Hamstring strain injury (HSI) is the most common injury in sports involving high-speed running. Most HSIs affect the biceps femoris long head (BFlh) and occur at or near the proximal musculotendinous junction (MTJ). Each HSI typically results in between 6 and 26 days lost from training and competition which reduces performance and presents a significant financial burden to professional sporting organisations. High rates of HSI recurrence are especially problematic given that re-injuries are often more severe and result in greater recovery times than the initial insult. An emerging body of work suggests that hamstring muscle-tendon geometry may have important implications for injury risk and performance. For example, a narrower BFlh proximal aponeurosis relative to the muscle width has been estimated to increase muscle fibre strain adjacent to the MTJ. However, limited research has examined how hamstring aponeuroses and free tendons are affected by prior HSI and no work has explored how they respond to short- or long-term mechanical loading. Therefore, the overall aim of this thesis was to investigate the adaptation of hamstring muscle-tendon geometry to HSI and training. Study 1 aimed to identify differences in BFlh geometry between individuals with and without a history of HSI. This was achieved through a secondary analysis of previously collected magnetic resonance imaging (MRI) data from n=26 recreationally active males with (n=13) and without (n=13) a history of unilateral HSI. The results revealed that previously injured limbs displayed significantly larger BFlh aponeurosis volumes and smaller muscle-to-aponeurosis volume ratios than control limbs with no history of injury. Study 2 aimed to synthesise the current evidence regarding the effect of mechanical loading on lower limb tendons' geometric (morphological; cross-sectional area, CSA), material (modulus), and mechanical (stiffness) properties in a systematic review and meta-analysis. Results from 61 articles demonstrated that mechanical loading induced moderate increases in tendon stiffness, large increases in modulus, and small increases in CSA. Resistance training interventions were most common and of these, high strain protocols led to the largest changes in stiffness and modulus. Notably, no published intervention studies examined the effect of mechanical loading on the hamstrings. The primary purpose of Study 3 was to examine hamstring geometric adaptation following 10-weeks of training with the Nordic hamstring or a hip extension exercise. Despite evidence for heterogenous patterns of muscular hypertrophy within and between the hamstrings, limited aponeurosis or free tendon CSA and volume was induced following these interventions. The relationship between force-generating capacity (volume) and force transmitting structures (aponeurosis interface area) was also examined, with neither exercise group inducing significant change in ratio. Study 4 examined the differences in hamstring muscle-tendon unit geometry and muscle fibre type between elite athletes and recreational athletes. Fifteen state- to international-level sprinters or jumpers and n = 15 recreationally active individuals underwent assessments of hamstring muscle-tendon geometry via MRI and BFlh and gastrocnemius fibre typology via MR spectroscopy. Participants also underwent assessments of maximal sprint velocity (during a 60-m effort) and maximal knee flexor strength. Elite athletes were faster, stronger, possessed a greater proportion of Type II muscle fibres, and had non-uniformly larger muscles, free tendons, and aponeuroses, than the recreational group. Semimembranosus and semitendinosus muscle and tendon size, alongside fibre typology, were associated with sprint velocity. Semimembranosus and BFlh muscle and semitendinosus tendon volumes were predictors of knee flexor strength. Collectively, these results suggest that hamstring muscle-tendon geometry may be altered in athletes with a history of HSI and in those with a history of habitual loading. In contrast, short-term training (i.e., 10 weeks) promotes significant hamstring muscle hypertrophy but limited corresponding change in tendon-aponeurosis size, at least in healthy, recreationally active athletes. These findings may have implications for strategies targeted at reducing HSI and re-injury and improving athletic performance.