Bodybuilding exercises involve both compound movements (involving several muscle groups at the same time) and isolation movements (focusing in on specific muscles or muscle groups). For example, a barbell bench press is a compound exercise that involves the pectorals, triceps, and front delts. A dumbbell flye is an isolation exercise which, done correctly, isolates just the chest.

But bodybuilders have traditionally tried to use techniques to isolate one specific head or part of a complex muscle group. Some try varying their foot position while doing squats to hit the outer head of the quadriceps and develop a more pronounced sweep to the thighs. The problem is there is a physiological principle that tells us this simply isn’t effective.

The principle of non-contiguous innervation tells us that motor neurons don’t stimulate specific muscle fibers in limited locations or groups, but innervate muscles over widespread areas. So, there is a network of connections emanating from the motor neurons that cause contraction of muscle fibers in the quadriceps that connect to all the different heads of the muscle group, not specifically to any one muscle in the group. So, these muscles will always be activated as a group, and you can’t really train one of the heads by itself.

This means that training at different angles or changing the position of your feet during squats or leg presses doesn’t allow you stimulate a specific area of the quads. You can use a technique that isolates the inner thigh adductors apart from the quads, because they’re a different muscle group. But when you do squats, you’re inevitably involving some adductor contraction along with working the quads. The body has evolved to allow muscles to work in whatever combination that allows you to function.

As another example, the upper and lower abs pretty much work as a single unit. When you do a primary ab exercise like crunches, it’s better if you can work through a longer range of motion, but because of non-contiguous innervation, all parts of the abdominals will respond as a connected unit. You can’t isolate upper or lower abs.

The same problem exists if you try to isolate one head of the biceps or triceps or the upper or lower chest. The exercises you use may feel different, but the motor neurons involved don’t know what you think you’re accomplishing. They’re doing their physiologically designated function and don’t limit stimulating muscle fiber to a given physically limited area.

In fact, including a wider variety of exercises for a body part does not itself automatically lead to increased development. The biceps are a relatively simple muscle group and almost any curl movement has the same result. A curl is a curl is a curl.

Additionally, a small muscle group like this is fairly easy to overtrain. Remember, the modern approach to bodybuilding training is short bursts of intense workouts followed by sufficient periods of rest and recuperation. You don’t grow when you are training, you grow when you rest.

So why do bodybuilders continue to do so many different exercises? Primarily to avoid boredom and over-habituation.

When you do the same routine week in and week out, it’s hard to keep up your enthusiasm. Plus, you do the movements by sheer habit, don’t give them your full attention and start to lose intensity. When you do a new exercise, use a new technique or try a new piece of equipment, it tends to help focus your attention.

Arnold used to occasionally do his training partner’s workout or do his own in reverse order, both of which would force him to pay more attention to what he was doing.

The other reason is that bodybuilders really like training. If feels good to them. They love the pump. With a really good bodybuilder, you never have to motivate him or her to go to the gym. You need to encourage them to limit their workouts to avoid overtraining.

That’s also a benefit of using free weights. Work on a leg press machine as heavy as you want, but it is not the same as trying to balance and deal with the stress of a barbell squat. Free weights involve stress on the joints you don’t get from machines or cables. Research has shown the whole nervous system lights up training with free weights in a way that is not true for machine and cable training.

When it comes to variety of exercises, there are special exceptions. For example, the back is really a complex set of interacting muscles. So, you need movements for the lats, the middle back, the lower back and the traps. Lat development requires an amount of resistance too great for you to get your shoulders shrugged far back enough to engage the muscles of the middle back. So, you need a different set of exercises to develop both.

To sum up, because of the principle of non-contiguous innervation, don’t waste our time and energy trying to isolate one head of a multiple head muscle. Instead, in general you should limit the number of different exercises you do for a body part, train in short but more intense workouts, then rest and recuperate to give your body a chance to rest and grow.

Bill Dobbins is a prolific bodybuilding photographer and co-author of Arnold’s New Encyclopedia of Modern Bodybuilding.