By Soey Park for Body1
Injuries sustained to the anterior cruciate ligament (ACL) are considered to be one of the most common problems faced by athletes. A recent epidemiological study conducted by the NCAA found a significant increase in the rate of ACL injuries (1.3%/year) over the past two decades. Though ACL injuries accounted for only 3-5% of all injuries, 88% of these injuries resulted in 10 or more days of lost playing time.
In the past few years, we have seen an increase in the number of studies being conducted to better understand and prevent ACL tears. Recently, the role of fatigue in knee injuries has emerged as a possible causal factor. The study of the relationship between fatigue and ACL injuries has gained momentum as epidemiologic data gathered from a number of different sports has shown that during the off-season, injuries are more likely to occur during the first few practices, whereas during the season, injuries occur more towards the later moments of a game.
Fatigue can be explained as occurring in two basic mechanisms. The first involves the peripheral nervous system (i.e. motor neurons, peripheral nerves, motor endplates, muscle fibers).The other involves the central nervous system (i.e. the brain and spinal cord). Peripheral fatigue is probably the easier of the two to understand, involving the inability of the body to sufficiently supply the contracting muscles with the necessary energy or other metabolites needed. Central fatigue is a little more complex. It involves a drop in neural drive (the exchange of signal/impulses between neurons) as nervous impulses from receptors in the fatigued muscles are inhibited. This inhibition acts on the motor pathways of the central nervous system, decreasing the outflow of motor impulses to all muscles.
Things to keep in mind
Research shows that ACL injuries usually result in 10+ days of lost time
Even if only one knee is fatigued, both knees will move dangerously, increasing the likelihood of ACL-related injuries
Include variability in training to sharpen anticipatory skills for unexpected moves/plays.
Ramp up warm up/work out sessions prior to the first day of practice to lower risk of ACL injury. Give the muscles in your knee time to get back into the game by easing into workouts rather than going from zero to sixty
Both central and peripheral fatigue may occur together or separately, depending on the situation. In terms of sports-related knee injuries, peripheral fatigue is the most common explanation for weakening muscles leading to ACL related injuries, but recent studies have pointed to the need to better understand how central fatigue affects athletes.
Numerous studies find that fatigue significantly affects the likelihood of ACL-related injuries. The usually also cite a clear need to focus on preventative measures. A recent study conducted by University of Michigan researchers found that fatigue contributes directly to ACL injury due to potentially harmful changes in the movements of the knees. In this study, participants were asked to perform repetitive one-legged squats until they could do no more. Then they were asked to perform various jumping and movement challenges to test for differences between the two legs. In the end, the study found that though only one leg was exercised, both legs showed signs of being tired. The potentially dangerous movements found in both the legs point to the need to better understand the role of central fatigue in sports-related injuries.
Another study conducted by researchers at the University of Michigan examined the role that fatigue and unexpected stimuli have on non-contact ACL injuries, or injuries sustained without direct physical contact with an object or fellow athlete. The subjects of this study were asked to perform a series of single-leg squats followed by a randomly ordered landing. A light stimulus presented prior to landing was used to signal unanticipated landings. The results of this study found that the unanticipated stimuli significantly and negatively affected the subjects’ knee kinetics.
In other words, a number of studies have found that the changes in lower limb mechanics induced by fatigue stem from substantial degradation in central processing and control pathways. These changes account for the occurrence of non-contact ACL injuries, which are then exacerbated when the movements are unanticipated.
Such findings have big implications for training programs. Trainers should focus on the interaction between fatigue and the central nervous system. Training athletes to sharpen their anticipatory skills in conjunction with routine training and strengthening exercises could greatly decrease the likelihood of ACL injuries.