Acute and Delayed Effects of Fatigue on Ground Reaction Force, Lower Limb Stiffness and Coordination Asymmetries During a Landing Task

Preparing to Land: Hamstring Preactivation Is Higher in Females and Is Inhibited by Fatigue

The hamstring plays an important role in reducing loads born by the anterior cruciate ligament. As anterior cruciate ligament injuries occur rapidly after ground contact, how the hamstring is activated prior to landing can influence injury risk. The purpose was to determine sex-related differences in hamstring activation immediately before landing and the effect of fatigue on "preactivation." Twenty-four participants (13 males and 11 females, age = 24.3 [6.5] y, mass = 72.2 [19.3] kg, height = 169 [9.7] cm) participated in this study. Participants completed a drop-vertical jump protocol before and after a lower body fatigue protocol. Hamstring electromyography (EMG) amplitude at 5 periods prior to landing, peak vertical ground reactions forces (in newtons/body weight), rate of loading (in body weight/second), and landing error scoring system were measured. Females had higher EMG amplitude before and after fatigue (P < .024), with decreased EMG amplitude for both sexes after fatigue (P = .025). There was no change on vertical ground reaction force, rate of loading, or landing error scoring system. Males and females demonstrated similar landing performance before and after fatigue but have different hamstring neuromuscular coordination strategies. The acute reduction in hamstring EMG amplitude following fatigue may increase loading on the anterior cruciate ligament.

The Effect of Repetitive Drop Jumps among Different Heights on Bilateral Asymmetry of Countermovement Jumps

Background: The study explored the influence of repeated drop jumps (DJs) from different drop heights on the lower extremity bilateral asymmetry and muscle activation of countermovement jumps (CMJs). Methods: Eighteen male athletes performed 200 drop jumps (DJs200) from three drop jump height (DJH30, 40 and 50 cm). The CMJs were performed before the first DJ and after the 50th, 100th, 150th and 200th DJs, recording them as pre-CMJ, CMJs50, CMJs100, CMJs150 and CMJs200. One-way repeated measures ANOVA was used to compare differences among the three drop heights at pre-CMJ, CMJs50, CMJs100, CMJs150 and CMJs200, respectively. Results: The peak ground reaction forces (PGRF) of CMJs100, CMJs150 and CMJs200 at DJH50 were greater than at DJH30 and DJH40 (all p < 0.05). The muscle activation during CMJs50 at DJH50 was greater than at DJH30 and DJH40 (all p < 0.05). The muscle activation during CMJs100, CMJs150 and CMJs200 at DJH50 was smaller than at DJH40 and DJH30 (all p < 0.05). The PGRF had no significant difference among the three different drop heights during CMJs50 (p > 0.05). Conclusions: The DJs50 at DJH50 had no effect on the bilateral asymmetry and increased muscle activation of CMJs. The excessive DJs100 at DJH50 increased bilateral asymmetry and decreased CMJ muscle activation during CMJs.

Different Drop Heights in Bilateral Asymmetry and Interjoint Coordination during Repetitive Drop-Jumps

The difference of drop heights will affect the biomechanics of lower extremities during drop-jump (DJ) landing. Therefore, this study explored the effects of drop heights and training volumes on interjoint coordination and the side-to-side asymmetry of the lower extremities during landing. Twenty males were randomly assigned to perform 200 DJs (DJs200) from 30, 40 and 50 cm (drop-jump height (DJH) 30, DJH40 and DJH50) platform. One-way ANOVA repeated measure, using MATLAB software, was used to compare the differences of interjoint coordination, side-to-side asymmetry of ground contact time (GCT) and the maximum impact in vertical ground-reaction forces peak (I-vGRFpeak) in the 1st, 50th, 100th, 150th and 200th jumps (DJ1, DJs50, DJs100, DJs150 and DJs200). To examine whether significant differences exist, the least significant difference’s (LSD) method was used for post-hoc comparison. The mean absolute relative phase (MARP) and deviation phase (DP) of hip–knee were lower than DJH50 at DJH30 and DJH40, while side-to-side asymmetry of GCT and I-vGRFpeak were greater than DJH30 and DJH40 at DJH50 within DJs200 (all p <0.05). However, there was no significant difference in MARP and DP of hip–ankle and knee–ankle. Therefore, training at DJH30 may effectively improve jumping performance and reduce musculoskeletal injury risk.