for full article and charts Do differing types of training have different effects on biomechanical risk factors for ACL injury?
Resistance training programs can help reduce anterior cruciate ligament (ACL) injury risk by helping athletes perform “softer” landings after jumps.
However, can core stability programs or plyometric programs also help reduce ACL injury risk by modifying the biomechanics of drop landings? If so, how do they work?
This study set out to find out…
What’s the background?
Non-contact ACL injury is much more common in female athletes than in males. Indeed, studies indicate that females may be up to 3 – 4 times more likely to incur an ACL injury than males when playing the same sport (e.g. Grindstaff, 2006).
Certain biomechanical features of jump landings have been identified as more risky for ACL injury and some of these are more commonly observed in females than in males. Very generally, greater transverse and frontal plane joint angle ranges of motion (ROMs) and joint moments at the hip and knee (i.e. greater knee abduction, greater hip adduction and greater hip and knee internal rotation) are thought to place athletes at a greater risk of ACL injury.
Moreover, knee valgus is often considered a significant risk factor, as this involves hip internal rotation and adduction with knee abduction. On the other hand, greater sagittal plane joint ROM at the hip and knee (i.e. greater hip and knee flexion) are thought to reduce the risk of ACL injury by allowing for a “softer” landing.
Consequently, most injury prevention programs are constructed in order to help reduce the extent to which these biomechanical conditions are fulfilled during athletic movements. Such programs frequently include balance, lower body strength, plyometric and agility training in order to improve neuromuscular control. However, it is difficult to assess the effectiveness of the various elements of such programs and it is unclear which aspects are most useful for making positive changes to jump landing biomechanics and which are superfluous.
In the previous study review, we saw that resistance training can indeed increase the amount of knee flexion. This suggests that resistance training is a valuable part of an ACL injury prevention program. Some previous research indicates that plyometrics may be similarly helpful.
For example, Lephart (2005) tested the effects of a plyometric training program and of a resistance-training program on lower body joint angle movements over an 8-week period and found that both programs led to significantly increased hip flexion ROM at initial contact, peak hip flexion ROM, peak knee flexion ROM and time to peak knee flexion ROM. They also noted that both programs led to decreased peak knee flexion moments and peak hip flexion moments.
This suggests that we might expect plyometric training to have a similar effect as resistance training on sagittal plane lower body joint angle ROMs and joint moments.
What did the researchers do?
Who were the subjects?
The researchers wanted to compare the effects of a 4-week core-stability program with those of a 4-week plyometric program on lower body and trunk biomechanics during a drop jump. They therefore recruited 23 girls from 3 area high schools, aged 14.8 ± 0.8 years. The subjects were not randomly allocated to the groups and the control and core stability groups comprised athletes from both lacrosse and soccer teams, while the plyometric group comprised only lacrosse players.
What was the intervention?
The groups performed a 4-week intervention involving either no additional training (control), additional plyometric training (plyometric group) or additional core training (core stability group). The researchers designed the plyometric and core stability programs to last 20 minutes and to require no equipment.
What did the plyometric program comprise?
The plyometric program comprised a series of bilateral and unilateral jumps and skipping exercises focused on takeoff and landing form involving soft, balanced, and controlled movements. The exercises in the first 2 weeks comprised: forward/backward single-legged line jumps, side-to-side single-legged line jumps, high skips, distance skips, broad jumps, tuck jumps and alternating single-legged lateral jumps.
The exercises in the second 2 weeks comprised: forward single-legged hop, hop, hop, and stick, squat jumps, single-legged maximal vertical jumps, single-legged jump for distance, broad jump, jump, jump, vertical jump, 180-degree jumps, and single-legged lateral jump.
What did the core stability program comprise?
The core stability program was intended to improve coordination of the abdominal and lumbar stabilizers and hip extensors, external rotators, and abductors. The exercises in the first 2 weeks comprised: abdominal draw-in, side plank knee bent, side-lying hip abduction, side-lying hip external rotation (clam shells), crunches, lumbar extension with hands on head and walking lunges with hands on hips.
The exercises in the second 2 weeks comprised: hamstrings bridge with abdominal draw-in, side plank legs extended with abdominal draw-in, quadruped hip extension with external rotation and abduction, crunches with opposite elbow to knee, lumbar extension with upper extremities straight, squats with upper extremities overhead, and lunges with ball toss.
What tests did the researchers perform?
Before and after the 4-week training intervention, the researchers tested a number of variables during jumping including lateral trunk-flexion angle, hip-flexion, adduction, and internal-rotation angles, knee-flexion, abduction, and internal- rotation angles, hip flexion, adduction, and internal rotation external joint moments, and knee flexion, abduction, and internal rotation moments.