follow-up visits to refresh coaching skills or give players feedback
on their performance. Throughout the season it was up to
the coaches to make sure the exercises were performed properly
with high quality. Although the programme proved to
reduce the risk of several injury types, follow-up visits during
the season could have proved helpful in ensuring the quality
of the exercise performance and might possibly have resulted
in an even higher preventive effect.
The coach of each team recorded the injuries, the exposure
and the compliance. We did not monitor the validity and reliability
of their recordings. In cases in which the registration
form was not completed during or immediately after a training
session or match the coach had to complete the registration
form in a retrospective manner. However, recall bias is presumably
small, because the majority of the coaches followed the
protocol and submitted their registration forms on a weekly
basis. Also, all teams were offered an incentive, provided they
recorded all data throughout the study period. It is possible
that coaches completed and submitted the registration forms
merely to receive the reward, without ensuring the accuracy
of the recorded data. This may have impaired the reliability of
the submitted data.