Ruchelle Brown-Calvert | Muscular imbalances – a risk factor for musculoskeletal injuries

MUSCULAR IMBALANCE defines a state where the original tension within muscles with opposing action is altered such that one muscle or group of muscles is too short or tight and the other is too weak or lengthened. This changes the strength and mobility of muscles and is a common risk factor for musculoskeletal injuries in athletes.

They generally arise on the background of athletes inadvertently increasing strength exercises for a specific muscle group but to the detriment of others.

To explore this phenomenon further, we will take a closer look at quadriceps and hamstring muscle imbalance and how this increases the risk of hamstring injuries.

Primary function

The hamstrings are a group of muscles at the back of the thigh and their primary function is to straighten the hip and bend the knee. The quadriceps are a group of muscles located at the front of the thigh and their main action is to straighten the knee joint. Essentially, they have opposing functions.

The prevalence of hamstring injuries is quite significant and over a 10-year period (2007-2016), approximately 30 per cent of the clients seen at the University of the West Indies Sports Medicine Clinic were diagnosed with a hamstring injury. Sports that involved sprinting and kicking, such as track and field and football, were most commonly implicated.

Reduced hamstring flexibility, muscle fatigue, poor running style (overstriding) and a previous hamstring injury are well known risk factors, but often overlooked is the effect of quadriceps and hamstring muscular imbalance.

Athletes who participate in sports with a specific demand for sprinting and kicking often engage in resistance training to increase muscular strength primarily of the quadriceps with the assumption that this equates to explosive power, with little or no attention given to increasing hamstring strength.

While the association between the value of the hamstring to quadriceps ratio (H:Q ratio) and its effect on sports performance remains to be elucidated, a value greater than 0.6 is proposed to reduce the risk of hamstring injuries with a value closer to one in the athletic population considered as ideal.

Nerve activity

Strength training is a sensory stimulus that signals the brain to increase the (neural) nerve activity to muscles. When one increases the strength exercise of the quadriceps, the increased nerve activity causes it to increase in size and strength, but it also becomes overactive and shortens. If the quadriceps remain in that partially contracted state for long periods, its ability to generate maximum force is reduced.

On the other hand, the hamstrings which oppose the action of the quadriceps, if not strength-trained receive less neural activity and becomes underactive, lengthened, and weak.

The quadriceps and the hamstrings should work in an agonistic and antagonistic fashion whereby when the quadriceps shorten to straighten the knee joint, the hamstring should lengthen.

However, when an imbalance such as that described earlier exists, there is a greater pulling/lengthening of the weaker hamstrings, which is the perfect set up for a hamstring injury.

Hamstring-quadriceps imbalance also results in a pull on the knee joint into suboptimal positions that increases the risk of injuries such as anterior cruciate ligament injury.

In a case where hamstring-quadriceps muscular imbalances are identified, injury prevention strategies become very important and employing specific interventions, guided by trained exercise and fitness professionals such as sports physiotherapists will be warranted.

The National Academy of Sport Medicine describes four phases on the corrective exercise continuum to aid with correction of such dysfunctions.

First, inhibition techniques are utilised to reduce the overactivity of the muscles, with the aim of increasing mobility and flexibility. This is followed by lengthening techniques. Concomitantly, there is activation of the underactive muscle; the primary aim of this phase is to increase muscle strength. Finally, once the desired effect is obtained in the opposing muscle (groups), exercises are done to integrate their action.

Early identification of muscle imbalances is integral in mitigating injury risk, but client education on correct exercise programmes is a vital component in the armamentarium of injury prevention strategies.

Sport Pulse and Sport Matters are fortnightly columns highlighting advances that impact Sport. We look forward to your continued readership.

Dr. Ruchelle Brown-Calvert is the Head of Sports and Exercise Medicine and can be reached at ruchelle.brown@uwimona.edu.jm