Optimal cut-off values of the active knee extension test for diagnosing hamstring tightness

Background

Active knee extension (AKE) is widely used to assess hamstring length, and an 'AKE >20°' is often used as the criterion for diagnosing tightness. However, the scientific evidence for this is unclear.

Objective

This study measured and analysed AKE according to sex in two groups with different exercise participation levels to identify the appropriateness of current criteria.

Methods

In the supine position, 90° flexion of the hip and knee joints was performed. The maximal knee extension was measured using a goniometer. Cut-off values for diagnosing hamstring tightness were calculated in three different ways as 'mean-1SD', 'mean-2SD', and 'mean-3SD'.

Results

There were significant differences in AKE between groups and between sexes. In group A, representing normal healthy adults, cut-off values defined as mean-1SD and mean-2SD were 33.5° and 19.7° in male participants and 28.0° and 15.7° in female participants, respectively. There was a weak correlation between AKE and body mass index, but not between AKE and height and between AKE and weight.

Conclusion

This study found that the level of exercise participation significantly affects AKE, and female participants consistently had high flexibility regardless of exercise participation. Therefore, the current cut-off value, widely used in research and clinical settings, may not be a suitable criterion for diagnosing hamstring tightness.