TB-Long) of the TB. 7 men and 10 women performed force matching isometric tasks at 20%, 40%, 60%, and 80%maximum voluntary contraction (MVC) at 60°, 90°, and 120° of extension. During the submaximal force matching tasks, the surface EMG signals of the TB-Med, TB-Lat, and TB-Long were recorded and calculated the root mean square (RMS). RMS of each force level were then normalized by RMS at 100%MVC. For the TB-Med, ultrasonography was used to determine the superficial region of the muscle that faced the skin surface to minimize cross-talk. The joint angle was monitored using an electrogoniometer. The elbow extension force, elbow joint angle, and surface EMG signals were simultaneously sampled at 2 kHz and stored on a personal computer. The RMS did not significantly differ between the three muscles, except between the TB-Med and TB-Lat during 20%MVC at 60°. The RMS during force levels of ≥ 60%MVC at 120° was significantly lower than that at 60° or 90° for each muscle. The sum of difference, which represents the difference in RMS from the identical line, did not significantly differ in any of the assessed muscles in the present study. This suggests that a relatively smaller neuromuscular activation could be required when the elbow joint angle was extended. However, neuromuscular activation levels and relative force levels were matched in all three TB synergists when the elbow joint angle was at 90° or a more flexed position.

Highlights

The force exerted by the muscle depends on the motor unit (MU) activation patterns and the mechanical properties of the muscle fibers and muscle-tendon complex
We found that the root mean square (RMS) of each muscle increased linearly with increasing force (Figs 4 and 5), suggesting that force control was finely modulated by the recruitment and/or firing rates of the MUs of each muscle
Another study that assessed the normalized RMS-force relationship of the four individual quadriceps muscles found that the normalized RMS-force relationship of the vastus intermedius gradually shifted downward with knee extension compared with the other synergistic muscles [14]

Summary

Introduction

The force exerted by the muscle depends on the motor unit (MU) activation patterns and the mechanical properties of the muscle fibers and muscle-tendon complex. Harwood et al [13] showed that recruitment thresholds of motor units (MUs) of the long head of the TB (TB-Long) were significantly higher than that of the lateral head (TB-Lat) This suggests that the TB-Long and TB-Lat have different MUs recruitment properties with changes in muscle force. In the knee joint, the normalized RMS-force relationship of the quadriceps femoris was shifted down by extension of joint angles [14], implying changes in joint angle affects EMG activation patterns. It is not clear whether the normalized RMS-force relationship significantly differs among the three synergistic TB muscles with changes in the elbow joint angle. Accumulating data on the neuromuscular activity of the TB muscle with changes in elbow joint angle will be important for establishing accurate neuromusculoskeletal modelling for estimating muscle force from EMG in the future

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Conclusion