Effects of unstable loads and surfaces on core and lower limb muscle activation during Bulgarian squats

<b>Background:</b> Unstable training surfaces and loads are often used to enhance neuromuscular activation, but their comparative effects on core and lower limb muscle activity during unilateral exercises remain unclear. <b>Objective:</b> To compare the effects of Bulgarian split squats performed under unstable surface conditions (USC) and unstable load conditions (ULC) on core and lower extremity muscle activation. <b>Methods:</b> Twenty-one physically active adults performed Bulgarian split squats under three conditions: stable condition (SC), USC using a balance pad, and ULC using a water-filled aqua bag. Surface electromyography (EMG) measured muscle activation during both descending and ascending phases. Data were normalized to %MVIC and analyzed using repeated measures ANOVA or non-parametric tests depending on data normality. <b>Results:</b> USC significantly increased core (rectus abdominis, external oblique, internal oblique, erector spinae) and selected lower limb muscle (gluteus maximus, gluteus medius, biceps femoris) activation compared to SC (p < .05). ULC elicited the greatest activation across both phases (p < .001). No significant differences were found in vastus lateralis and vastus medialis among conditions. <b>Conclusions:</b> Bulgarian split squats with unstable loads result in greater activation of core and selected lower limb muscles than unstable surfaces or stable conditions. Unstable load training may be an effective method to enhance muscular activation and stability in unilateral lower body exercises.Clinical trial registrationKCT0009349 (approval date: April 19, 2024).