Reliability and validity of active and passive pectoralis minor muscle length measures.

Background: Pectoralis minor muscle length is believed to play an important role in shoulder pain and dysfunction. Current clinical procedures for assessing pectoralis minor muscle length may not provide the most useful information for clinical decision making.

Objective: To establish the reliability and construct validity of a novel technique to measure pectoralis minor muscle length under actively and passively lengthened conditions.

Design: Cross-sectional repeated measures.

Methods: Thirty-four healthy adults (age: 23.9, SD=1.6 years; 18 females) participated in this study. Pectoralis minor muscle length was measured on the dominant arm in three length conditions: resting, actively lengthened, and passively lengthened. Based upon availability, two raters, out of a pool of five, used a caliper to measure the distance between the coracoid process and the 4th rib. The average of two pectoralis minor muscle length measures was used for all muscle length conditions and analyses. Intraclass correlation coefficients determined intra-and inter-rater reliability, and measurement error was determined via standard error of measurement and minimal detectable change. Construct validity was assessed by ANOVA to determine differences in muscle length across the three conditions.

Results: Our intra- and inter-rater reliability values across all three conditions ranged from 0.84 to 0.92 and from 0.80 to 0.90, respectively. Significant differences (p<0.001) in muscle length were found among all three conditions: rest-active (3.66; SD=1.36cm), rest-passive (4.72, SD=1.41cm), and active-passive (1.06, SD=0.47cm).

Conclusions: The techniques described in this study for measuring pectoralis minor muscle length under resting and actively and passively lengthened conditions have acceptable reliability for clinical decision making.