Background The hamstring muscles are the most injured muscles in athletes practicing sprints. In addition, they seem greatly contributing to the production of horizontal force, a key element in the sprint acceleration performance.

Objective To experimentally analyse the relationship between horizontal force production and hamstrings strength during maximum-velocity treadmill sprints with and without fatigue.

Design Descriptive cross-sectional study.

Setting Laboratory study on competitor-level athletes.

Participants 14 healthy male athletes trained for sprint running.

Main Outcome Measurements Comparison analyses of the following parameters before and after 12 6-seconds sprints separated by 44 seconds of recovery on an instrumented treadmill sprint: horizontal ground force production, EMG activity of the vastus lateralis, rectus biceps femoris and gluteus muscles, maximum isokinetic strength of the flexors and extensors of the hip and knee measured at 120°/s in concentric and eccentric before and just after the 12 sprints.

Results For the best performance sprint, the multiple linear regression analysis showed a significant relationship (p=0.02) between the horizontal force and the combination of EMG activity of the biceps femoris in late swing phase of the stride and the eccentric isokinetic strength of the knee flexors. After the 12 sprints, there was a significant decrease (p<0.05) in sprint performance (−6.6%), horizontal force (−8.6%), isokinetic strength (−5.8 to -11.9%) and muscle activity (−5.0 to −38.9%), and there was a significant correlation between the horizontal force and eccentric strength of the knee extensors (r=0.640; p<0.02), and concentric strength of hip extensors (r=0.676; p<0.01).

Conclusions For a high-performance acceleration, it would seem necessary to be able both to strongly activate the hamstrings just before ground contact and have a great capacity eccentric strength of the hamstrings. In fatigue conditions, horizontal force reduction would seem not only related to the strength of decreased hamstring, but with the knee extensors eccentric strength decrease.