This study examined seasonal changes in countermovement jump (CMJ) performance among NCAA Division I male and female soccer athletes across competitive and off-season phases. Twenty-three male and 16 female athletes completed CMJ testing at 6 in-season and 6 off-season time points using uniaxial force plates. Variables assessed included jump height, peak power, countermovement depth, modified reactive strength index, and eccentric-specific metrics such as force at zero velocity, eccentric rate of force development, and eccentric duration. Linear mixed models were used to analyze within-season changes. Concentric performance metrics remained stable during the in-season for both sexes, while significant declines in eccentric variables were observed. In men, countermovement depth and eccentric velocity decreased in the latter half of the season ( p < 0.05). Women exhibited more pronounced fatigue responses, including reductions in eccentric rate of force development, force at zero velocity, and increased eccentric duration ( p < 0.05). In the off-season, both men and women demonstrated significant improvements in peak power, relative power, and concentric impulse ( p < 0.05), with female athletes also improving jump height and eccentric metrics. These findings highlight the utility of CMJ force plate metrics in detecting neuromuscular fatigue and performance adaptation. While resistance training may help preserve concentric performance during the season, eccentric function seems more vulnerable, particularly in women, underscoring the need for targeted eccentric training and monitoring. The off-season presents a valuable window for restoring and enhancing lower-body power.
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