Mike Mentzer is an advocate of productive exercises,
such as the Nautilus Behind Neck machine shown here.
There are a variety of factors which go into determining what exercises a person should perform in a given session. The most important factor is safety. The exercise must be biomechanically correct, and it must not place undue stress on the joints. The exercise must be able to be performed in a slow and controlled fashion through a full range of motion, and it should not place the body in an awkward position where an injury could occur.
The second factor is the confidence of the athlete to perform a particular exercise. The barbell squat is a very productive exercise when performed through a full range of motion - to the point where the calves contact the backs of the thighs - and taken to a point of MMF. However, some athletes may have trouble learning the movement, and some may not be able to perform it well at all due to poor leverages. If an athlete is not comfortable doing an exercise because he is too concerned about his safety or form, then invariably he will not work as hard.
Instead of insisting that the athlete performs barbell squats, it's much more sensible to choose an alternative exercise which is comparable in effectiveness, and which the athlete is confident performing all-out to MMF. If the athlete is allowed to perform a leg press or machine squat exercise in place of the barbell squat, he will not have to worry about all of the technical details associated with performing a barbell lift correctly, he will not have to worry about balancing the bar on his back, and he will not have to worry about how he is going to dump the bar when he reaches MMF. He can concentrate all of his mental energy on training as hard as humanly possible and providing an adequate stimulus for increased strength and muscle size. If there are two exercises which are comparable in their effectiveness and efficiency, always choose the one that the athlete is more comfortable performing to a point of total exhaustion.
Another factor in selecting exercises is their effectiveness and efficiency. A productive exercise must be able to exhaust a relatively large amount of muscle mass in a short amount of time. The most productive exercises - the ones which will stimulate the largest amount of muscular growth - are compound exercises, or ones which involve rotation around more than one joint and therefore involve a large number of muscles. Your program should be built upon these movements, and augmented with single joint exercises.
Single joint exercises should address muscles which are not trained to a sufficient degree with compound exercises. For example, the lattimus muscles of the upper back are at the mercy of the weaker arm muscles in the close grip chin. Because the biceps will reach MMF well before the upper back, in many cases it is necessary to address the lattimus muscles with an isolation exercise such as the machine pullover or manual row so that they receive sufficient stimulation. Also, some muscles are all but ignored with a program made up of compound exercises. The neck and the forearms are examples of muscles which need direct stimulation in order to make them stronger.
When you are choosing exercises, always pick compound movements as the basis of your program. Basic exercises like the squat, chin, and dip give the most bang for your buck, and they will provide you with a good foundation on which to build. Use single joint exercises like the shrug, four way neck, and wrist curl to address smaller muscles not hit by the compound exercises. However, keep in mind that the more exercises you do, the more of an inroad that the workout will make into your ability to recover, and beyond a certain point you will be unable to recover at all.
