Hypohydration reduces vertical ground reaction impulse but not jump height

This study examined vertical jump performance using a force platform and weighted vest to determine why hypohydration (~4% body mass) does not improve jump height. Measures of functional performance from a force platform were determined for 15 healthy and active males when euhydrated (EUH), hypohydrated (HYP) and hypohydrated while wearing a weighted vest (HYP_v) adjusted to precisely match water mass losses. HYP produced a significant loss of body mass [−3.2 ± 0.5 kg (−3.8 ± 0.6%); P < 0.05], but body mass in HYP_v was not different from EUH. There were no differences in absolute or relative peak force or power among trials. Jump height was not different between EUH (0.380 ± 0.048 m) and HYP (0.384 ± 0.050 m), but was 4% lower (P < 0.05) in HYP_v (0.365 ± 0.52 m) than EUH due to a lower jump velocity between HYP_v and EUH only (P < 0.05). However, vertical ground reaction impulse (VGRI) was reduced in both HYP and HYP_v (2–3%) compared with EUH (P < 0.05). In conclusion, this study demonstrates the failure to improve jump height when HYP can be explained by offsetting reductions in both VGRI and body mass.