Reliability of ground reaction forces during a vertical jump: implications for functional strength assessment.

OBJECTIVE: To determine the reliability of ground reaction force during a vertical jump. DESIGN AND SETTING: Two test sessions 48 hours apart in which subjects performed five maximal vertical jumps with their right lower extremity on a force platform without arm movement. Applied Biomechanics Laboratory at the University of Toledo. SUBJECTS: Nineteen healthy males (n = 12) and females (n = 7), with an average age of 21.3 years and 23.2 years, respectively, from the University of Toledo participated in this study. The average height for males and females was 70.0 and 66.6 inches, respectively. The average weight for males was 170.5 lbs., while the average weight for females was 132.4 lbs. MEASUREMENTS: Reliability of the peak vertical ground reaction force and vertical impulse was assessed using the formula for intraclass correlation coefficient (2,1) (ICC [2,1]). RESULTS: Measurement of peak vertical ground reaction force was demonstrated to be very reliable (ICC [2,1] r(xx) = .94; SEM = .003% BW), whereas the reliability estimate for vertical impulse was not very reliable (ICC [2,1] r(xx) = .22; SEM = .24% BW seconds). Furthermore, no significant relationship was found between peak vertical ground reaction force and vertical impulse. (BW = body weight; SEM = standard error of measurement). CONCLUSIONS: We conclude that peak force measured during a one-legged vertical jump is reliable and may provide an alternate method of evaluation of lower extremity functional strength.