Voluntary activation of the infraspinatus muscle in nonfatigued and fatigued states

Hypothesis

Failure of voluntary activation is an important source of weakness in several different muscles after injury or surgery. Despite the high prevalence of shoulder rotator cuff disorders and associated weakness, no test currently exists to identify voluntary activation deficits for the rotator cuff. The purpose of this study was to develop a test to quantify voluntary activation of the infraspinatus. We hypothesized that there would be a consistent relationship between the voluntary activation level and different force levels and that reduced voluntary activation would partially account for reduced force with fatigue.

Materials and methods

Twenty healthy volunteers underwent assessment of voluntary activation using an electrical stimulus applied to the infraspinatus muscle during active isometric external rotation. Voluntary activation was assessed across several levels of external rotation effort and during fatigue.

Results

The voluntary activation-percent force relationship was best fit using a curvilinear model, and the fatigue test reduced both force and voluntary activation by 46%.

Discussion

In the nonfatigued state, the voluntary activation-percent force relationship is similar to that reported for the quadriceps. After fatigue, however, greater failure of voluntary activation was observed compared with reported values for other upper and lower extremity muscles, which may have implications for the understanding and treatment of rotator cuff pathology.

Conclusion

A measure of voluntary activation for the infraspinatus varied with the percent maximum force in a predictable manner that is consistent with the literature. The infraspinatus may be more susceptible to failure of voluntary activation during fatigue than other muscles.

Level of evidence: Basic Science Study

Keywords: Voluntary activation, infraspinatus, rotator cuff, fatigue, young adults, exercise, isometric contraction