In this study, 90 participants with an acute hamstring strain injury will participate. Age restrictions will be delimited between 18 and 35 years old to rule out age-related confounding factors. Potential participants will be excluded from this study if they had a history of hamstring injuries, or a recent severe trauma or surgery in the lower extremity. A hamstring injury is defined as a football-related injury in the posterior thigh muscle region, preventing a player from participating in training or competition for at least 1 week. In addition, any contra-indication for occluding vascular flow (such as hypertension, risk factors for and/or history of cardiovascular pathologies or thromboembolism, surgery, fractures, severe obesity,...) will entail an immediate exclusion from the study.

Within 4 days after the onset of a suspected HSI the participant will undergo a magnetic resonance (MR) scan in order to acquire more insight in the severity, location and extend of the injury. The British Athletics Medical Team Classification will be used to classify HSI on the basis of grade (1: small tear, 2: moderate tear, 3: extensive tear or 4: complete tear) and injury site (a: myofascial, b: MTJ or c: tendon). \[Pollock et al.2014\] Each participant will be assigned an unique number and subsequently randomly assigned to either the control, experimental treatment or sham group by using an online research randomizer (http://www.randomizer.org) which is already used in previous literature \[van der Horst et al. 2015\]. This will result in a total of 30 participants per treatment group. The experimental group will perform, on top of a classic rehabilitation protocol, exercises while occluding the vascular blood flow. The use of low-intensity exercises with vascular blood flow restriction (BFR) has been found, in previous studies, to induce the same hypertrophic muscle adaptations as high-intensity exercises without BFR. The combination of increased metabolic stress, due to an Ischemic and hypoxic muscular environment, and exercise induced mechanical tension is suggested to be responsible for this effect. \[Hughes et al. 2017\] The patients in the sham BFR group will receive the same protocol as the experimental group with the exception of not inflating the Blood flow restriction cuff, causing no real vascular occlusion. Furthermore, the intervention in the control group will consist out of a classic rehabilitation program suitable for treating a hamstring injury. Shortly after the MR scan, still in the first week after injury onset, a shear wave elastography measurement of the injured site and at exact same location at non-injured site will be performed. Appendicular to the first, three more elastographic measurements will be performed at fixed moments in time: between day 12 and 16, at moment of return to play (RTP) and at week 12 after injury onset. All the participants will undergo a standardized rehabilitation protocol in one of the three groups until the medical team cleared the patient for returning to sport participation, based on clearly defined and standardised criteria. Tests and questionnaires for evaluation of hamstring strength, flexibility, pain and functionality will be conducted at three major moments in the rehabilitation process: within the first therapy session, weekly during the complete rehabilitation process and at moment of return to play. After RTP, athletes will be contacted regularly, until six months after injury onset, to determine the number of re-injuries. A questionnaire concerning demographic and anthropometric data, sport activities and injury history will be completed by the participants, prior to the testing protocol, as these factors will be additionally investigated for their possible influence on the primary outcome measures. The aim of the study is to investigate the effect of vascular occlusion (Blood Flow Restriction Training), within the rehabilitation program, on the healing proces (outcome measures predictive for the duration of the rehabilitation: average pain at each therapy session, eccentric mid and outer range strength, total painful palpation length, maximal hip flexion active knee extension test) and especially on the elasticity of the injured hamstring muscle tissue (measured with shear wave elastography).