Strength Measurements in Acute Hamstring Injuries: Intertester Reliability and Prognostic Value of Handheld Dynamometry

STUDY DESIGN: Cohort study, repeated measures. BACKGROUND: Although hamstring strength measurements are used for assessing prognosis and monitoring recovery after hamstring injury, their actual clinical relevance has not been established. Handheld dynamometry (HHD) is a commonly used method of measuring muscle strength. The reliability of HHD has not been determined in athletes with acute hamstring injuries. OBJECTIVES: To determine the intertester reliability and the prognostic value of hamstring HHD strength measurement in acute hamstring injuries. METHODS: We measured knee flexion strength with HHD in 75 athletes at 2 visits, at baseline (within 5 days of hamstring injury) and follow- up (5 to 7 days after the baseline measurement). We assessed isometric hamstring strength in 15 degrees and 90 degrees of knee flexion. Reliability analysis testing was performed by 2 testers independently at the follow-up visit. We recorded the time needed to return to play (RTP) up to 6 months following baseline. RESULTS: The intraclass correlation coefficients of the strength measurements in injured hamstrings were between 0.75 and 0.83. There was a statistically significant but weak correlation between the time to RTP and the strength deficit at 15 degrees of knee flexion measured at baseline (Spearman r = 0.25, P = .045) and at the follow-up visit (Spearman r = 0.26, P = .034). Up to 7% of the variance in time to RTP is explained by this strength deficit. None of the other strength variables were significantly correlated with time to RTP. CONCLUSION: Hamstring strength can be reliably measured with HHD in athletes with acute hamstring injuries. The prognostic value of strength measurements is limited, as there is only a weak association between the time to RTP and hamstring strength deficit after acute injury.