Intervention
All teams in the intervention and control groups followed
their usual training program. In addition, the teams in the
intervention group performed 27 sessions of the Nordic hamstring exercise in a 10-week period during the midseason
break (Figure 2). The Nordic hamstring exercise is a partner
exercise. The athlete starts in a kneeling position, with his
torso from the knees upward held rigid and straight. A training partner applies pressure to the athlete’s heels/lower legs
to ensure that the feet stay in contact with the ground
throughout the movement. The athlete then attempts to
resist a forward-falling motion using his hamstring muscles
to maximize loading in the eccentric phase. The participants
were asked to brake the forward fall for as long as possible
using the hamstrings. The athletes were asked to use their
arms and hands to buffer the fall, let the chest touch the surface, and immediately get back to the starting position by
pushing with their hands to minimize loading in the concentric phase.22 The exercise was conducted during regular
training sessions and players were supervised by their (physical) coach, who was informed about the exercise orally and
had received written descriptions and illustrations of the
exercise. The team coaches decided when the exercise was
performed during the training session but they were advised
not to use the Nordic hamstring exercise before a proper
warm-up program. The teams followed the training protocol
presented in Table 1. After the start of the second half of the
2007-2008 playing season (spring 2008), the exercise was
conducted once a week with 3 sets of 12, 10, and 8 repetitions,
respectively. However, the exercise was not conducted during
the vacation period in the preseason (2-3 weeks). Compliance
of the training program and adverse effects were registered
for each team on a weekly basis by contact with the coaches
during the 10-week training period