The popliteus muscle is short and deeply situated in the posterior aspect of the knee, and contributes to control knee joint position. Little is known how activation of the popliteus is controlled. We hypothesized that for the purpose of securing knee joint position, the popliteus would be activated prior to the prime movers of the knee, and that activation would occur earlier and be amplified with increased structural laxity due to joint loading direction and position. Surface and fine wire electromyography (EMG) was used to measure onset and amplitude of muscle activity in the popliteus and three parts of the quadriceps in 10 healthy women (age 25 AE 4 years). Subjects performed seated isometric knee extensions in 308 and 908 knee flexion in open (OKC) and closed kinetic chain (CKC) in a reaction time task. The popliteus was activated after the quadriceps in all tasks, but with shorter latency relative to the quadriceps in CKC, independent of knee flexion angle. EMG amplitude was greatest for all muscles in OKC in the 308 knee flexion. Biomechanical variables alone do not explain popliteus activation. In addition to biomechanics, behavioral, and habitual aspects need to be considered in further studies. ß

The purpose of this study was to determine whether m. popliteus (POP) activity would contribute to the control of knee joint position in unpredictable and in self-initiated provocations of standing balance. Ten healthy women (age 25.2 AE 4.5 years, means and SD) without known knee pathology were tested for postural reactions (1) to unpredictable support surface translations in anterior and posterior directions, and (2) in self-initiated balance provocations in a reaction time (RT) forward reach-and-grip task. Electromyographic activity was recorded from POP and other leg muscles plus the deltoid muscle. Three-dimensional kinematics were captured for the knee joint and the body centre of mass was calculated. POP was active first of all the muscles recorded, regardless of translation direction, and knee joint movements elicited were either knee extension or external rotation of the tibia. In the RT task, the POP was active after initiation of reaching movement, and there was little consistency in the kinematic response. POP activity was not direction specific in response to support surface translation, but appeared triggered from reactive knee joint movement. The response to the support-surface translation suggests that POP served to control knee joint position rather than posture. In the RT task, we could not deduce whether POP activity was attributed to knee joint control or to postural control. ß

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