Synergist

A synergist is a muscle that assists the agonist during a movement.^{1International Sports Sciences Association, Strength and Conditioning, p. 43.} In elbow flexion, the brachialis acts as a synergist alongside the biceps brachii.^{2International Sports Sciences Association, Corrective Exercise, 1st ed., p. 36.}

The NSCA describes indirect assistance: muscles that stabilize the scapula help upper-arm movement because many arm muscles originate on the scapula and wouldn’t work effectively without that support.^{3Haff and Triplett (eds.), Essentials of Strength Training and Conditioning, 4th ed., p. 36.}

Synergists can also counter unwanted motion.^{3Haff and Triplett (eds.), Essentials of Strength Training and Conditioning, 4th ed., p. 36.} The rectus femoris crosses the hip and knee, so it flexes the hip as it extends the knee.^{3Haff and Triplett (eds.), Essentials of Strength Training and Conditioning, 4th ed., p. 36.} When rising from a low squat without inclining the trunk forward, hip extensors such as the gluteus maximus act synergistically to counter that hip flexion.^{3Haff and Triplett (eds.), Essentials of Strength Training and Conditioning, 4th ed., p. 36.}

In practice

Even elbow flexion needs help from muscles beyond the upper arm, including the neck, back and core muscles that stabilize the spine.^{1International Sports Sciences Association, Strength and Conditioning, p. 43.} During a right-arm dumbbell curl, muscles crossing the wrist hold it neutral while shoulder muscles contract to prevent shoulder movement.^{2International Sports Sciences Association, Corrective Exercise, 1st ed., p. 36.}

How textbooks define synergist

The ISSA bodybuilding text uses a narrower definition: it calls the hamstrings an “assistant mover” in hip extension and reserves “synergist” for muscles that cancel movement.^{4International Sports Sciences Association, Bodybuilding: The Complete Guide to Unlocking Muscle Hypertrophy, 1st ed., p. 34.} In that text, helping synergists cancel each other’s normal movement; true synergists cancel an unwanted movement of a prime mover without joining the desired one.^{4International Sports Sciences Association, Bodybuilding: The Complete Guide to Unlocking Muscle Hypertrophy, 1st ed., p. 34.}