The study design did not allow us to determine whether warm-up or static stretching had the greatest effect on flexibility. Both were performed on the same day to reflect the pre-participation situation, however further studies may differentiate between the effect of these factors. The warm-up only used a subjective instruction to the subjects to guide intensity, which limits reproducibility. The exclusion criteria meant that not all subjects with previous hamstring strain were eligible for the study. Therefore the results cannot be extrapolated to all athletes with previous injury e.g. those with good flexibility post-injury. Similarly, the participants were mostly young and male, and further study is needed in more diverse populations. The small sample size limits the conclusions that can be made, and further larger studies are needed. The sample size is however similar to previous similar trials [
29,
39] and the number in each group is in line with previous studies [
28]. All subjects were seated for 15 minutes on a standard chair with their feet on the floor after stretching, however their exact knee and hip angles were not standardised, which could have influenced the results after 15 minutes rest. Neither the subjects nor investigator were blinded to the stretching interventions performed. This bias was minimised however by the use of an independent observer to measure the ROM. We acknowledge that other research indicates these short-term increases are only maintained by following a suitable ongoing training programme [
53]. We attempted to have both groups perform a similar magnitude of stretching (3 × 30 seconds), however the nature of dynamic stretching means the dynamic group spent less time in a lengthened position. In addition, the number of repetitions of the dynamic stretch performed may have varied between individuals. Despite this, the amount of dynamic stretching was standardised to time to allow comparison with the static stretch, and to reflect the reality of using dynamic stretching in clinical practice. The alternative of asking each participant to perform a defined number of leg swings could have caused considerable variation in the duration of stretching performed, due to individual variations in the rate of leg swings performed. As stated already, this study considered only the effect stretching had on hamstring flexibility. It is acknowledged that other factors apart from flexibility (e.g. immediate performance) must be taken into consideration when choosing the type of stretching programme suitable for a client. No objective measure was available to verify the history of hamstring strain e.g. ultrasound. However, self-report has shown to be moderately valid for diagnosis of previous injury [
54] and been used in previous research on hamstring injury [
55,
56]. In addition, the length of time since injury, the duration of time away from sport, or rehabilitation protocols used since injury are unknown. Subjects with an injury in the past month were excluded as it was felt that natural recovery of ROM after injury in these subjects may have confounded the results. Similarly, those with no injury in the past 12 months were deemed to be less likely to display reduced flexibility. We did not examine all static and dynamic stretching techniques, and results may vary using different stretching protocols. There is still considerable debate about the number of repetitions and duration of stretches that is optimal [
1,
36,
57], but the protocol chosen here reflects common clinical practice. This study did not examine the effectiveness of other methods of increasing hamstring flexibility. Further studies are needed to investigate the role of other means of increasing hamstring flexibility e.g. eccentric training [
58] or other types of stretching e.g. PNF [
59]. This is particularly significant as eccentric training can also influence other potential risk factors (e.g. muscle strength) during rehabilitation [
60]. This study did not examine if increased flexibility resulted in an improved clinical outcome. Despite these limitations, the strengths of the study must be acknowledged. It would appear that the current study is the first to assess the effects of warm-up and static/dynamic stretching on hamstring flexibility in both previously injured individuals and closely matched uninjured controls. The reliability of the protocol was established, and the inclusion/exclusion criteria were clear and clinically relevant.