If you were to read many professional exercise science certification manuals, you would find diagrams showing what muscle groups an exercise works. However, you must know there is a big difference between "working" a muscle and "isolating" a muscle, which makes many of these manuals somewhat misleading. Click the Slideshow to learn 5 important differences between working and isolating a muscle:
For example, while it is true (and many professional manuals stress this) that squats "work" the hamstrings and calves, it is not true that they isolate these groups. Likewise, while doing rows for the back will "work" the biceps, they will not isolate them. Another example is how a chest press "works" the deltoids and the triceps, but it does not isolate them. There is a big difference between the two terms.
First off, only "working" a muscle will not actually strengthen it the way desired, and it will definitely not sculpt it and define it in the way most people want. First of all, these kinds of results are just not that simple to begin with- for starters, reps and sets, along with weight and technique all play more pivotal roles in gaining strength, tone, and definition or size gains than just exercise selection.
Secondly, working a muscle does not lead to any kind of dramatic definition or toning of that muscle- no separation, striations, or rippedness can occur only by "working" a muscle; these can only occur by isolating a muscle.
While some strength could be gained for a muscle worked instead of isolated, it would more pertain to the strength one has on the actual exercise, not necessarily that individual muscle group- for example, one would get stronger on squats due to The SAID principle- 'Specific Adaptation to Imposed Demand', but to say that the synergistic muscles strengthened as a result would be misleading. This is not to say that they will not improve in strength, just not equally with the protagonist muscles of the exercise (the quads and glutes), which is the way they generally make it seem.
This brings up the main point- a synergistic muscle is not equal to the protagonist during an exercise. It is only meant to "help" and stabilize the protagonist with the movement. Only the protagonist muscle is actually isolated. Sometimes the antagonist muscle, like the hamstrings to the quads on squats, can also work and increase relatively slightly in strength, but the there is still no isolation.
So to think as many textbooks would lead a beginner to believe that working a muscle is the same as isolating it and leads to similar or equal results as one would benefit from through isolating it is misleading, and uninformed.
The truly best results for toning, shaping, defining, strengthening, or building a muscle can only be attained through isolation, and the more isolated the muscle during an exercise, and the more intensity of the isolation, will trigger the fastest and most rapid transformation of that muscle group.
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