Friday, April 2, 2010

Activating the Quadriceps and Hamstrings – Implications for ACL Injury

Some estimate that women’s risk of anterior cruciate ligament (ACL) injury is eight times that of men. Why are women at such greater risk? Several ideas have been proposed and researched but the question remains. It may be that women have different neuromuscular control patterns when performing movements that are associated with ACL injury. The timing and degree to which the muscles surrounding the knee are activated can either increase or decrease the stress placed on the ACL. More stress = greater injury risk. A new study from researchers at Marquette University tested this idea and found that men and women do in fact control hamstring and quadriceps muscle contractions differently.

The researchers focused on how men and women activated their hamstring and quadriceps muscles during soccer-type activities – landing and cutting. To do this, they recoded electromyographic signals from two hamstring muscle (medial and lateral) and three quadriceps muscles (vastus medialis, vastus lateralis and rectus femoris). They were interested in both the timing of activation and the degree of activation both before (anticipation) and after (during force production) ground contact.

The key findings were that during both exercises, women activated the hamstring muscles to a lesser extent than men. In particular, the post-contact activation level was smaller. Second, the duration of quadriceps activation was longer in the women than in the men. Third, the ratio of hamstring to quadriceps activation tended to be greater in man than in women. These data indicate that immediately after ground contact, women generate less force with the hamstrings and contract their quadriceps slightly longer than men.

What does all of this mean? There are gender differences in how the quadriceps and hamstring muscles are activated during cutting and landing. In both maneuvers, women tend to be more quadriceps dominant whereas men tend to be more hamstring dominant. That is, women rely on their quadriceps to a great extent when changing direction whereas men involve their hamstrings to a greater extent.

There is little doubt that this quadriceps dominance has important implications for ACL injury risk. Greater quadriceps activation results greater force during landing or cutting. This could pull the tibia forward, placing added stress on the ACL and increasing the risk of injury. On the other hand, greater hamstring dominance in the men could have the opposite effect. Increasing hamstring activation during these movements would tend to prevent forward tibia movement which in turn, would unload the ACL and help stabilize the knee.

What can be done of overcome this possible liability in women. That’s not entirely clear. However, it seems logical that strengthening the hamstring muscles along with balance and plyometric exercises would be useful. Several ACL injury prevention programs have been very effective in lowing both risk and incidence of ACL injuries (CLICK HERE). A common theme of these programs is increasing hamstring strength and altering activation patterns through plyometric and balance exercises.

The bottom line, men and women differ in how they activate their hamstring and quadriceps muscle during soccer-type maneuvers. Focusing on correcting these differences could narrow these differences and lead to reduced risk of ACL injuries in women.

Reference:

Ebben WP, Fauth ML, Petushek EJ, Gargeau LR, Hsu BE, Lutsch BN, Feldman CR(2010)Gender-based analysis of hamstring and quadriceps muscle activation during jump landings and cutting. Journal of Strength and Conditioning Research, 24:408-415.