In elite soccer, muscle injuries represent 18% to 37% of all time-loss injuries and hamstring injuries have become the most common single injury. This paper investigates the utility of one of the most commonly used measures to guide return to play (RTP), muscle strength1.
Hamstring strength is generally considered to be an important criterion when determining whether a player is ready to RTP following a hamstring injury. Side-to-side strength differences are often used in decision making however there is little evidence to support this. A side-to-side difference of <10% is often suggested as an important criterion. To date, no study has determined whether differences exist when comparing the strength of the injured limb at RTP with the preinjury level.
This study included 41 professional male soccer players with an MRI proven hamstring injury. The isokinetic strength of the injured limb at RTP was compared with preinjury screening results, and the side-to-side difference was determined at both time points. Interestingly, side-to-side differences in isokinetic hamstring strength at RTP were similar to preinjury levels. At RTP, the average strength of the injured limb was >95% compared with preinjury strength. An interesting finding was that a 10% difference in side-to-side strength (positive or negative) is present in a large proportion of players at RTP after hamstring injury and is even observed in a similar proportion of players pre-injury (63.4% weaker, 57.9% stronger). The distribution of positive and negative isokinetic strength differences when comparing strength at RTP with preinjury measures were similar, urging clinical caution when interpreting these results.
The use of isokinetic strength testing, in isolation, as a criterion for RTP is not supported by this study. Importantly, side-to-side isokinetic strength differences appear to be common and should be interpreted with caution. The previous accepted cut-off of 10% may be inappropriate, as side-to-side strength differences of 5%–20% are commonly found, both at RTP and pre-injury.
1. van Dyk N, Wangensteen A, Vermeulen R, et al. Similar Isokinetic Strength Preinjury and at Return to Sport after Hamstring Injury. Med Sci Sports Exerc. 2019;51(6):1091-8.