Synergistic dominance occurs when synergist muscles compensate for a weak or inhibited prime mover, taking over its primary function. In this case, the erector spinae-normally a spinal stabilizer-are performing hip extension instead of the gluteus maximus, which is the prime mover for that action.
The NASM CPT7 Study Guide explains that altered length-tension relationships and poor neuromuscular recruitment patterns can cause helper muscles to dominate, leading to inefficient movement and potential injury.
This is distinct from reciprocal inhibition (agonist-antagonist relationship), autogenic inhibition (GTO- mediated relaxation), and neuromuscular efficiency (optimal recruitment patterns).