Friday, July 13, 2012

Fatigue, Balance and injury - Learning from Gymnasts

There is no doubt that a loss of balance can lead to injuries. As players lose their balance, unwanted stresses are placed on the ankle, knee and hip. Fatigue can also raise the risk of injury. Muscles lose their ability to produce the necessary force at the correct times to stabilize and protect joints and ligaments from injury. Most of the studies on fatigue and joint injury focused on the limb muscles (hamstrings, quadriceps and ankle muscles). But what about the core muscles? Does fatigue of these muscles affect balance and raise injury risk? Researchers from the University of Amsterdam addressed this question by looking at core fatigue and balance in young gymnasts. They found that when the core muscles experience fatigue, balance and core stability is affected. This bit of information from gymnasts has important implications for other athletes, especially soccer players.
In this study, the researchers measured balance and “core stability” in a group of young, elite gymnasts from The Netherlands (average age 12 years). The test required the gymnasts to stay seated as still as possible on a wobble seat and they were asked to regain balance after the wobble seat was suddenly moved. On one occasion, measurements were made before the start of a training session. On the other, they were made after the gymnasts did a series of “dump handstands”. This exercise is a regular part of training and was designed to fatigue the core muscles (abdominals, obliques and back muscles).

In the fatigued state, the gymnasts showed decreased balance and reduced ability to control their trunk position. They showed increased wobble and reduced ability to regain their balance. Thus fatiguing the trunk or core muscles reduced stability and balance in a group of trained gymnasts.

So, why post an article about gymnastics on a soccer site? This paper actually implications for soccer players. Several research studies show that trunk or core stability is a key risk factor ACL injury, especially females. Balance is achieved when to body’s center of gravity remains within the base of support. Balance is lost as the player leans to one side, forwards or backwards. When this happens and the center of gravity passes outside of the base of support, a torque is produced and movement occurs. If this position is not corrected, the player will fall in the direction of the lean. Thus, a critical component of balance is the ability to maintain the trunk’s center of gravity within the player’s base of support. This is the concept of “core control” – the ability to control the core to avoid an unbalanced position.

Unfortunately, the correction for lost core control often occurs at the hip and knee (the third figure). To maintain balance, the player with typically flexes the hip and moves her knee inward into a knock-kneed position. This shifts the center of gravity back inside of her base and provides balance. However, it is this knee movement, called knee valgus or abduction that places stress on the ACL and greatly raises the risk of damage (link). Thus, a lack of core control and failing to maintain trunk stability can cause unwanted movements that greatly raise the risk of ACL injury.

As for knee stability and ACL injury prevention, a second key component is the strength of the muscles surrounding the knee. Specifically, the knee flexors (hamstring and gastrocnemius muscles) and extensors (quadriceps) all help stabilize the knee and protect the ACL. Earlier, we discussed how fatigue of these muscles can affect knee stability (link). When these muscles are fatigued, the biomechanics of landing and cutting are changed (increased knee valgus), placing the ACL at risk of injury.

Back to the question of gymnasts. Gymnasts spend hours training the core muscles and developing balance – both key traits of an elite gymnast. One could argue that if fatigue of core muscle in gymnasts affects their trunk stability, a lack of core strength could certainly reduce trunk stability in a soccer player. During the run of play, the player’s body position is constantly changing – stopping, starting cutting and turning. Without adequate trunk stability, the player may not be able to control the trunk’s position and keep its center of gravity within the base. That is, balance is easily lost and some corrections are needed. Add to that, fatigue of the thigh muscles, and stability of the knee and stress on the ACL could become a problem. Thus, weak abdominals and weak back muscles can lead to balance problems. Couple that with weak and fatigued hamstring muscles, and the player may be at increased risk of injuring the ACL. Again, if this occurs in gymnasts who focus training on core muscles and balance, it can certainly happen in soccer players.

For this reason, the various neuromuscular training programs such as the FIFA 11+ emphasize core strength and fitness. By increasing core strength and balance, players are much better able to maintain trunk stability. That is, they are less likely to allow the trunk’s center of gravity to slip outside of the base of support. This, should lower their rise of knee and ankle injury.

Reference

van Dieën JH, Lugar T, van der Eb J (2012) Effects of fatigue on trunk stability in elite gymnasts. European Journal of Applied Physiology, 112: 1307-1313.