Field monitoring of sprinting power-force-velocity profile before, during and after hamstring injury: two case reports

Very little is currently known about the effects of acute hamstring injury on over-ground sprinting mechanics. The aim of this research was to describe changes in power-force-velocity properties of sprinting in two injury case studies related to hamstring strain management: Case 1: during a repeated sprint task (10 sprints of 40m) when an injury occurred (5th sprint) in a professional rugby player; and Case 2: prior to (8 days) and after (33 days) an acute hamstring injury in a professional soccer player. A sports radar system was used to measure instantaneous velocity-time data, from which individual mechanical profiles were derived using a recently validated method based on a macroscopic biomechanical model. Variables of interest included: maximum theoretical velocity (V-0) and horizontal force (F-H0), slope of the force-velocity (F-v) relationship, maximal power, and split times over 5 and 20m. For Case 1, during the injury sprint (sprint 5), there was a clear change in the F-v profile with a 14% greater value of F-H0 (7.6-8.7N/kg) and a 6% decrease in V-0 (10.1 to 9.5m/s). For Case 2, at return to sport, the F-v profile clearly changed with a 20.5% lower value of F-H0 (8.3 vs. 6.6N/kg) and no change in V-0. The results suggest that the capability to produce horizontal force at low speed (F-H0) (i.e. first metres of the acceleration phase) is altered both before and after return to sport from a hamstring injury in these two elite athletes with little or no change of maximal velocity capabilities (V-0), as evidenced in on-field conditions. Practitioners should consider regularly monitoring horizontal force production during sprint running both from a performance and injury prevention perspective.