Non-contact anterior cruciate ligament injuries in females: The effects of gender and developmental stage on landing biomechanics

The purpose of this study was to identify neuromuscular and biomechanical differences between pre and post pubertal males and females during landing from a vertical jump (VJ). Fifty-nine subjects with no history of back or lower extremity injuries were divided into age and gender groupings (15 girls, age=9.19, VJ=27.94cm; 16 boys, age = 9.63, VJ=31.75cm; 14 women, age=24.22, VJ=41.91cm; and 14 men, age=23.57, VJ=55.79cm). Subjects were familiarized with a functional jumping and landing task after providing informed consent. Surface electromyographic (EMG) electrodes were applied to the vastus medialis (VM), biceps femoris, medial hamstrings (MH), and gastrocnemius of the dominant leg. Twenty-three retro-reflective markers were applied to each subject to create an eight-segment model. Subjects jumped for a ball set at 50% of their maximum VJ in two jump conditions: straight (S) and offset (O). Condition order was randomized. For S, the ball was positioned directly in front of the subject’s midline. In O, the ball was offset from midline (children =12 inches, adults =18 inches) in the direction of the dominant leg, thereby requiring an upper body twist or bend and return to a forward facing position prior to landing. For all trials, subjects started from the same self-selected takeoff position and were instructed to land on two feet facing forward with only their dominant foot on the force plate. Subjects completed four successful trials of each condition. EMG signals were normalized to peak activity during the landing phase of each jump. Analyses focused on the dominant leg. Multivariate statistical analyses (a = .05) were used to compare the following parameters between groups and conditions: ankle, knee, hip and trunk angles at initial contact (IC) and 250 milliseconds (ms) following IC, maximum vertical ground reaction force (GRFz), mean GRFz from IC through 250 ms, and mean normalized EMG activity from IC through 250 ms. Post-hoc analysis for S revealed that girls had less knee flexion (57.50º ± 15.16) 250 ms following IC compared to women (76.16º ±19.75, p=.046) and men (77.00º ± 25.27, p=.034) and less maximum knee flexion (67.26º ± 10.81) compared with women (80.75º ± 15.72, p=.044). Girls took less time (0.16s ± .04) to reach maximum knee flexion compared to women (0.24s ± .09, p=.033) and men (0.25s ± .11, p=.019). Girls (7.12º ± 5.22, p=.005) and boys (6.51º ± 6.22, p=.002) had less hip flexion at IC compared to women (14.09º ± 5.12). Girls also had less hip flexion (25.58º ± 10.74) 250 ms after IC compared to women (45.95º ± 18.21, p=.004) and men (41.08º ± 19.28, p=.045) while boys had less hip flexion (28.25º ± 12.48) than women (p=.017). For O, girls reached maximum knee flexion earlier (0.17s ± .03) than women (0.29 ±.11, p=.003) and men (0.26 ± .10, p=.038). Compared to women (18.47º ± 4.21), girls (11.46º± 7.18, p=.014) and boys (10.33º ± 6.21, p=.003) had less flexion at IC and 250 ms after IC ( Girls: 31.96º ± 12.29, p=.012, Boys: 33.80º ± 10.24, p=.030, Women 49.02º ± 15.66). Boys had a greater hip abduction angle at IC (90.38º ± 6.81) compared to women (84.16 º ± 6.57, p=.038). Independent-samples t-tests (a = .05) were performed to compare VM and MH onset timing during the descent phase, prior to IC for the men and women. Onset was defined as the resumption of muscle activity after at least 10 ms of activity below 2% of peak EMG. No significant differences were found. For most children there was no identifiable off time, consequently, they were not included in this onset analysis. The brevity of the descent phase among prepubescent subjects may account for the lack of a discernable muscle rest period. These findings lend support to developmental differences in hip and knee kinematics during landing from a vertical jump, but do not support gender differences in landing biomechanics. Further analysis of joint kinetics may indicate the importance of these differences in ACL injury. Further research investigating the effect of gender and developmental level upon a functional task, such as landing from a vertical jump, is warranted.

2003 Decoster LC, Vailas JC: Functional Anterior Cruciate Ligament Bracing: A Survey of Current Brace Prescription Patterns. Orthopedics, Vol. 26, No. 7, July, 2003, 701-706.