When looking at muscle function, people often look at one joint, consider the muscles that act on that joint, and identify the muscles that perform opposite actions as antagonists to one another. In some cases, this is a suitable explanation. However, some muscles cross more than one joint. The word used to describe muscles that cross two joints is biarticulate (bi =2, articulation = a joint, biarticulate = 2-joint). These 2-joint muscles create synergy in muscle groups that are often thought of as antagonistic. Watch the video below.
Synergy occurs when multiple joints are flexing together or extending together. This occurs naturally during all task-oriented movements. Running, jumping, cutting, pushing, pulling, picking things up, throwing... all these movements feature synergistic coordination between muscle groups.
When it comes to enhancing those movements by adding neurological drive, using strength exercises that feature similar coordination provides good training transfer. Thus exercises like squats, hip hinges, lunges, loaded jumps, loaded throws, pullups, and presses are the fundamentals of strength training. We do not need to overthink exercise selection or seek out fancy training. The simple exercises are actually the best exercises.
This does not mean less functional movements are bad or worthless. We also have to consider structural strength along with neurological. An isolation exercise can feature extremely high muscle tension and thus provide a great structural stimulus. With the lack of neurological transfer, we cannot expect great performance enhancement over time from non-synergistic exercises, but they can still play a role in training.
Why is this is important to recognize? There are a thousand ways to work a muscle. But in training for performance, we have to develop strength that carries over to sport. Achieving that is not a matter of just working the right muscles. It requires neurological transfer. We have to develop strength in movements that feature similar large scale coordination to the sport.
People have long known that primarily developing muscle groups in isolation is not optimal for improving sports performance. Biarticulate muscles and synergy give us insight as to why.