Assessing Isometric Midthigh Pull, Countermovement Jump, And Dynamic Strength Index In Active-duty Air Force Members

Dynamic strength index (DSI) is often calculated as countermovement jump (CMJ) propulsive peak force divided by isometric mid-thigh pull (IMTP) peak force, and can provide insights into ballistic and peak force characteristics and inform training priorities. PURPOSE: To examine differences in CMJ and IMTP performance between high and low DSI groups, and assess relationships between CMJ, IMTP, and DSI. METHODS: 15 male and 4 female active-duty Air Force members performed three trials of the IMTP and CMJ as part of a larger cohort-based training study. Independent samples t-tests assessed differences between the high and low DSI groups with effect sizes (d), and Pearson correlations (r) evaluated relationships between the performance measures. RESULTS: No significant differences were found between DSI groups for CMJ jump height, CMJ peak power, IMTP force at 100 ms, IMTP force at 200 ms, or IMTP peak force. The high DSI group showed greater values for CMJ peak force (p = 0.004, d = 1.52) and CMJ RSImod (p = 0.04, d = 1.07). For the low DSI group, IMPT peak force showed a large negative correlation with DSI (r = -0.69, p = 0.03). For the high DSI group a large positive relationship was found between IMTP force at 200 ms and CMJ peak force (r = 0.68, p = 0.045), and IMTP peak force showed very large relationships with MJ jump height (r = 0.75, p = 0.02, CMJ peak force (r = 0.74, p = 0.02), CMJ peak power (r = 0.89, p = 0.001), and CMJ RSImod (r = 0.77, p = 0.02). CONCLUSIONS: As both groups showed similar levels of IMTP peak force, the difference in DSI between high and low was dynamic production of peak force during the CMJ. The high DSI group also showed greater RSImod, which reflects the relationship between jump height and concentric contraction time. As jump height was similar, the high DSI group may have spent less time in the countermove and takeoff of the CMJ, reflecting a different jump strategy. Correlations support previous literature showing negative relationships between DSI and IMTP peak force and highlighted positive relationships between CMJ performance and IMTP force production. Data from this study can provide normative data for Air Force personnel, and DSI taken with other measures such as RSImod can inform practitioners for training purposes and remain important lower body performance measures.