An upper/lower split can last you forever. A lot of massive, strong powerlifters stick with that throughout their entire lifting careers. However, if you’re older and/or have some trouble recovering, you may prefer a push/pull/legs split that has you training everything directly once per week. This is how most famous bodybuilders have trained in the past and many still do.
Want to know one of the top reasons runners end up hurt? Look at your backside. Underutilized gluteal muscles are to blame for a large percentage of injuries, says Nirav Pandya, M.D., assistant professor of orthopedic surgery at the University of California San Francisco Medical Center. And it’s a weakness that often occurs because runners are hyperfocused on building strong quads and hamstrings.
Longer rest periods are more ideal for making progressive tension overload happen, and shorter rest periods are more ideal for generating metabolic fatigue. So, if you’re doing an exercise that is better suited for progressive overload (i.e. primary compound exercises), you’re going to want to rest longer between sets to maximize strength output. And if you’re doing an exercise that is better suited for metabolic fatigue (i.e. isolation exercises), you’re going to want to rest less between sets to make that happen. And if you’re doing an exercise that is suited equally for a combination of the two (i.e. secondary compound exercises), you’re usually going to want a moderate rest period somewhere in between.
That pump is tangible, real-time biofeedback to let you know that blood is flowing to your muscle cells, beginning a chain of events that stimulates protein synthesis. Maybe that's why it's easy to overlook how important good nutrition is in the mass-building equation. When you choose to eat, say, chicken instead of ice cream, there's no immediate muscle gratification -- no pump to keep you motivated.
It has been argued that purposely overtraining for a brief period can be beneficial. One article published by Muscle & Fitness magazine stated that you can "Overtrain for Big Gains". It suggested that if one is planning a restful holiday and does not wish to inhibit their bodybuilding lifestyle too much, they should overtrain before taking the holiday, so the body can recuperate and grow during the prolonged rest period. Overtraining can be used advantageously, as when a bodybuilder is purposely overtrained for a brief period of time to super compensate during a regeneration phase. These are known as "shock micro-cycles" and were a key training technique used by Soviet athletes.