Ninety male subject will be divided into three group: a younger group (18-22y), an older group (30-40y) and a control group (18-40y). The study comprises 3 phases. PHASE 1: baseline strength and stiffness measurements of the hamstring muscles, PHASE 2: 12 week Nordic Hamstring strength training program (not for the control group) and PHASE 3: final strength and stiffness measurements. The aim of this study is twofold, namely to investigate (1) whether there is a difference in baseline stiffness of the hamstring muscle complex between different age categories and (2) the evolution of hamstring muscle stiffness during an eccentric strength training program within the different age groups. These two research questions could ultimately lead to a possible answer on the question if the higher probability of HSI (=hamstring strain injury) in the older age category with the lowest levels of eccentric hamstring strength could be due to an age-related difference in hamstring stiffness.

Ninety male subjects will be equally allocated, according to their age, to either the younger NHE (Nordic Hamstring Exercise) group (aged between 18 and 22 years), the older NHE group (aged between 30 and 40 years) or the control group (aged between 18 and 40 years old). Potential candidates will be excluded from this study if they have a history of hamstring injuries, or a recent severe trauma or surgery in the lower extremity. All participants must perform at least 1 hour of weekly sport participation and do not participate in more than 3 training sessions a week. To ensure valid measurements of eccentric strength and muscle shear modulus, subjects will be instructed not to engage in strenuous training or sport activities 48 h prior to testing. All participants will be informed regarding the aim, content and possible risks associated with the experimental approach before they will be asked to sign an institutionally approved informed consent document, indicating voluntary participation in this study. Prior to the first training session, participants will undergo both a bilateral isokinetic eccentric and concentric strength protocol and an ultrasound elastographic assessment of the hamstring muscles to determine baseline strength (eccentric and concentric) and shear modulus of the posterior thigh muscles, respectively. Participants will be instructed not to engage in strenuous training or sport activities 48 h prior to testing to ensure a valid measure of the hamstring muscle-tendon elasticity. After completion of all the pre-training assessments, participants in the NHE groups will be familiarized with the correct execution-modalities of the eccentric hamstring exercise (NHE). For a period of 12 weeks, both experimental groups will perform a progressive eccentric strength training program targeting the hamstring muscles, solely consisting of the NHE. On the other hand, the control group will be instructed not to engage in any lower limb strength training. After completing the final training session, hamstring eccentric strength and shear modulus of all participants will be reassessed. The isokinetic strength protocol will be systematically conducted after all shear modulus measurements have been finished in order to avoid acute influences of previous exertion on the elasticity measurements.

Inclusion Criteria:

• Minimum 1h sport/week and Maximum 3 sport sessions a week
• Male
• Injury-free at the moment of testing.

Exclusion Criteria:

• History of injuries or surgery lower extremity
• History of hamstring injuries