Synergist
Short definition
A muscle assisting the agonist: contracting in the same direction, adding force or taking over part of the work in a specific phase.
Full definition
A synergist contracts together with the agonist, assisting the movement by adding torque in the same direction or balancing moments at adjacent joints. Synergist contributions are phase-dependent: in the bench press the triceps and anterior deltoid assist the pectoralis major at the bottom and mid-range, yet near lockout the triceps becomes the prime mover. Ignoring synergists overstates the load on the agonist, which is why Samson-fit sums all muscle contributions from the ExerciseMuscleAnatomy table.
Examples
In the bench press the synergists are the triceps and anterior deltoid (15–25% contribution).