In general, IO and TrA muscles have anatomical similarities in that both are among the muscles comprising the structure of the lower abdominal wall and they have similar functions. However, unlike the TrA, the IO affects the spine because of its direction.4 The IO attaches

to the posterior layer of the thoracolumbar fascia. Contraction of this muscle creates a lateral tension force on the thoracolumbar fascia, which creates the intrinsic translational and rotational stabilization of the spinal unit.20 This should be a reason for the higher activation of this muscle during the bird-dog position. Also, Huang23 stated that the unilateral multifidus activation increased in the bird-dog Inhibitors,research,lifescience,medical position. Inhibitors,research,lifescience,medical Because the body weight is supported by one knee and elbow in the bird-dog position, which is an unstable status, the torque of the arm and leg is greater and the multifidus muscle shows a higher activation potential. On the other hand, lumbar multifidus and erector spine muscles have a relatively high proportion of type 1 (slow twitch)

muscle fibers, which makes them well-suited for endurance or sustaining contraction activities. During the bird-dog exercise, these muscles produce EMG signal amplitudes of 29% maximal isometric voluntary contraction (MIVC).23 According to Arokoski,16 Inhibitors,research,lifescience,medical in the bird-dog exercise with weights on the hand, unilateral leg extension, and unbalanced limb movements, there is an increase in the trunk muscle activation, which contributes to the preservation of the spinal

stability. Inhibitors,research,lifescience,medical The increase in the activation of left IO and left and right multifidus muscles in the bird-dog exercise may be related to the unstable body position. Accordingly, the core muscles are recruited for proprioception, balance, and energy transfer from the lower extremity to the upper extremity and hold a neutral abdominal posture.24 In the present study, the left TrA activation in left leg extension was higher than that in right arm extension. Callaghan reported that in order to reduce loads on the spine in exercises such as leg extension with the Inhibitors,research,lifescience,medical spine being held isometrically, the trunk muscles exhibited asymmetrical activity.25 In single leg extension, while the subject is others on the hands and knees, he/she produces mild extensor activity and lower spine compression less than 2005 N. Raising the contralateral arm (bird-dog) increases extensor muscle activation and also spine compression to more than 3000 N. Sufficient stability is ensured with mild abdominal bracing.26 According to a Selleck JQ1 recent study, the reciprocal index of the TrA depended on the magnitude and direction of perturbation, refuting the “corset hypothesis” as a normal mechanism for the TrA prior to arm movement. Therefore, the feed-forward activity of the TrA is asymmetric and depends on the direction of perturbation due to arm movement.