Effects of fatigue on running mechanics: Spring-mass behavior in recreational runners after 60 seconds of countermovement jumps

The purpose of this study was to investigate the effects of acute fatigue on spring-mass model (SMM) parameters among recreational runners at different speeds. Eleven participants (5 males and 6 females) performed running trials at slower, self-selected, and faster speeds on an indoor track before and after performing a fatigue protocol (60 s of countermovement jumps). Maximal vertical force (Fmax), impact peak force (Fpeak), loading rate (LR), contact time (Tc), aerial time (Ta), step frequency (SF), step length (SL), maximal vertical displacement of the center of mass (ΔZ), vertical stiffness (Kvert), and leg work (Wleg) were measured using a force plate integrated into the track. A significant reduction (-43.1 ± 8.6%; P <.05) in mechanical power during jumps indicated that the subjects became fatigued. The results showed that under fatigue conditions, the runners adjusted their running mechanics at slower (≈ 2.7 ms-1;ΔZ -12% and SF +3.9%; P < -05), self-selected (≈3.3 ms-1; SF +3%, SL -6.8%, Ta -16%, and Fmax-3.3%; P < -05), and faster (≈3.6 ms-1SL -6.9%, Ta -14% and Fpeak-9.8%; P < -05) speeds without significantly altering Kvert (P >.05). During constant running, the previous 60 s of maximal vertical jumps induced mechanical adjustments in the spatiotemporal parameters without altering Kvert.