# Influence of Isometric and Dynamic Fatiguing Protocols on Dynamic Strength Index
###### Abstract  
Background: Strength and conditioning experts widely recognize the dynamic strength index (DSI) as a tool for assessing an athlete's ability to utilize strength in dynamic actions. The DSI is calculated as the ratio of peak force in dynamic actions versus isometric ones. To date, the influence of fatigue on the DSI is still not fully understood. This study aimed to explore the effects of both dynamic and isometric fatigue tasks on the DSI. Methods: A total of 24 physically active participants underwent fatigue tests involving repeated countermovement jumps (dynamic) and repeated isometric mid-thigh pulls (isometric) in separate visits. Results: The results revealed a marked drop in performance, with dynamic force showing a more significant reduction (\(p<0.001\); d = 1.57) than isometric force (\(p\) = 0.015; d = 0.30). After the isometric fatigue task, the DSI increased, indicating a more substantial decline in isometric force (\(p<0.001\); d = 1.75) compared to dynamic force (\(p\) = 0.313; d = 0.08). Following this trend, the DSI decreased post-dynamic fatigue (\(p<0.001\); d = 0.99) and increased post-isometric fatigue (\(p<0.001\); d = 3.11). Conclusion: This research underscores the need to consider fatigue's task-specific effects on the DSI, enabling more tailored training methodologies for athletes. dynamic strength; vertical jump; mid-thigh pull; fatigue; task specificity Article Influence of Isometric and Dynamic Fatiguing Protocols on Dynamic Strength Index  
[PERSON], D.  
To the best of our knowledge, no study has investigated the influence of fatigue on the DSI. The changes in isometric and dynamic force production under the influence of fatigue are not necessarily uniform. Diverging responses in these capacities have been noted after specific fatiguing tasks like cycling at varying intensities [13]. In addition, after a running event, lower limb isometric force and maximal jump power decreased by \(\sim\)25-30% and \(\sim\)14%, respectively, with only a moderate correlation between the two [14]. Similarly, a significant decrease in knee extension and plantar flexion maximal force (22% and 17%, respectively) was observed after a marathon run, with no change in CM] force [15]. In addition, throughout a 12-week strength training program with deliberate overarching, no relationship was found between changes in CM] performance and changes in maximal voluntary force [16]. More specific to the DSI evaluation, another recent study reported larger decrements in both CM] height and IMIP force after heavy resistance training compared to isometric training [17]. Furthermore, it has been suggested that the jumping strategy can be altered to maintain the same performance despite losses in maximal voluntary force [18]. If fatigue differently affects isometric and dynamic force capacities, it could distort the DSI values. Assessing the DSI under fatigue conditions might yield insights distinct from the measurements at rest. It is conceivable, though still speculative, that athletes showing the most pronounced DSI decrease under fatigue (indicating a more substantial drop in dynamic compared to isometric force) might benefit from enhancing their dynamic strength endurance. Given the absence of research on fatigue's impact on the DSI, our study was conceived as a proof-of-concept to examine whether the DSI decreases after fatiguing dynamic contractions. To further investigate whether these changes depend on the fatigue task, we also examined the DSI following an isometric fatigue task. Our primary objective was to ascertain the effects of dynamic (repeated CMJ) and isometric (repeated IMTP) fatigue tasks on the DSI, that the repeated CMJ task would disproportionately reduce force-production in CMJ compared to IMIP, leading to a lowered DSI. An opposite effect (increased DSI) was expected following repeated IMTPs, which would show that changes in the DSI depend on the type of fatiguing task. The findings of our study will have significant implications for physical fitness assessments in sports environments.