Mark Rippetoe

Humans are not physically normal in the absence of hard physical effort.

The human body functions as a complete system – it works that way, and it likes to be trained that way. It doesn’t like to be separated into its constituent components and then have those components exercised separately, since the strength obtained from training will not be utilized in this way.

Machines, on the other hand, force the body to move the weight according to the design of the machine. This places some rather serious limitations on the ability of the exercise to meet the specific needs of the athlete. For instance, there is no way for a human being to utilize the quadriceps muscles in isolation from the hamstrings in any movement pattern that exists independently of a machine designed for this purpose. No natural movement can be performed that does this. Quadriceps and hamstrings always function together, at the same time, to balance the forces on either side of the knee. Since they always work together, why should they be exercised separately? Because somebody invented a machine that lets us?