Introduction:​ ACL injuries have an annual incidence rate of 1 per 3000 people for the general population. Seventy to eighty percent of these injuries occur in noncontact maneuvers that involve sudden deceleration. The likelihood of sustaining an ACL injury for females is two to eight times greater than males. Movement studies and cadaver simulation studies suggested the biomechanical risk factors for ACL injuries increased with greater proximal anterior tibial shear forces, lower knee flexion angles, greater knee valgus angle/moments, greater quadriceps EMG, and lower hamstring EMG during deceleration movements. While the effects of different training programs on the biomechanical risk factors of ACL injuries were well investigated and were shown to be effective in preventing ACL injuries, no research has been conducted to investigate the effects of detraining on the knee biomechanics during functional movements in highly trained athletes.

Methods:​ Twelve NCAA Division I female volleyball players participated in two stop-jump tests before and after a one-month season interval. Twenty-one retroreflective markers were attached on the subjects and four surface electrodes were placed on the muscle belly of the subject’s right thigh. Isometric maximum voluntary contraction tests were conducted for quadriceps and hamstring muscle groups. Subjects were asked to perform five trials of a stop-jump task as fast and high as possible. Muscle electrical activity was collected by a surface EMG capture system. Three-dimensional coordinate data of retroreflective markers were recorded using eight optical video cameras. Ground reaction force data were collected by a force platform. The EMG linear envelopes of each muscle group during the stop-jump tasks were normalized as a percentage of the corresponding maximum EMG in maximum voluntary contractions. An inverse dynamics approach was used to calculate the 3-D knee joint moments and resultant forces during the stop-jump tasks using a rigid body model. Exact 2-tailed Wilcoxon signed-rank tests (α=0.05) were used to compare dependent variables between pre-detraining and post-detraining stop-jump tests.

Results:​ Jump height at the time of post-detraining was significantly lower compared with jump height at the time of pre-detraining (p=0.001). Subjects had significantly smaller knee flexion angles at initial foot contact with the ground (p=0.042) and smaller maximum knee flexion angles (p=0.042) during stance phase at the time of post-detraining compared with pre-detraining. No significant differences were observed for frontal plane kinematic and kinetic data. Smaller pre-landing biceps femoris EMG (p=0.002) was found at the time of post-detraining compared with pre-detraining. No significant differences were observed for pre-landing vastus medialis oblique, vastus lateralis, and semitendinosus EMG as well as all muscles’ landing EMG.

Discussion:​ The decreased biceps femoris EMG could be the cause of decreased initial knee flexion angle which consequently resulted in a reduced maximum knee flexion angle. The decreased knee flexion angle indicates a possible increased strain on the ACL and thus an increased risk for ACL injury. Proper neuromuscular training programs should be implemented for highly trained volleyball players after detraining to recover their performance and motor control patterns for preventing ACL injuries. Future studies with longer detraining durations, larger sample sizes and diversities of sports are needed to confirm the generalization of the results to other ACL high risk sports and thoroughly understand the detraining effects on ACL risk factors.