# Influence of Isometric and Dynamic Fatiguing Protocols on Dynamic Strength Index
## 4 Discussion  
This study was conducted to examine the influence of dynamic (repeated CMJ) and isometric (repeated IMTP) fatigue tasks on the DSI. As we hypothesized, the fatigue induced by repeated CMJ reduced PF to a greater extent in CMJ than in IMTP, which resulted in a greatly decreased DSI. This suggests that caution should be taken when interpreting DSI values under the influence of fatigue. We also observed an opposite effect (increased DSI) after repeated IMTP, demonstrating that fatigue effects are highly task-specific. To the best of our knowledge, this is the first study that assessed the effects of fatigue on the DSI. Future research should consider investigating the practical relevance of the DSI in the fatigued state. The findings of this study suggest that coaches should either (a) conduct DSI assessments at the outset of testing sessions or (b) allow sufficient recovery between tests to ensure accurate evaluation of an athlete's dynamic strength capabilities. Previous research has documented that the magnitude of fatigue is specific both to the fatiguing protocol and measurement tasks [13; 14; 15; 28; 29]. Therefore, our results were expected. Several underlying mechanisms could explain why notable decreases in performance were observed only when the measurement task corresponded to the fatiguing task. Although the main agonist muscles for both tasks were the same, the differing muscle activations between CMJ and IMTP could underlie our observations. For instance, vastus medialis was reported to exhibit \(\sim\)83.5% maximal voluntary activity during IMTP [30], while the values can be much higher (\(\sim\)150-190%) in CMJ [31]. In addition, CMJ involves an eccentric and a concentric contraction, and stretch-shortening cycle, and is partially underpinned by different mechanisms than isometric force production (e.g., musculotendinous stiffness [32] and rate of force development [33]). Given the importance of the rate of force development in CMJ, it could be hypothesized that motor unit firing rate, speed of motor neuron recruitment and excitation-contraction coupling are the main limiting mechanisms in this task [34; 35], while reductions in isometric force are largely explained by peak muscle activation [36]. While the exact mechanisms cannot be elucidated from our experiment, it is clear that CMJ and IMTP both highly induce task-specific fatigue effects, which are also reflected in the DSI. Specific differences in motor unit recruitment and motor commands between isometric and dynamic movements can provide further insight into the observed results in the dynamic strength index (DSI). Isometric muscle contractions, such as in the IMTP, require continuous motor unit recruitment and put a higher demand on the central nervous system. This demand can lead to central fatigue, involving neurotransmitter depletion, decreased excitability of motor neurons, and reduced voluntary drive to the muscles. Additionally, isometric contractions predominantly recruit both slow-twitch and fast-twitch muscle fibers, which contribute to decreased peak force output during IMTP and consequently higher DSI values after IMTP fatigue [37]. On the other hand, dynamic movements like the countermovement jump (CMJ) involve a broader range of motion and external movement, which can lead to more peripheral fatigue. During CMJ, eccentric contractions place high mechanical stress on muscle fibers, and repeated CMJs can cause the accumulation of metabolic waste products to a greater extent [38]. Performing repeated CMJs preferentially recruits fast motor units, especially during eccentric contractions. Furthermore, the concentric push-off phase in CMJ is predominantly affected by metabolic fatigue [38]. Consequently, the short-term performance depression due to the accumulation of lactate and other metabolic markers results in a greater impairment of explosive performance and a decrease in the DSI after repeated CMJ.