Fast voluntary trunk flexion movements in standing: primary movements and associated postural adjustments.

Movement patterns were studied during fast voluntary forward flexions of the trunk from an erect standing position. Three healthy subjects performed three series of six consecutive trunk flexions at maximum velocity and with successively increasing amplitude, covering a major part of the range of motion (range for all subjects: 13-97 degrees). Angular displacements of the trunk, hip, knee and ankle were measured together with the tilt of the pelvis and the flexion of the spine using a Selspot optoelectronic system. Trunk flexion was the result of a simultaneous forward pelvic tilt and flexion of the spine. For trunk movements up to 55 degrees, spine flexion dominated the movement, whereas for larger movements a major part of the amplitude was caused by pelvic tilt. During flexion of the trunk a simultaneous hip flexion and ankle extension was seen. At the knee there was an initial flexion and a subsequent extension. The net amplitude of the knee flexion showed a negative correlation with net trunk flexion amplitude for movements up to 50 degrees, whereas for larger amplitudes the correlation was positive. Time from onset of the trunk movement to peak knee flexion showed a weak correlation to net trunk flexion amplitude (r = 0.34) whereas the corresponding correlation was higher for pelvic tilt, spine flexion, hip flexion, ankle extension, and knee extension (r = 0.60-0.91). Each successive trial during a series of trunk movements was started from an increasing degree of knee flexion. This gradual adaptation was also present when successive trunk flexions were performed with constant movement amplitude.(ABSTRACT TRUNCATED AT 250 WORDS)