Table 1. Trunk muscles functional anatomy and directed co-contraction ratios with respect to trunk kinematics.

Flexion	Extension	Medial flexion	Lateral flexion	Rotation left	Rotation right
rectus abdominis L+R external oblique L+R	erector spinae L+R	external oblique L erector spinae L	external oblique R erector spinae R	external oblique R	external oblique L
Agonists	Antagonists	Agonists	Antagonists	Agonists	Antagonists
DCCR trunk flexors		DCCR trunk medial flexors		DCCR trunk rotators
If agonist mean EMG > antagonist mean EMG : DCCR = 1 – antagonist mean EMG / agonist mean EMG If agonist mean EMG < antagonist mean EMG : DCCR = agonist mean EMG / antagonist mean EMG – 1

Trunk kinematics is flexion/extension, medial/lateral flexion (i.e. respectively a lean to the left or right during the cutting maneuver to the left), and rotation to the left/right (i.e. respectively towards or opposite to the new movement direction during the cutting maneuver to the left). Left (L) and right (R) rectus abdominis, external oblique and erector spinae muscles during a bilateral contraction (L+R) or a unilateral contraction (L or R) yielded different trunk motion in 3D. Accordingly, directed co-contraction ratios (DCCR) served as information for trunk control.