Background The Nordic hamstring exercise (NHE) has been utilised within Hamstring Strain Injury prevention programmes. However, there is a concern amongst some that once a football season has commenced, there is insufficient time to include the NHE due to short turn around between matches. To date, changes in strength have not been documented over a season.

Objective To determine if changes in strength measured during the NHE using a commercially available device can occur as the season progresses.

Design Time series design that involved measures of Nordic exercise strength (NES) obtained throughout the 2015–2016 season whist undertaking a hamstring strengthening intervention.

Setting Professional Football Academy.

Patients (or Participants) 33 male football academy players aged between 16 and 21 years.

Interventions (or Assessment of Risk Factors) One session of 2 sets 3–6reps NHE in addition to normal posterior chain strengthening exercises weekly.

Main Outcome Measurements NES (mean of left and right limbs) and between limb difference (%) were analysed by phase of the season (i.e., pre-season pre-Christmas, post-Christmas and end of season periods).

Results At pre-season the NES and between limb difference baseline data was 337±68N and 9±6%. Increases were observed in NES from pre-season to pre-Christmas (16N, P=0.0126) and post-Christmas to end of the season (15N, P=0.0180). Over the Christmas period the increase was smaller (5N, P=0.3871). The between limb scores, however, remained stable (P=0.6010). Post-hoc analysis revealed a negative association (r=-0.65) between preseason NES and the change in NES from baseline to end of the preseason.

Conclusions Most players increased their NES over the season, yet imbalance between limbs remaining stable throughout. Stronger players of the squad at the start of pre-season did not improve as much as their weaker colleagues. The weekly low volume stimulus may offer an adequate stimulus to develop strength.