HALHAM°: a Novel Device for Nordic Hamstring Exercise Assessment

Hamstring strain injuries remain among the most common and recurrent injuries in sport, even in 2025. Despite a well-established base of evidence regarding the mechanism of hamstring injury, the reported incidence of injuries nearly doubling from 2001 to 2021 is alarming. While the Nordic hamstring exercise has substantial evidence supporting its effectiveness for reducing the recurrence of hamstring injuries, current assessment devices fail to adequately capture hamstring performance at extended muscle lengths, which is critical given the common injury site during the late-swing phase of sprinting. The thesis aimed to design, validate, and implement a novel hamstring assessment device capable of modifying and measuring the knee flexors torque-length relationship across Nordic hamstring exercise inclinations, preferentially targeting the biceps femoris long head at longer muscle lengths while maintaining similar torque. A prototype system (namely the HALHAM°) was engineered to enable concurrent kinetic, kinematic and electromyographic measurement during the Nordic hamstring exercise. Chapter 1 reviewed the impact, aetiology and mechanisms underlying hamstring strain injuries, and established the need for standardised hamstring testing. Chapter 2 outlined the HALHAM° design, detailing how it addressed the limitations of current systems, including the inability of isokinetic dynamometers to measure torque beyond 145-165° of knee extension. Chapter 3 (Study 1; n=15) established the break-torque angle as a reliable and reproducible metric representing the proxy length at which muscle failure occurs, demonstrating that eccentric hamstring assessment must extend beyond peak torque alone. Chapter 4 (Study 2; n=18) showed that performing the Nordic hamstring exercise at an incline significantly increased break-torque angle compared to both the flat and decline conditions, F(2,34)=63.85, p<0.01, ω2=0.78, without affecting peak torque, F(2,34)=0.952, p=0.389, ω2<0.01, indicating a rightward shift in the torque-length relationship. However, large variability in hip flexion (0.4 53.8°) and angular velocity of the knee joint at peak torque (3.6–96.3deg·s-1), highlighted inconsistent individual Nordic exercise technique. Chapter 5 presented device hardware and software upgrades, including IMU-based angular tracking validated against a gold standard Polhemus system (r=0.99, p<0.0001), and introduced a real-time biofeedback interface for exercise technique control. Chapter 6 (Study 3; n=24) demonstrated that biofeedback significantly improved exercise control, reducing hip flexion, t(23)=2.98 p<0.01, d=0.51, and descent velocity, t(23)=3.67, p<0.01, d=0.825 compared with verbal feedback alone. Chapter 7 (Study 4; n=21) found greater bicep femoris long head activation at longer muscle lengths during incline testing, t(20)=9.74, p<0.0001, d=1.573 and a higher lateral-to-medial hamstring activation ratio t(20)=7.30, p<0.0001, d=0.750, confirming preferential recruitment of injury-prone muscle regions. Chapter 8 consolidated the findings, evaluating the extent to which the overall thesis aims and objectives were achieved, proposing future directions and practical applications. The HALHAM° is a developed, validated system capable of measuring torque-length dynamics, muscle activation, and exercise quality under multiple Nordic hamstring exercise conditions. Incline variations preferentially target the bicep femoris long head at longer muscle lengths while maintaining torque production, overcoming the key limitations of existing devices. The HALHAM° system represents a significant advancement in hamstring assessment and Nordic hamstring exercise technique monitoring.