Monday, July 28, 2014

Hamstring Strength, Fatigue and Knee Stability

The incidence of anterior cruciate ligament injuries is all too common. In the U.S., it is estimated that as many as 200,000 injuries occur per year. Most of these are non-contact, meaning that movements associated with activity (landing, stopping, turning and cutting) stress the ACL, causing it to rupture. Some also estimate that women are five to eight times more susceptible to ACL injury than their male counterparts. Over the past 20 years, we have begun to understand the mechanisms of these types of injuries as well as ways to reduce risk and prevent ACL tears. A key risk factor is weak hamstring muscles.  Weaker hamstrings increase risk.  A new study shows that when hamstring strength is reduced by fatigue, the ACL is subjected increased stress, particularly when planting the foot and cutting.  the study emphasizes the need for improving hamstring strength as an important means of lowering injury risk.

Seventeen active females performed a series of sidestep cutting maneuvers before and after undergoing a fatiguing bout of hamstring contractions. For the cutting maneuver, they were asked to run towards a force place, plant their right foot and cut to the side at a 45 degree angle. This was done several times. After the first round of trials, the subjects performed a series of hamstring contractions (hamstring curls) designed to induce fatigue and reduce hamstring strength. Afterwards, they repeated the cutting maneuvers.

The subjects were fitted with several joint markers and electrodes that were used to measure movements and muscle activity. Computer measurements of muscle activity, limb and joint movements were then entered into a three dimensional knee model to estimate loads placed on the ACL.

The figure (taken from the study) shows the load on the ACL during the sidestep cutting maneuver. When the foot touches the ground, ACL load increases, reaches a peak, then decreases at the athlete pushes off and the foot leaves the ground. The red line represents ACL load before the hamstring muscles were fatigued. The black line is the load immediately after. Notice the difference in peak force. The data show that when hamstring strength was reduced, the load on the ACL was increased by 36%. Most importantly, the forces exerted in the sagittal plane (viewed from the side as the tibia slides forward) and in the frontal plane (as the tibia undergoes knee valgus) were increased.

What seems to contribute to increased load on the ACL is decreased hamstring strength. During the cutting maneuver, peak forces produced by the hamstrings were reduced by 27%. With the hamstrings providing less force to stabilize the knee, the ACL has to shoulder more of the load.

The researchers also found that cutting while fatigued reduced the amount of hip, knee and ankle flexion. This in turn, increased the impact force as the foot hit the ground. This is, planting the foot with less hip, knee and ankle flexion causes it the “stop” more quickly, increasing impact. Increased impact forces add to joint stress and increase the load placed on the ligaments.

The key emphasis of the study is that when players execute a sidestep, cutting maneuver in a fatigued condition, weakened hamstring muscles can result in greater load placed on the ACL as well as higher joint impact forces. This increased load elevates risk of sustaining an ACL injury, possibly a ruptured or torn ligament.

We’ve known for years that weak hamstrings (especially relative to the quadriceps muscles) are a risk factor for ACL injures (link   . While the hamstrings flex the knee joint, they also play an important role in stabilizing the knee and protecting the ACL. During a stopping or cutting maneuver, external forces along with the quadriceps muscle force can cause the tibia to slide forward, twist or undergo valgus movements (abduction or a knock-kneed position). This is referred to as “valgus collapse” and is one of the primary causes of non-contact ACL injury. The hamstrings are designed to counter these unwanted movements, pulling to tibia backwards and limiting rotation and valgus. Thus, weakened or fatigued hamstrings are less able to stabilize the knee and protect the ACL. As shown in the study, stress on the ACL is increased.

The take home message of this study is that strengthening the hamstrings and preventing fatigue should help stabilize the knee and prevent ACL injuries. It is this reason that ACL injury prevention training programs include strengthening the hamstring muscles. All of the programs advocated by trainers and therapists include some kind of hamstring exercises like the Nordic curls. By strengthening the hamstrings AND preventing them from fatiguing during a match, we may be able to reduce load placed on the ACL and lower the possibility of injury.

Research has clearly shown that ACL injury prevention programs such as the FIFA 11+ do help. Through persistent and diligent use of these programs, both the risk and incidence of injuries can be lowered (link, link, link). As mentioned earlier, more than 200,000 ACL injuries occur each year in the US. This is tremendous personal and financial burden. Also the potential for complications later in life such as osteoarthritis are high. Thus, any program that lowers injury risk should be strongly encouraged. In this case, an ounce of prevention is well worth a pound of cure.


Weinhandl JT, Earl-Boehm JE, Ebersole KT, Huddleston WE, Armstring BSR, O’Coner KM (2014) Reduced hamstring strength increases anterior cruciate ligament loading during anticipated sidestep cutting, Clinical Biomechanics, DOI: 10.1016/j.clinbiomech.2014.05.013.