External Load, More than Surface Instability, Drives Post-Activation Performance Enhancement in Split Squat Conditioning Activity: Phase-Specific EMG Responses

External load primarily drives acute CMJ potentiation, whereas instability mainly redistributes recruitment toward the hamstrings without improving jump performance beyond the stable condition. These findings indicated that when the goal is acute jump enhancement, external load should be prioritized, whereas unstable surfaces may be used to selectively target posterior chain activation.