OBJECTIVE: To measure and compare latency, electromechanical delay, and speed of motor response of the peroneus longus muscle in a large sample of subjects with healthy or unstable ankles. DESIGN AND SETTING: Subjects with healthy or unstable ankles underwent identical test procedures consisting of 6 consecutive, sudden 50 degrees ankle-inversion movements in the standing position with full weight on the tested leg. Latency, first and second decelerations, and total inversion time were monitored. In a separate setting, electromechanical delay was measured during a voluntary ankle eversion. SUBJECTS: We tested 81 subjects (27 males, 54 females) with healthy ankles or unilateral or bilateral ankle instability: 40 individuals with 48 unstable ankles (18 males, 22 females; age range, 18 to 23 years), and 41 individuals with 46 healthy ankles (9 males, 32 females; age range 15 to 29 years). MEASUREMENTS: Using surface electromyography, the latency of the peroneus longus muscle was measured during sudden inversion. Simultaneously, using accelerometry, first and second decelerations were measured. Separately, the electromechanical delay was measured using electromyography and accelerometry. RESULTS: The time of the first deceleration was significantly shorter in subjects with unstable ankles (25.5 milliseconds versus 28.4 milliseconds for those with healthy ankles). The latency, total inversion time, second deceleration time, and electromechanical delay were not shown to be significantly different between healthy and unstable ankles. CONCLUSIONS: The significantly shorter first deceleration during sudden inversion in the standing position in subjects with unstable ankles compared with subjects with healthy ankles is explained by less control of inversion speed in the first phase of the sudden inversion in unstable ankles. Our study does not confirm the results of the systematic literature search showing a longer latency in subjects with unstable ankles.