The purpose of this study was to compare the immediate effect on young athletes of a single session of RM (versus PSS and PL) on their hamstring muscles’ flexibility and strength. The study’s results suggest that PSS, RM, and PL interventions all increase hamstring muscles flexibility straightaway, but only the improvement related to RM intervention was found to be significantly higher compared to the PL intervention. Moreover, according to our results, none of the considered interventions had a significant acute effect on MVC.
RM intervention showed an immediate effect on hamstring flexibility, with a significant increase in the sit-and-reach test scores compared to the PL intervention. Our findings are similar to those of Sullivan and colleagues , who compared a group of healthy participants undergoing four different durations of RM (1 set of 5 s, 1 set of 10 s, 2 sets of 5 s, and 2 sets of 10 s) to a control group (no intervention). In that study, the subjects had to perform a single-leg sit-and-reach test for ROM, an MVC force test, and a measure of muscle activation of the biceps femoris (electromyography) for the muscle strength. The authors found an increase in the hamstring ROM up to 4.3%, with no change in MVC and EMG activity. Moreover, their results seem to indicate that there is a time-related effect between RM intervention exposure and increases of muscle flexibility. The improvement in ROM found in our study was 5%. This slightly different result could be attributed to the different population (young athletes, instead of recreationally active persons), the alternative testing modalities (two-legged sit-and-reach test, instead of a single-leg test), or the different duration of the intervention (60 s, instead of a maximum of 20 s). Our findings could be considered as an evolution of the findings of Sullivan et al. , suggesting that a prolonged RM intervention can lead to an augmented acute increase of ROM, without affecting the muscular performance. However, findings from our pilot test suggested that the maximum increase in flexibility is reached between 30 and 90 s of treatment, and a longer exposure did not further augment muscle flexibility.
Conversely, Monteiro and colleagues  compared four self-massage interventions, differentiated in terms of type of massage and volume of exposure. In their study, self-massage of the hamstring resulted in increased short-term hip flexion and extension ROM, which became more evident with higher dose volumes. Monteiro et al.  strengthened the hypothesis of time- and dose-related change in ROM for longer interventions as well. However, the RM was self-applied and the pressure was not controlled. Differences in time- and dose-related change could therefore be attributed to the different treatment intensities.
In addition, studies conducted on other muscles have found similar increases in muscle flexibility. Among these, Halperin and colleagues  studied the effect on passive ankle ROM, MVC, and balance of static stretching (“SS”) and self-massage with a RM on the calf muscles. They found that both interventions increased ankle ROM at 0 and 10 min after exposure to the intervention. Moreover, they found that RM led to a small improvement in MVC at 10-min post-intervention.
To our knowledge, no previous studies on roller massager on hamstring have included a placebo-treated group applied to the target muscles. However, Phillips et al.  and Griefahn et al.  investigated the effect of rolling and a placebo intervention on the flexibility and performance of quadriceps and triceps surae muscles and of the thoracolumbar fascia, respectively. Similar to our results, both studies found an increase of flexibility of the target muscles/fascia [29, 30]. Contrary to our findings, though, Phillips and colleagues  found a negative effect of 5 min of rolling on muscle performance.
PSS intervention showed an acute increase of hamstring flexibility, but the effect was not superior to that from the placebo intervention. The results of our study do not totally share those found in previous studies. In fact, more studies have affirmed that SS of hamstring muscles is effective in increasing their flexibility [14, 40,41,42]. For example, Michaeli and colleagues  examined the effect of two different stretching interventions and a placebo intervention on hamstring flexibility, immediately after the intervention and 1-h post-intervention. In contrast to our results, they found that SS intervention is effective in enhancing acute hamstring flexibility, compared to a placebo intervention. The difference between our results and those of Michaeli’s group could be attributed to the different population, the different testing modalities, or even the different placebos. In fact, as the authors underline , the placebo that was applied in their study (a sham ultrasound on the dorsal face of the foot) was not properly blinded to the participants. Conversely, the participants of our study believed that they were receiving a treatment on their hamstring muscles.
Kataura and colleagues  studied the effect of SS on hamstring muscle flexibility and muscle force in relation to the application intensity. The authors defined the stretching intensity with a percentage of the ROM recorded at the point of maximum tolerable intensity of stretching without pain. They pointed out that only stretching interventions with an intensity greater than 80% significantly increase hamstring flexibility. Moreover, they found that with 100% and 120% intensity of stretching, there is a significant reduction of isometric muscle force .
Our PSS intervention was in line with that of Kataura et al. in terms of intensity and/or time. Therefore, the differences between our results and theirs could be attributed to different study populations, different testing modalities, or even the presence of a placebo in our study.
To our knowledge, no prior studies have compared the effect of SS with that of an RM on hamstring muscles. The only study featuring SS and RM on hamstring muscles is that of Hodgson et al. , which combined the two techniques and compared the effect on hamstring flexibility and performance of SS alone and of SS with one or more adjunctive treatments with RM. The authors concluded that SS increases flexibility, and that the addition of RM makes the change more significant or prolongs it over time . Their results also suggest that SS reduces muscle performance whereas the addition of RM does not influence it .
Our results are in contrast with those of this earlier study. Although we tried to standardize the application of the stretching procedure (i.e., reporting the degree of perceived pain and maintaining a stretching duration in line with those proposed in the literature), we cannot exclude that our application was under-dosed. Moreover, we used different testing modalities and included a placebo-treatment to compare with the interventions.
Our results also showed that the group treated with the sham ultrasound increased the flexibility of their hamstring muscles, supporting the “placebo effect” already reported in previous trials [44, 45]. The observed improvement may be explained by positive expectations associated to the placebo ultrasound intervention.
The magnitude of the improvement and the non-significant difference with PSS may appear surprising, but the nature of the outcome measure should be considered. The sit-and-reach test score can be influenced by the subject’s performance and by painful sensations, while the central pain-modulatory systems activated by the placebo effect can play a role too. Additionally, the control group underwent a placebo-treatment. Therefore, the between group differences represent the real effect of the interventions, since both treatments increased the sit-and-reach value.
The present study has a number of limitations that must be considered when interpreting the results. First, the sit-and-reach test is not specific, and did not allow us to discriminate the flexibility of hamstring muscles from the flexibility of the lumbar spine of participants. Although we treated the hamstring bilaterally and we did not perform interventions on the lumbar spine, even if we could hypothesize that the changes seen in the test were attributable to changes in hamstring flexibility, and acknowledging that the sit-and-reach test shows a high reliability , we cannot exclude the influence of other factors. Moreover, the enhancement of flexibility observed in our study could be attributed to the repetition of the testing modalities. Participants were asked to repeat the sit-and-reach test three times pre-intervention and three times post-intervention, which could have affected the viscoelastic properties of their tissues and enhanced the ROM of their hips.
Second, the population of this study included young athletes of which we did not record the level of activity and who practice different sports. The variability of these sports could influence the differences of physical and biological features of the participants and subsequently their results. This diversity is not only a limitation, though, because it better represents the variability of the population of athletes with whom a clinician, a fitness instructor, or a trainer can work.
In addition, the RM used in the study was designed for self-massage, which could bring about more variability and less standardization of the treatment. The results could be therefore less reliable if the device is managed and used by the individuals themselves.