Age and sex influence the activation-dependent stiffness of the pectoralis major.

The pectoralis major fiber regions contribute uniquely to the mobility and the stability of the shoulder complex. It is unknown how age and sex influence the stiffness of these regions during volitional contractions, but this knowledge is critical to inform clinical interventions targeting the pectoralis major. The aim of the present study was to determine if the activation-dependent stiffness of the pectoralis major fiber regions differs between the sexes and if it is altered with age. Ultrasound shear wave elastography was used to acquire shear wave velocity from the clavicular and the sternocostal fiber regions of 48 healthy participants, including 24 younger (12 males, 12 females, mean ± SD age 25 ± 4.1 years) and 24 older adults (12 males, 12 females, 55 ± 3.6 years). Participants performed vertical adduction and horizontal flexion torques in neutral and 90° externally rotated shoulder positions, and one of the two shoulder abduction positions (60° and 90°) at varying torque magnitudes (passive, 15% and 30% of maximal voluntary contraction). Separate linear mixed-effects models were run for each fiber region and shoulder position to determine if the activation-dependent stiffness differed between the sexes and was altered in older adults. Age-related alterations in stiffness during volitional contractions were observed in both fiber regions and were dependent on the task. Alterations in activation-dependent stiffness due to age were more pronounced in females than males. Additionally, females had greater stiffness than males during volitional contractions in both fiber regions. The present findings provide the first line of evidence that the activation-dependent stiffness of the pectoralis major fiber regions is influenced by sex and changes with age.