The COVID-19 pandemic forced elite football leagues into extended breaks followed by prompt resumption of competitive match-play. Inadequate periods of on-pitch football-specific training may underlie the increased injury incidence reported following the restart in a non-peer reviewed report. Detraining effects from isolated training are expected, but existing models do not completely describe the unprecedented conditions imposed by COVID-19 on elite footballers. Providing insight into specific neuromuscular qualities affected by extended absence of football-specific training, we share performance and kinetic data from an elite team`s countermovement jump (CMJ) assessments performed immediately pre and post 15 weeks of isolated training. To contextualise these trends, off-season changes with no training are also presented. The isolated circuit resistance and aerobic interval training largely maintained jump height, Reactive Strength Indexmodified and other variables associated with athletic performance, but there were moderate magnitude (p=0.01 to 0.04) changes in eccentric "yielding" and landing "loading" phase variables. These changes suggest a compromised initiation of deceleration during countermovement and impact attenuation actions and could reflect altered coordination/motor control strategies or muscle-tendon properties. This analysis may have revealed kinetic markers specifically stimulated by football-specific training/competition, relevant to post-quarantine monitoring. As lower landing forces may be favourable from an overuse injury risk perspective, while yielding phase alterations suggest a negative effect on reactive performance characteristics, the overall effect on vulnerability to injury is unclear.
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