Torque deficits may put limbs at risk
By Jordana Bieze Foster
Changes in muscle strength in athletes who have experienced a concussion may help explain the apparent association between concussion and lower extremity injury, according to research presented in May at the annual meeting of the American College of Sports Medicine (ACSM), held in Orlando, FL.
In 25 collegiate football players who experienced a concussion during the competitive season, researchers from the University of Florida in Gainesville found significant differences in isokinetic knee extension torque, knee extension torque asymmetry, and hamstring:quadriceps strength ratio at the end of the season compared to preseason testing.
Each athlete lost an average of 7.6 days of practice and competition time while recovering from the concussion. The average time from concussion to postseason testing was 95.6 days.
Knee extension strength decreased significantly from preseason to postseason, and was significantly more asymmetrical between limbs. Knee flexion torque and knee flexion torque symmetry did not differ significantly between tests, but hamstring:quadriceps ratio was weighted significantly more heavily in favor of the hamstrings at the postseason assessment than in the preseason.
The changes in knee extension torque and asymmetry may increase the risk of lower extremity injury following concussion, which the University of Florida group reported at the 2013 meeting of the American Medical Society for Sports Medicine in San Diego, CA (see “Study links concussion to increased incidence of lower extremity injury,” May 2013, page 13).
“Following concussion, athletes do demonstrate decreases in strength. This may be why they are at increased risk of lower extremity injury,” said Dominique F. DuBose, ATC, a graduate student in the university’s Rehabilitation Science Program, who presented the new findings at the ACSM meeting. “We may need to include measures of strength in determining return to play after concussion.”
The strength study did not include a control group, so the authors can’t be certain that similar strength changes might not have occurred in athletes with no history of concussion. Multiple studies have demonstrated that athletes tend to lose lean body mass over the course of a season, possibly because coaches tend to deemphasize strength training once competition begins.
Although those studies have not looked at the lower extremities specifically, a study published in the November 2004 issue of the Journal of Strength and Conditioning Research (JSCR) found that lower body strength decreased 14% during the course of a field hockey season. And an April 1989 study published in the International Journal of Sports Medicine found that both quadriceps and hamstring torques decreased significantly during a season of amateur ice hockey.
Conversely, however, a study published in the November 2006 issue of JSCR found that lean tissue mass in the legs and lower body power both significantly increased during the course of a season in collegiate soccer players.
All of this suggests that the University of Florida group’s finding that knee extensor torque decreased while knee flexor torque did not, and the finding of increased knee extensor asymmetry, appear to be different from what has been reported in typical athlete populations.
A second study presented at the ACSM meeting offered some positive news for concussed athletes, finding that the light aerobic exercise typically done in the first stage of most return-to-play protocols does not significantly affect gait termination or dynamic postural control.
Researchers from the University of North Carolina at Charlotte found that peak propulsive force during gait termination was significantly lower in 50 athletes with a recent concussion than in 17 controls, but 15 minutes of light exercise did not make it worse.
DuBose DF, Tillman SM, Moser MW, et al. Deficits in lower extremity strength following sports-related concussion in collegiate football players. Med Sci Sports Exerc 2014;45(5 Suppl):Abstract 133.
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Oldham JR, Evans KM, Munkasy BA, et al. Initiation of exercise following concussion alters planned gait termination strategy but not dynamic postural control. Med Sci Sports Exerc 2014;45(5 Suppl):Abstract 130.