Precise Quantification of the Onset and Cessation of Flexion-Relaxation in the Erector Spinae Muscles: A Comparison of 1 Versus 2 Inertial Motion Sensors.

Precise quantification of the lumbar flexion-relaxation phenomenon can provide a better understanding of the load transfer between active and passive tissues and its implications for spinal stability. This study aimed to compare the precision of instantaneous angles using 1 versus 2 inertial motion sensors to determine the deactivation (electromyography [EMG]-off) and reactivation (EMG-on) of left and right erector spinae muscles during trunk flexion and extension. Electromyography and kinematic data were collected from 12 participants who performed 20 slow, controlled trunk flexion-extension motions. The angle derived from 1 motion sensor was the pitch angle of the T12 sensor, and the angle derived from 2 sensors was the difference between the pitch angles of the T12 and S1 sensors. The EMG-off (or EMG-on) angle was defined as the angle at which unilateral erector spinae muscle activity dropped below (or exceeded) 3 times the muscle activity recorded at full trunk flexion during trunk flexion and extension. Results revealed that absolute and relative variability in 20 EMG-off and EMG-on angles were greater when measured with 1 inertial motion sensor than with 2 sensors (P ≤ .003), suggesting that using 2 sensors can improve the precision of flexion-relaxation phenomenon determination compared with using 1 sensor.