Rugby union is a popular and fast-growing sport played internationally that involves high levels of physical contact and running activities. Unfortunately, these physical requirements can often lead to soft-tissue injuries. Some of the most common injuries resulting from rugby union occur to the triceps surae muscle-tendon unit (MTU). The calf raise test is one of the most commonly used tests in clinical practice to assess triceps surae muscle function, especially endurance. Clinicians and researchers have also used variations of this test in the assessment and management of individuals with Achilles tendinopathies or Achilles tendon ruptures, such as eccentric-concentric power tests. These data, however, generally derive from general or injured populations. To date, limited research has assessed triceps surae muscle function in rugby union athletes. Chapter One explores literature focusing on the triceps surae MTU (anatomy and injuries), rugby union, rugby-related injuries, and calf muscle testing procedures, specifically the calf raise test and eccentric-concentric power tests. Chapter Two aims to establish normative calf muscle function values in uninjured male rugby union athletes, examine the test-retest reliability of key measures across repeated testing sessions, and assess the validity of clinical testing methods using a novel Calf Raise application against research-grade laboratory equipment. Finally, Chapter Three summarises and discusses the key findings, limitations, and strengths of the experimental chapter, and addresses potential future research directions. In Chapter Two, 120 rugby union athletes participated in establishing normative calf muscle function values, and were categorised into playing position (forwards and backs) and level of professionalism (International, Super Rugby, Provincial, and Club), while also considering age, leg dominance, body mass index (BMI), and previous MTU injuries. Eighteen athletes participated in test-retest reliability and performed an additional two testing sessions 1 week apart, whereas 20 athletes participated in validation of the Calf Raise application outcomes. All participants completed three calf muscle tests: 1) eccentric-concentric bodyweight power test; 2) eccentric-concentric weighted power test; and 3) concentric-eccentric endurance test. Reliability was acceptable across outcomes following familiarisation (coefficient of variation < 10%, intraclass correlation ≥ 0.83). Validity of the application outcomes against 3D motion capture and force plate data was also acceptable (coefficient of variation ≤ 6.6%, intraclass correlation ≥ 0.84). Forwards produced superior power during the bodyweight (59 W, p = 0.007) and weighted (73 W, p < 0.001) power tests, with playing level significantly influencing outcomes (p < 0.009). Super Rugby players were more powerful than Club and Provincial players in both power tests, and International in the bodyweight test. Backs completed more repetitions (3 repetitions, p = 0.001) and positive displacement (30 cm, p = 0.001) than forwards during endurance testing, with no influence of level. When accounting for the clinically relevant factors, BMI, age, and previous injury explained some of the differences observed between positions and levels. These findings support previous research highlighting differences in rugby union athletes based on position and level, and provide initial benchmark values of calf muscle function for rugby union players in New Zealand. Given the results stem from uninjured male rugby union athletes, results cannot be generalised to females, different rugby codes, and those with a current or rehabilitating MTU injury.