Background The incidence of hamstring injuries (HSI) in professional rugby union is high. Nordic hamstring exercise (NHE) has been utilised within HSI prevention programmes. To date, little is known about the effect of NHE on HSI in professional rugby.

Objective To examine the effect of a HSI prevention programme on Nordic eccentric strength (NES), injury incidence and severity in professional rugby union.

Design Prospective time series design that involved measures of NES obtained throughout the 2015–2016 rugby season whist undertaking a hamstring strengthening intervention.

Setting Professional rugby union.

Patients (or Participants) 50 professional male rugby union players.

Interventions (or Assessment of Risk Factors) One session of 3 sets 3–6reps NHE in addition to normal posterior chain strengthening exercises weekly.

Main Outcome Measurements NES was measured at 3 monthly intervals through the season using a commercial device. HSI incidence and severity (days lost) were compared between two seasons.

Results From June 2015 to June 2016 NES remained stable (n=31; 5N; 95%CI=−52 to 42N; P=0.821; ES=0.06). Imbalance, however, reduced by 9% (95% CI=3 to 15% (n=31; P=0.005; ES=0.72). For season 2014–2015, number of HSI were 5.2/ 1000 match hours and 0.25/1000 training hours (total of 0.67/1000 hours). During the intervention season, HSI incidence was halved during match play (2.63/1000 match hours) yet, training HSI were 0.37/1000 hours giving a total incidence of 0.56/1000 hours. HSI severity reduced from 124 d in the 2014–2015 season to 99 d in the 2015–2016 season.

Conclusions One weekly NHE session successfully reduced match incidence, overall incidence and severity of HSI in professional rugby union. NES remained stable through the intervention and imbalance demonstrated significant reductions. Retention testing at the end of the 4 week off-season period revealed NES had returned to baseline, however imbalance remained significantly reduced.