A simple on-field fast knee-flexion test to assess acute knee flexor fatigue

Purpose: In a practical setting, outside the laboratory, acute muscle fatigue may be underestimated because substantial recovery occurs during the elapsed time between the end of exercise and fatigue assessment. We introduce a simple field test to assess knee flexor contractile function quickly after exercise cessation. Methods: Fourteen young amateur football players performed maximally fast knee flexions (FKFs) in the prone position with their dominant leg, before (pre) and 20 s after finishing a series of fourteen fatiguing 40 m sprints (post) and again following 6 min recovery (rec). Peak angular acceleration (PAA) about the knee joint was measured with a small inertial measurement unit (IMU) firmly attached to the shin. Results: Although participants only practiced the FKFs for 1 min in the warm-up, the reliability of PPA was good with coefficients of variation of 3.0% (pre), 2.7% (post), and 3.6% (rec). Sprint time increased from 5.96 ± 0.40 s to 6.55 ± 0.37 s (p < 0.001, f = 0.89), PAA decreased from 107.1 ± 11.5 rad.s⁻² to 94.1 ± 11.7 rad.s⁻² (p < 0.001, f = 0.50) and following recovery (p < 0.05) values were 6.15 ± 0.39 s and 103.1 ± 10.7 rad.s⁻², respectively. The percentage decrease in PAA during FKFs was linearly related (r² = 0.48, p = 0.01) to the percentage increase in 40 m sprint time. In addition, PAA (pre) was related to the time of the first sprint (r² = 0.33, p = 0.03). Conclusion: The proposed FKF test is reliable and can easily be executed to evaluate acute knee flexor muscle fatigue on the field. The presented relations between (changes in) sprint performance and peak knee angular accelerations during isolated fast knee flexions are promising but need confirmation in larger-scaled studies.