Differences in strength between ‘agonist’ and ‘antagonist’ muscles are thought to heighten the risk of injury in athletes. For example, a popular belief is that if the quadriceps muscles are disproportionately stronger than the hamstrings, hamstring strains are more likely to occur. Along similar lines, imbalances in muscular strength between the legs are believed to produce both reduced athletic performances and an increased risk of injury, with the lower-strength leg at risk. However, it has not been entirely clear how differences in leg strength should be determined. Seated, one-leg-at-a-time exertions on exercise machines which isolate particular leg muscles are inadequate for assessing imbalances, since the movements carried out have little applicability to complex, weight-bearing, sport-specific motions in which the muscles of the entire leg work together in concert.
In a unique new study, researchers in the Biomechanics Laboratory at Ball State University in the US have taken a stab at measuring leg-strength differences in a more relevant way (‘Determination of Strength Imbalance of the Lower Extremities,’ Journal of Strength and Conditioning Research, Vol. 16(4), p. 7 (Supplement), 2002). In the research, 14 collegiate female softball players carried out back squats with the barbell loaded to 80% of the one-repetition maximum (the maximum amount of weight which could be squatted one – and only one – time). As the athletes squatted, individual force platforms placed under each foot recorded ‘vertical ground reaction forces’ (the amount of force each leg was applying to the ground) during three sets of three repetitions. This Ball-State investigation is the first study to actually measure vertical ground reaction forces under each foot during strength training.
As it turned out, significant differences in force production were found between legs, with the stronger leg producing about 6% greater average and peak vertical ground reaction forces. Such differences might well be related to performance and the risk of injury; one might reasonably argue, for example, that the weaker leg would be more vulnerable to injury and thus would be in need of shoring up. Further research is required, however, since there is no firm evidence that this is actually the case. In addition, athletes tend to use their legs in different ways and thus it may be natural for strength differences to be present. Many soccer players, for example, ‘plant’ with their left legs and kick with their right lower limbs, and many basketball players dunk or make powerful movements to the basket from their left legs. In addition, any unilateral (one-leg) strength-training (as part of an effort to make a weak leg as strong as the dominant one) would have to be very sport-specific. Otherwise, the unilateral gains in strength would only be apparent during the strength-training and not when they are really needed – during competition.