A novel device using the Nordic hamstring exercise to assess eccentric knee flexor strength: a reliability and retrospective injury study.

STUDY
DESIGN: Reliability and case-control injury study.
OBJECTIVES: To determine if a novel device designed to measure eccentric knee flexor strength via the Nordic hamstring exercise displays acceptable test-retest reliability; to determine normative values for eccentric knee flexor strength derived from the device in individuals without a history of hamstring strain injury (HSI); and to determine if the device can detect weakness in elite athletes with a previous history of unilateral HSI.
BACKGROUND: HSI and reinjury are the most common cause of lost playing time in a number of sports. Eccentric knee flexor weakness is a major modifiable risk factor for future HSI. However, at present, there is a lack of easily accessible equipment to assess eccentric knee flexor strength.
METHODS: Thirty recreationally active males without a history of HSI completed the Nordic hamstring exercise on the device on 2 separate occasions. Intraclass correlation coefficients, typical error, typical error as a coefficient of variation, and minimal detectable change at a 95% confidence level were calculated. Normative strength data were determined using the most reliable measurement. An additional 20 elite athletes with a unilateral history of HSI within the previous 12 months performed the Nordic hamstring exercise on the device to determine if residual eccentric muscle weakness existed in the previously injured limb.
RESULTS: The device displayed high to moderate reliability (intraclass correlation coefficient = 0.83-0.90; typical error, 21.7-27.5 N; typical error as a coefficient of variation, 5.8%-8.5%; minimal detectable change at a 95% confidence level, 60.1-76.2 N). Mean ± SD normative eccentric flexor strength in the uninjured group was 344.7 ± 61.1 N for the left and 361.2 ± 65.1 N for the right side. The previously injured limb was 15% weaker than the contralateral uninjured limb (mean difference, 50.3 N; 95% confidence interval: 25.7, 74.9; P<.01), 15% weaker than the normative left limb (mean difference, 50.0 N; 95% confidence interval: 1.4, 98.5; P = .04), and 18% weaker than the normative right limb (mean difference, 66.5 N; 95% confidence interval: 18.0, 115.1; P<.01).
CONCLUSION: The experimental device offers a reliable method to measure eccentric knee flexor strength and strength asymmetry and to detect residual weakness in previously injured elite athletes.