Neuromuscular fatigue has been implicated as a significant problem for individuals returning to sport following an anterior cruciate ligament (ACL) injury and reconstruction. Due to the high rate of re injury in those that have had an ACL reconstruction, one hypothesis is that neuromuscular fatigue will negatively impact strength performance, postural stability (single leg balance), and biomechanics during jumping and landing. It blows my mind that a female athlete with an ACL tear is 16 times more likely than a healthy female athlete to tear an ACL again.
The interplay between a previous injury, the resulting changes to the input to the brain, modified motor planning, and re injury is an interesting development in research. Today I wanted to dive a little more into fatigue, the impact on biomechanics, and how physical therapy and strength training can start to augment the problem.
Neuromuscular fatigue is a decrease the ability of an athlete to produce voluntary force in a muscle or group of muscles (McLean), which is combination of central and peripheral fatigue. Peripheral fatigue is a related to muscle damage and metabolic factors distal to the connection between the motor neuron and muscle fiber. Central fatigue can occur anywhere prior to the connection between the muscle and motor neuron (Figure 1). On the surface, quantifying or even getting a clear view of central fatigue seems impossible to examine in detail with any moving or jumping test. When I was reading over all the information on fatigue, I kept imagining when I ran a 400 meter race in track (just a horrible experience). The first two hundred meters felt fine but by the final curve, my ability to run and pick my legs up was basically gone. This fatigue was due to the metabolic byproducts produced from all out effort along with bumping against a central governor.
A repeated finding for individuals with ACL reconstruction is a consistent deficit of strength in the quads, hamstring, and glutes. This has been measured in variety of ways. It can be isometric, strength exercise, or a functional pattern. At Smith Performance Center, we use hand dynamometer, step down test, and strength norms with lifts.
Single leg balance is also a critical component that can capture brain changes that are not expected. We commented on this before that balance can become impaired if visual dominance develops. Our preferred method of single leg balance testing is the Y balance test however this requires the eyes to be open and will not quantify visual dominance unless stroboscopic glasses are used. This has been an interesting tool that I will dive into later. If you want more information, I wrote an article for lower extremity review here.
There are also numerous landing tasks that can be performed. A consistent finding has been that an athlete who tears their ACL demonstrates risky movements during jumping, landing, cutting, and pivoting. These movements appear to remain even after physical therapy and the rehabilitation process. This is a large problem since most ACL injuries are occurring during these types of movements and the rehab process is not successfully improving the movements. Our research has been focused on the tuck jump assessment and hop testing, but there are multiple ways to examine the strategies that an individual is using.
Overlaying all of these findings is the fact that performance deteriorates with fatigue. Unfortunately neuromuscular fatigue appears to hit those with a peripheral joint injury more (Ie. ACL injury).
Lets dive into the research.
Thomas, A. C., Lepley, L. K., Wojtys, E. M., McLean, S. G. & Palmieri-Smith, R. M. Effects of neuromuscular fatigue on quadriceps strength and activation and knee biomechanics in individuals post-anterior cruciate ligament reconstruction and healthy adults. J Orthop Sports Phys Ther 45, 1042–1050 (2015).