Introduction Hamstring muscle strains can lead to an increased risk of reinjury. We recently proposed a clinical classification system for acute hamstring strains. We studied the effect of the grade of the initial injury on the subsequent risk of reinjury. We hypothesised that there would be no difference in reinjury rate between acute low grade (grade I and II) and high grade (III and IV) hamstring muscle strains.

Methods Between 1994 and 2007, we managed 165 elite Greek track and field athletes with acute, first-time unilateral hamstring muscle strains. Using a validated classification system, strains were classified into four grades (I, II, III and IV) based on objective clinical criteria (knee active range of motion deficit at 48 h). The same rehabilitation protocol was prescribed to all athletes and the rate of reinjury was recorded during the following 24 months.

Results At follow-up, 23 of the 165 athletes (13.9%) had experienced a second hamstring muscle strain. Of the 75 athletes presenting with a grade I injury, 7 (9.3%) had experienced a recurrence after 24 months. Of the 58 athletes presenting a grade II injury, 14 (24.1%) experienced a recurrence. Of the 26 athletes presenting a grade III injury, 2 (7.7%) experienced a recurrence and of the 6 athletes presenting a grade IV injury, none had experienced a recurrence after 24 months.

Conclusions According to our classification, athletes with acute grade II hamstring muscle strains experience a higher risk of reinjury than athletes with grade I or grade III strains. Low grade hamstring muscle sprains lead to a higher risk of reinjury than high grade hamstring muscle sprains. Objective clinical findings can accurately predict the risk of reinjury following acute hamstring muscle strains.