BACKGROUND: A countermovement jump (CMJ) is routinely used in many sporting settings to provide a functional measure of neuromuscular fatigue. However, the variables that are most sensitive to fatigue remain somewhat unclear. The purpose of this study was to determine the acute changes in neuromuscular fatigue in rugby union players during a period of preseason training. METHODS: Nine male (age: 19.0 ± 1.5 years) academy rugby union players performed five CMJ trials on three occasions, at baseline, 24 hours and 48 hours post-baseline. The fatiguing protocol consisted of multiple high-intensity training sessions commensurate with the period of preparation and the sport. A total of 14 CMJ variables were derived from the force-time curve. Meaningful differences in CMJ performance were examined using the magnitude of change (effect sizes; ES) compared to baseline. RESULTS: Most variables, 9 of the 14, showed substantial decreases at 24 hours post-baseline. Mean concentric power, peak velocity, jump height and force at zero velocity were impaired by the greatest magnitude (ES = -0.98 to -1.57). At 48 hours post-baseline, substantial increases in eccentric duration, concentric duration and total duration were first observed (ES = 0.48 to 0.61). Concomitantly, peak power, peak velocity and jump height, recovered to baseline levels.CONCLUSION: During the late regeneration phase, neuromuscular fatigue can manifest itself as an altered movement strategy, rather than as a simple reduction in physical output such as jump height. Practitioners are therefore advised to incorporate a wide range of variables when trying to identify subtle changes in the bimodal recovery pattern associated with stretch-shortening cycle induced fatigue.