Force direction and arm position affect contribution of clavicular and sternal parts of pectoralis major muscle during muscle strength testing.

STUDY DESIGN: Cross-sectional study.

PURPOSE OF THE STUDY: The study aims to determine the effects of force direction and arm position in differentiating the clavicular (PMc) and sternal (PMs) parts of the pectoralis major (PM) muscle during maximal voluntary isometric contraction (MVIC) to provide basic evidence to support the clinical thinking behind muscle strength testing of PM.

METHODS: Nine experimental conditions with 3 force directions of horizontal adduction (+30° oblique, horizontal, and -30° oblique to the transverse plane) and 3 arm rotation positions (0°, 45°, and 90° shoulder external rotation from the transverse plane) were randomly tested for 26 healthy male participants. The MVIC force level was monitored and measured with a fixed dynamometer, and the surface electromyographic (EMG) signals of the PMc, PMs, anterior deltoid, middle deltoid, and latissimus dorsi were collected during the test for each condition. The PMc/PMs EMG ratio and normalized EMG amplitude were used to quantify the contribution of the tested muscles.

RESULTS: The MVIC force level significantly declined when the arm's external rotation increased (P < .01; the grand mean decreased from 106.7 N ± 27.8 N to 89.5 N ± 22.6 N). The PMc/PMs EMG ratio showed that the best test condition to differentiate the PMc and PMs was the force direction of +30° oblique to the transverse plane and the 45° arm rotation position. Other muscles contributed less than 40% of their MVIC activity levels, with a higher activation level found in the anterior deltoid muscle (P < .01).

CONCLUSIONS: Arm rotation position should be considered as a predominant factor when clinically examining the strength of horizontal adduction movement. All tested conditions failed to fully separate PMc and PMs activation during MVIC and suggested that functional differentiation of the PM might not be applicable to maximal exertion.

LEVEL OF EVIDENCE: NA.