Introduction
Hamstring injuries represent the most common injury in many sports involving high-speed running.1–4 Most are thought to occur during the late swing phase when the hamstrings muscles must produce high eccentric force to decelerate the extending knee and flexing hip,5 therefore low eccentric hamstring strength has been considered a risk factor. Data from prospective cohort studies examining the association between hamstring strength and hamstring injury risk are equivocal,6–9 but systematic eccentric strength training reduces the risk of hamstring injuries.10–13 Testing maximal eccentric hamstring strength has therefore become common practice both in research and in the real-world setting in these sports.
The eccentric strengthening exercise most used in research aiming to prevent hamstring injuries is the Nordic hamstring exercise.10 12 13 Consequently, a test device that measures the eccentric hamstring force during the Nordic hamstring exercise was developed14 and is now commonly used in both research15 and by elite teams in various sports. During the test, the player’s ankles are secured by braces attached to load cells. These measure the maximal eccentric hamstring force when the athlete leans forward and resists falling forward for as long as possible (ie, performing the Nordic hamstring exercise). The Nordic hamstring exercise is intended to be supramaximal, and a prerequisite for Nordic hamstring test devices is that the subject reaches a ‘critical point’ where the external load from gravity acting on the upper body exceeds their maximal eccentric hamstring strength.14 However, some athletes are able to control the forward falling motion throughout the full range of motion of the exercise. These players will never reach the ‘critical point’ and the test will therefore not be able to measure their maximal eccentric hamstring strength as intended.
Some studies have addressed this problem. To ensure that supramaximal intensity is achieved, study participants who were able to control the final phase of the forward falling movement before full extension have performed training while holding weight plates, increasing the weight over the intervention period.16–22 Several of these studies have used the same criterion when testing maximal eccentric hamstring force with Nordic hamstring test devices.17–21 As maximal eccentric hamstring force measured during Nordic hamstring strength testing correlates with body mass,23 it is possible that performing the test with added weight could affect the outcome, irrespective of changes in eccentric hamstring strength. This would be critical, especially if using different weights for pre-tests and post-tests when evaluating the effect of a training intervention or comparing groups that have performed the testing with different weights. Currently, no studies have examined how performing Nordic hamstring strength testing with added weight affects the maximal force measured.
Therefore, we aimed to investigate how adding extra weight when performing Nordic hamstring strength testing affected the maximal eccentric hamstring force recorded in male and female football players, and if there were any differences between players who were able to control the forward falling movement during the final phase of the range of motion and those who could not.