## 4 Discussion
In the standing-prone position, the leg rise reduces the base of support to the contralateral foot plant, leading to two effects on the oblique muscles: (1) a movement-facilitating effort of spine rotation towards the side of the raised leg, with greater activation of the EO ipsilateral and the IO contralateral to the supporting leg; (2) a rigid trunk rotation and lateral translation that bring the body center of mass above the base of support, with higher activation of the oblique muscles ipsilateral to the raised leg (as in the case of the ipsilateral arm-leg rise from the quadruped position). The result is a heightened activation of the OE ipsilateral to the raised leg compared to the contralateral OE, with a minimal difference in the activation of the IO muscles (Figure 1c). In contrast, the arm rise does not impact the base of support, resulting in minimal imbalance and a negligible difference in the activation of contralateral oblique muscles (Figure 2c). Consequently, the leg mainly determines the effect of contralateral arm-leg rise on the oblique muscles (Figure 3c). In the ipsilateral arm-leg rise, only the second effect influences the difference in oblique muscle activation between the sides of the body, as the arm rise and leg rise tend to induce opposite spine rotations that counterbalance each other, maintaining the spine in a neutral position in the transverse plane. This led to increased activation of the oblique muscles ipsilateral to the raised limbs (Figure 4c). The ES consistently exhibited higher activation on the side of the raised arm. This is likely due to the posterior scapular tilt and slight spine extension linked with full shoulder flexion. Isolated leg rise resulted in higher activation in the contralateral ES in prone and quadruped positions, to compensate for the dominant activity of the ipsilateral MF (Figure 1a,b). Conversely, standing leg rise led to higher activation of the ES ipsilateral to the raised leg (Figure 1c). In the standing-prone position and single leg support, the gravity-induced lateral spine flexion towards the side of the supporting leg is resisted not only by the EO and IO on the side of the raised leg (as previously discussed), but also by the ES and MF of the same side. In all body positions, leg rise enhanced the ipsilateral MF activity due to the posterior pelvic tilt and slight spine extension associated with full hip extension. Conversely, arm rise enhanced the contralateral MF activity to compensate for the opposite dominant behavior of the ES. The MF activity during contralateral or ipsilateral arm-leg raise was mainly influenced by the effect of the raised leg in the quadruped and standing-prone positions. However, during the prone contralateral/ipsilateral arm-leg raise, the MF activity was higher on the side contralateral to the raised leg. This unexpected behavior observed in the prone position might stem from the participant's attempt to maintain ground contact on the anterior pelvic side opposite the raised leg (counteracting the effect of gravity), with the aim of maximizing the base of support and stabilizing the body. Previous studies examined the bilateral activity of ES, MF, EO, and IO during only 4 of the 12 exercise conditions investigated in the current study: contralateral arm-leg rise in the quadruped [18; 19; 20; 21; 22; 23; 24; 25; 26; 27] and prone [22; 27] positions, and isolated lower limb [18; 19; 20; 21; 26] or upper limb [26] rise from the quadruped position (Table 1). Overall, these prior findings are in good agreement with our results, particularly regarding the differences in muscle activation between the body's sides. The 12 exercise conditions analyzed in this study generated a wide spectrum of muscle activations, reaching up to 69%, 64%, 34%, and 24% MVIC for ES, MF, OE, and OI, respectively. This extensive dataset of muscle activity levels can aid in optimizing the planning and progress of asymmetric lumbar stabilization exercises.