The functional popliteal angle test can detect features of hamstring spasticity.

BACKGROUND: Hamstring spasticity is a common impairment in patients with central neurological lesions, and accurate diagnostics is crucial for effective treatment of associated gait abnormalities, such as crouch gait. Current diagnostics, including 3D clinical gait analysis during normal and fast walking, have limitations that may lead to misdiagnosis, possibly explaining the current poor treatment outcomes. To address these limitations, we introduced the functional popliteal angle test-a knee-extension motion with flexed-hip performed while standing on one leg at both slow and fast speeds. This study aimed to assess this test's construct validity, showing its capacity to detect features of hamstring spasticity.

METHODS: Kinematics and hamstring EMG data from patients with central neurological lesions and healthy participants were retrospectively extracted. Two criteria-EMG-Velocity and Peak Muscle Length ratios-were defined to capture key spasticity components: a disproportionately large increase in muscle activity with higher muscle-tendon lengthening velocities and a shorter maximum muscle length during fast vs. slow lengthening. Ratios were calculated individually for patients' and healthy participants' limbs, with values above one indicating spasticity.

FINDINGS: Eight out of nine healthy limbs scored below one on both criteria. Among 35 patient limbs (age: 16 ± 12 years), 18 scored below one on both criteria (categorized as non-spastic), while 14 scored above one on both criteria (categorized as spastic). Discrepancies arose in three limbs.

INTERPRETATION: The results suggest that the functional popliteal angle test can detect features of hamstring spasticity and holds potential for enhancing clinical diagnostics of hamstring spasticity.