Background
Hamstring injuries are among the most common sports injuries and strengthening exercises are often used for injury prevention. Knowledge about the muscle forces and fascicle behavior during these exercises can help optimize exercise prescription, but information on these outcomes across different exercises is lacking.

Objective
This study aims to characterize and compare muscle forces, and biceps femoris muscle fascicle behaviour between three hamstrings exercises: the Nordic hamstring exercise (NHE), single-leg Roman chair (RCH), and single-leg deadlift (DL).

Design
Ten male participants performed the NHE, RCH and DL in a randomized order with a 1RM load, while full-body kinematics, ground reaction forces, lower-limb muscle activation and biceps femoris muscle fascicle behaviour were measured. Muscle forces were determined using a musculoskeletal model.

Results
The increase in fascicle length was significantly larger in the NHE (30.8 ± 11.7 mm), compared to the DL or RCH (18.5 ± 5.2 and 17.8 ± 7.9 mm, respectively). Mean fascicle length was largest in the DL, followed by the RCH and NHE (130.1 ± 39.6, 92.8 ± 26.1 and 72.4 ± 14.6 mm, respectively), with all pairwise comparisons being significant. Peak muscle forces were generally highest in the NHE, followed by the RCH and DL. NHC peak forces however coincided with low muscle activation for the biceps femoris long head and semimembranosus.

Conclusion
The NHC generally exhibited the highest peak muscle forces and results in more eccentric fascicle lengthening along with a higher velocity when compared to the DL and RCH. The NHC may therefore be most effective to promote increases in fascicle length. While the NHC may be effective to promote strength adaptations for the biceps femoris short head and semitendinosus, the RCH and DL may be more effective to promote strength increases in the biceps femoris long head and semimembranosus.