Knee angle and force vector-dependent variations in open and closed kinetic chain for M. popliteus activation.

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 +/- 4 years). Subjects performed seated isometric knee extensions in 30 degrees and 90 degrees 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 30 degrees 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.