Postural strategy to keep balance on the seesaw.

This work investigates the kinematic and electromyography (EMG) strategy used by the central nervous system (CNS) to keep equilibrium during anterior-posterior balance on seesaws with different degrees of instability. The movement of hip, knee, and ankle were reconstructed using a 3D motion-analysis system and the EMG activities of selected ankle, knee, and hip muscles were recorded. Balance was kept mainly at the ankle joint. The EMG patterns of the gastrocnemius and anterior tibialis alternated between agonist and antagonist bursts. The agonist burst started before the end of the lengthening phase and was prolonged until the end of the shortening phase. The EMG activities of the muscles crossing the knee and hip joints were characterized by a pattern of generalized co-activation. The movements at these two joints were very small, suggesting a neural or biomechanical constraint underlying the operations of the equilibrium control. Our results also indicate that the strategy to keep balance on the seesaw is qualitatively the same for the different levels of mechanical demands in terms of the seesaw's instability.