The main focus of this thesis is the investigation of structural muscle parameters (i.e., fascicle length (FL) and passive muscle stiffness (PMS)) of the biceps femoris long head (BFlh) due to their relationship with hamstring injuries in sports. Chapters two, three and four contain two original experimental studies and one systematic review with meta-analysis focusing on the topic above. In the first systematic review with meta-analysis (chapter two), studies investigating the effects of eccentric training, including the Nordic hamstring exercise, have been screened and investigated based on the BFlh FL based on ultrasound assessment methods. The systematic review's findings indicated that eccentric training showed a large effect size on increasing the BFlh FL when it was measured using the trigonometric equation method. However, eccentric exercise did not largely increase the BFlh FL when measured using the manual linear extrapolation method or panoramic ultrasound scanning. Conclusions of the meta-analysis highlight that a "gold standard" measurement method is needed for comparable results between the studies investigating the effects of eccentric exercise on the BFlh FL. The first experimental study of this thesis (chapter three) compared the BFlh muscle structural parameters and knee extensors (KE) and knee flexors (KF) muscle volume ratios between rugby players and physically active non-athlete controls to explore the long-term effects of playing rugby on the mentioned muscular structure. The findings demonstrate that playing rugby and rugby-specific training led to increased BFlh PMS and lower BFlh FL/KE and KF/KE muscle volume ratios in the long term. Habitual rugby training and match-play lead to structural and morphological alterations in the KF and KE that may increase HSIs risk. Practitioners should administer long-term hamstring pre-habilitation training to reduce HSI risk in rugby players. The second experimental study of this thesis (chapter four) aimed to examine football-specific fatigue-induced alterations in risk factors of the HSIs, including biceps femoris long head fascicle length (BFlh FL), single-leg hop distance, hamstrings' maximal eccentric strength, and single-leg hamstring bridge test (SLHB) performance. Outcomes revealed significant decrements in SLHB performance and maximal eccentric hamstring strength immediately after ninety minutes of a simulated football match (TSAFT90). However, in this study, we could not observe any significant change in the BFlh FL and single-leg hop distance immediately after half-time and full-time of the TSAFT90. In conclusion, this study provided strong evidence for improving eccentric strength and SLHB performance in practitioners. In chapter five of this thesis, the systematic review – a meta-analysis of morphological effects of exercise on the upper limb muscles found that various exercise types induced large effect sizes on increasing muscle size in distinct upper extremity muscles. At least four weeks of training were necessary to induce the forementioned hypertrophic responses.