Effects of rear-foot instability devices on lower-limb muscle activation during the Bulgarian split squat in male football players

Unilateral resistance exercises such as the Bulgarian Split Squat (BSS) are commonly used to develop lower-limb strength, postural control, and neuromuscular coordination, depending on training variables (e.g., load and intensity). Although instability training increases muscle activation, few studies have examined the effect of rearfoot instability on neuromuscular responses during BSS. This randomized crossover study investigated the acute effects of three rear-foot instability devices on muscle activation during the ascent and descent phases of the BSS in 23 trained male football players. Participants performed body-weight BSS under four conditions: stable platform, BOSU ball, Swiss ball (Swiss), and TRX suspension. Surface electromyography (sEMG) recorded activation of the rectus femoris (RF), vastus lateralis (VL), vastus medialis (VM), biceps femoris (BF), semitendinosus (ST), and gluteus maximus (GM). Two‑way repeated‑measures ANOVA showed significantly greater activation during ascent for BF (p < 0.001), ST (p = 0.006), VL (p < 0.001), VM (p < 0.001), and GM (p < 0.001). Quadriceps activation during descent was highest on the Swiss: RF (Swiss vs. stable: p = 0.002; Swiss vs. BOSU: p < 0.001; Swiss vs. TRX: p = 0.006), VL (Swiss vs. stable: p = 0.017; Swiss vs. BOSU: p = 0.001), and VM (Swiss vs. stable: p = 0.024; Swiss vs. BOSU: p = 0.046). TRX increased ST activation during the ascent compared to the Swiss (p = 0.034), and the BOSU showed higher ST activation than the Swiss during the descent (p = 0.004). Surface significantly affected activation (ST: p = 0.018; RF: p < 0.001; VL: p < 0.001; VM: p = 0.013; GM: p = 0.042), and there was a significant surface × phase interaction for GM (p = 0.041). The findings highlight rearfoot instability as an effective programming variable to selectively enhance muscle activation without external loading, supporting its application in strength and rehabilitation programs.