Immediate effects of thoracic extension exercise on the mechanical response of the pectoralis minor assessed using shear wave elastography

Thoracic posture is considered a proximal factor influencing scapular alignment and shoulder mechanics. The pectoralis minor (PM), which anatomically links the thorax and scapula via the ribs and coracoid process, may play a role in this relationship. However, it remains unclear whether thoracic extension exercise can influence the mechanical response of the PM under standardized measurement conditions without direct stretching. In this randomized controlled exploratory pilot study, eighteen healthy adults were assigned to either an exercise group or a control group. The exercise group performed a standardized 10-minute thoracic extension protocol, while the control group remained seated. PM shear modulus was assessed using shear wave elastography (SWE) under standardized measurement conditions, and shoulder horizontal abduction range of motion (ROM) was measured using goniometry before and immediately after the intervention. A significant group × time interaction was observed for PM shear modulus, with a reduction in the exercise group and an increase in the control group. For ROM, a significant increase was observed within the exercise group; however, no significant group × time interaction or between-group difference was detected. Thoracic extension exercise was associated with a statistically significant reduction in PM shear modulus under standardized measurement conditions, even in the absence of direct stretching. However, the magnitude of change did not exceed the minimal detectable change (MDC95), and therefore its clinical relevance remains uncertain. Changes in ROM were not consistently supported at the between-group level. These findings should be interpreted cautiously, as they likely reflect condition-dependent mechanical responses rather than intrinsic changes in muscle properties. Further studies involving larger samples, repeated follow-up measurements, symptomatic populations, and direct assessment of thoracic posture and scapular kinematics are needed to determine whether these effects are clinically meaningful and reproducible. Trial registration : UMIN Clinical Trials Registry (UMIN-CTR: UMIN000059973).