Young women demonstrate less strength and neuromuscular control after puberty, and this may make them particularly susceptible to Anterior Cruciate Ligament (ACL) injuries, according to research presented this week at the Association of Academic Physiatrists Annual Meeting in Sacramento, Calif. The findings of this study suggest early strength training could help prevent this common knee injury that can often have a significant, long-term impact on musculoskeletal health and physical function.
The ACL is one of four ligaments that help hold the knee together. Females have up to eight times the risk of sustaining an ACL injury when compared to males, and this difference between the sexes typically appears during puberty (roughly ages 11 through 14). It is suspected this higher risk is based on alterations in neuromuscular control — the ability to control the body when in motion — and inadequate strength development in females during that time.
Researchers from the University of Florida recently studied neuromuscular control and strength development through different phases of female maturity to determine the role they play in females' increased risk of developing an ACL injury during physical activities, such as jumping.
"Compared to males, females tend to develop different strategies of movement during maturation, which place them at a higher risk of injury," explains Lead Investigator in the study, Daniel Herman, MD, PhD; assistant professor in the Department of Orthopaedics and Rehabilitation at the University of Florida. "Studies have shown that we can correct these strategies with injury prevention programs, but it would be best if we could prevent these high-risk strategies from developing in the first place. We feel that improving strength development during maturation may be the key."
Dr. Herman's team studied 56 healthy middle-school-aged females. Based on the Maturation Observational Scale - which differentiates puberty based on indicators such as growth, breast development, menstruation status, body hair, acne and sweating during physical activities - 18 of the participants were considered pre-pubescent (being just under 12 years old); 21 participants were considered in puberty (around the age of 12); and 17 were considered post-pubescent (being just under 14 years old).
The researchers asked each participant to perform three jump landings where they jumped from a box to the ground and immediately jumped vertically as high as possible after landing. Using the Landing Error Scoring System, or LESS, - which uses an 18-point checklist for evaluating motion from both the side of the body (e.g., knee bending) and the front of the body (e.g., knees coming together or apart) - and two-dimensional video, the researchers were able to assess the neuromuscular control of each participant during the exercise. Additionally, they tested the participants' strength in knee extension and flexion as well as hip extension and abduction.
Dr. Herman's team found that post-pubescent participants had higher knee flexion strength when compared to those who were pre-pubescent, but there were no other differences in strength between the groups. The post-pubescent group also had higher LESS grades, which indicates worse neuromuscular control, than both the pre-pubescent and pubescent group. Finally, the post-pubescent group had strong negative correlations between knee extension and LESS and hip abduction and LESS.
"Males tend to have significant increases in strength as they mature; this allows them to cope with the increases in weight and height that they also experience during maturation," says Dr. Herman. "The lack of group differences between the females in this study suggests a lack of similar increases in strength during maturation. These differences have been noted in other studies, but this is the first to our knowledge which demonstrates such a strong direct correlation between neuromuscular control and strength. We think that a relative lack of strength to cope with increases in size and height serves to drive the differences in motion patterns that we see develop between males and females during maturation."
Dr. Herman notes that more research should be conducted in this area, but these results already point to an opportunity to add to middle school physical education curricula by adding basic strength training during this critical time period of growth and development for young females.
Association of Academic Physiatrists (AAP)