As a rule, athletes and bodybuilders do exercises in a way that creates isolation of the muscle or involves several muscles when doing compound exercises. The isolation is needed for maximum muscle growth and is considered good practice.
It is also important to understand that a muscle does not work in isolation. The muscles work together in many different ways to enable you to produce the effect desired. This is muscle synergy. Because other muscles may be involved in a particular movement, it should not be considered a negative factor.
In many cases of muscle synergy, the greater the additional muscle involvement the greater will be the effect on an individual muscle. For example, when very heavy weights are used and the movement is slow, both the agonists (muscles mainly involved) and the antagonists (muscles opposed to the action) contract simultaneously.
This can be seen when doing a biceps curl. Not only does the biceps (and other elbow flexors) contract strongly, but the antagonist triceps also undergoes contraction, but not as strong as the biceps. This allows you to go through the full range of movement. Contraction of the triceps plays an important role in helping to stabilize and maintain elbow joint integrity.
If only the biceps, brachialis and brachioradialis muscles contracted, the force of their contraction would be so great that it could pull the elbow joint apart. But when the triceps contracts it holds the forearm in the elbow so that no damage to the joint occurs.
Since the movements are relatively slow when using heavy weights there is continuous feedback to the nervous system. This allows the muscles to work in close coordination with one another not only to accomplish the movement, but to protect the joint.
In more ballistic or speed-type movements there is proprioceptor feedback from the muscles and joints which monitors the movement and makes modifications if needed. For example, in most ballistic movements the movement is terminated when the antagonistic muscles come into play near the end of the movement.
In a slow movement the antagonistic muscles work together with the prime muscle movers during the entire movement. In a ballistic movement they come into play at the end to stop the movement and to maintain safety in the joint.
In many strength exercises the muscles work together (muscle synergy) to enable particular movements to occur. For example, the internal and external obliques are used mainly in trunk rotation and lateral flexion of the spine (side bending). However, when the internal and external obliques on both sides of the body contract simultaneously, they produce spinal flexion as in the sit-up and reverse sit-up exercises.
In this case, the external and internal obliques on one side oppose or cancel the rotational and side bending movements of the muscles on the other side of the trunk and as a result, aid the rectus abdominis in producing pure trunk flexion. This act of canceling an opposite action is known as muscle neutralization. You neutralize the opposing action of the muscle so that the muscles involved can all work toward one common objective. This is a classic example of muscle synergy.
A similar situation exists when doing lateral arm raises. The major muscle involved is the middle deltoid with some assistance from the anterior and posterior deltoids. However, the anterior deltoid is also a medial rotator of the arm, while the posterior deltoid is a lateral rotator of the arm. In addition, the medial head acts to pull the arm forward in an action known as horizontal adduction, while the lateral head tends to pull the arm backward in horizontal abduction.
Thus in order to have pure shoulder joint abduction (as needed in lateral arm raises), with no rotation of the arm, the medial and lateral heads cancel each others rotational and horizontal effects. They then assist in pure abduction (side raising) of the arms.
It can be said that all muscles in the body act synergistically, i.e., they help other muscles produce specific movements. But, synergy can also produce stability, i.e., the holding of the body or body part in position to allow for a specific movement to take place.
For example, when doing the overhead press you use the muscles of the arms and shoulders to raise the weight overhead, but the muscles of the midsection and legs also contract (isometrically) to maintain a stable body position. In this case, the muscles are known as stabilizers. In fact, it is the stabilizing action (the isometric contraction) that produces additional muscle growth and definition.
An example of this is when athletes and bodybuilders do very little abdominal training, but yet have well-defined abdominal muscles. The reason for this is that in many exercises the abdominal muscles contract isometrically to hold the midsection stable (together with the lower back muscles) and thus get this additional development when working the arms, chest, back or leg muscles.
In addition, there are many muscles, known as the anti-gravity muscles, that come into play in all standing and even seated positions when doing various exercises. For example even doing the standing biceps curl requires isometric contraction of the leg and midsection muscles in order to isolate the elbow joint action needed to work the biceps and other elbow flexors.
There are still other ways that the muscles work together to ensure one particular movement. For example, if you were doing a biceps curl with a supinated (palms up) grip, the pronator teres which is involved mainly in turning the hand palm down, also becomes involved. It cancels the second action of the biceps which is to supinate the hand. As a result, you can do the exercise in a supinated grip through the full range of motion.
To help ensure muscle isolation for improving strength of the primary and secondary muscles use the 1 X 20 RM strength training program. It is by far the best program available to ensure not only strength of all the muscles but strengthening of each and every joint worked.
For more information on this topic and the relationships between muscles , see Biomechanics and Kinesiology of Exercise and The Revolutionary 1 x 20 RM Strength Training Program.