Matthew J. Hermes1, Mandy E. Parra2, Andrew C. Fry1, & Trent J. Herda1

1The University of Kansas, Lawrence, Kansas; 2Baker University, Baldwin City, Kansas

The vertical jump (VJ) can assess lower body performance through force-time data analysis, but kinetic and kinematic events may not be easily explained from the force-time data alone. Yank, a time-derivative of force, represents instantaneous rate of force development and may be a viable method to understand kinematic and kinetic events during a VJ. How fatigue affects yank is unknown. PURPOSE: To assess the effect of fatigue on VJ yank-time characteristics. METHODS: Recreationally trained males (X±SD; n = 11, age = 22.7±3.3 yrs, height = 182.1±5.2 cm, mass = 86.9±11.8 kg) completed a repeated jump protocol on a force plate consisting of 10 sets of repeated jumps. Sets lasted 15 seconds, with 15 seconds rest between sets. Subjects were instructed to reach 90º knee flexion, jump maximally with no arm swing, and perform as many jumps as possible per set. The first jump of the first and last sets were analyzed. Yank-time data was derived from force-time data using a low-pass Hamming filter with a cutoff frequency of 10 Hz. Countermovement (CM) depth was integrated from force-time data. Yank events were defined as: 1 = jump initiation (yank drops below 0), 2 = first yank curve trough, 3 = lowest force value (yank moves above 0), 4 = force peak one (yank drops below 0), 5 = trough between force peaks (yank moves above 0), 6 = force peak two (yank drops below 0), 7 = flight initiation. Paired samples t-tests compared durations of yank-based VJ phases between nonfatigued and fatigued jumps (p < 0.05). RESULTS: Shorter VJ phase durations and moderate (g = 0.5-0.8), large (g = 0.8-1.2), or very large (g = 1.2-2.0) effect sizes were found in non-fatigued jumps for phases 4-5 (p = 0.004, g = 1.083), 5-6 (p = 0.07, g = 0.986), and 6-7 (p = 0.001, g = 1.369). Phases 1-2, 2-3, and 3-4 were not significantly different (p > 0.05), but phase 1-2 did have a moderate effect size (g = 0.561). Total jump time was significantly faster in non-fatigued jumps (p = 0.04, g = 0.684). Though non-significant (p = 0.112, g = 0.505), CM depth was shallower for fatigued jumps. CONCLUSION: VJ phase durations were shorter for some events in non-fatigued jumps, despite larger CM. This indicates yank is sensitive to fatigue. As yank has been suggested to help explain kinetic and kinematic events, yank-time assessments during fatigue may help to understand how fatigue affects kinetics and kinematics.

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