Improved hamstring strength is an oft-cited factor in preventing knee injuries to the anterior cruciate ligament. Strength imbalances between the knee extensors and the knee flexors in the same leg amplify the strain on this ligament, increasing the risk of a tear. Weaker hamstrings on one leg may predispose you to other muscle, tendon or cartilage injuries, such as hamstring strains. Previous injury increases the chances of inter-limb imbalances, leading to further risk of re-injury.
Meaning
The joints in the human body have muscles that act in contradiction to one another, called agonists and antagonists. During knee extension, the hamstring muscles, which are knee flexors, act as antagonists, helping to control the movement and prevent injury. Some muscle groups should be stronger than their counterparts, but imbalances can still occur and predispose you to injury. Muscle imbalances sometimes arise between the same muscles in different limbs. This is most evident in the feeling of limb dominance that healthy individuals experience, but it can become problematic over time.
Influencing Factors
Several factors affect the prevalence and severity of hamstring strength differences. In a study that examined functional strength imbalances in the lower extremity, Robert Newton and colleagues from Edith Cowan University in Western Australia found that handedness, specific sport demands for athletes, or a history of previous injury could contribute to the differences in knee flexor strength between limbs. The general public, whether active or sedentary, is also at risk for muscle imbalances through continued, repetitive demands, such as the use of one limb more than another during daily tasks.
Significance
It is unclear whether hamstring weakness on one leg is the cause of injury, the result of injury, or both. Kieran O'Sullivan and fellow researchers at the University of Limerick found hamstring weakness in athletes with a history of hamstring strains, but they were not able to discern if the weakness existed prior to the initial injury. Nevertheless, it is apparent that there is a correlation between injury and weakness, indicating that maintaining equal strength between the knee flexors on each leg could be an important aspect of injury prevention and should be a focus of rehabilitation efforts.
Considerations
Performing hamstring-specific exercises increases hamstring strength, but might not decrease an imbalance if one exists. When you do resistance training with both legs at the same time, there is a tendency for the stronger leg to compensate for the weaker leg, further emphasizing the difference in force production between limbs. Therefore, movements that challenge the hamstrings one leg at a time might be necessary to overcome compensation patterns and equalize the inter-limb strength.
Expert Insight
Differences in hamstring strength between legs are common and most likely occur through repetitive motions, limb dominance or previous injury. However, if not managed, these imbalances could lead to further injury or decreases in sport performance. Ensuring that both legs are performing an equal amount of work during exercising or activities of daily living is a vital aspect of reducing disparities between limbs. It might not be possible to accomplish this while strength training the lower extremities simultaneously, so care should be taken to challenge each leg separately as a part of a regular health and fitness program.
References
- "Journal of Strength and Conditioning Research"; Determination of Functional Strength Imbalance of the Lower Extremities; Robert U. Newton, Aimee Gerber, Sophia Nimphius, Jae K. Shim, Brandon K. Doan, Mike Robertson, David R. Pearson, Bruce W. Craig, Keijo Hakkinen and William J. Kraemer; 2006.
- "BMC Musculoskeletal Disorders"; The relationship between previous hamstring injury and the concentric isokinetic knee muscle strength of Irish Gaelic footballers"; Kieran O'Sullivan, Brian O'Ceallaigh, Kevin O'Connell and Amir Shafat; 2008.
