Any of you who have hung around a gym or fitness center and lifted weights have probably heard the controversy regarding whether to do heavier weights with fewer repetitions or less weight with more repetitions. What's the difference anyway?
This one fits into the "if you weight long enough, things that fall out of vogue will come back in vogue eventually." For years it was common wisdom to assume that if you lifted weights and wanted to bulk up or get bigger, you would try to lift heavier weights and do fewer repetitions. On the other hand if you wanted better definition or "cuts," that is to be able to discern the various curves, folds, ripples and relationships between your muscles, it was better to do lighter weight and more repetitions with a given weight lifting exercise. Later, that myth was partially dispelled and the feeling was that doing somewhere between 8-12 repetitions, with the last one or two being very challenging, was the best way to both build muscle bulk and help define muscle structure. Of course, keeping body fat content as low as possible would also serve to help the latter.
Fast forward to 2008, and the discussion now needs to involve what are known as "fast-twitch" and "slow-twitch" muscle fibers. What are they and who cares? All of our muscles have both of these types of muscle fibers. The fast-twitch fibers are the ones that are responsible for rapidly contracting for power - the ones you'd use to do the 40-yard dash or bench press a heavy barbell - at least for the first several repetitions. They give you a lot in a short amount of time but tire quickly and easily. Slow-twitch fibers serve the opposite purpose. They contract more slowly and with less power, but have better endurance. They're the ones you'd use while running a marathon, riding a bike or any more easily done repetitive activity. Each of these muscle fibers can be used in the same exercise. For instance, while bench pressing a moderate amount of weight (neither too heavy nor too light), the first reps, say 1-7, recruit mostly fast-twitch fibers, while those last few reps, say 8-12 or higher if possible, begin to recruit those slow-twitch or endurance fibers.
The bottom line is that both are important and both are stimulated by different types of activities. Neither should be ignored, and the more fibers that are recruited, the more fibers get "micro tears" that then need to repair themselves which causes them, and hence the entire muscle, to enlarge. Mixing up your workout is probably the best solution. If you go a bit heavier a couple of times a week, enough to get about 6-7 repetitions of an exercise, try going a bit lighter on the third workout of the week and squeezing around 12 reps out of each exercise. That will work both slow and fast twitch fibers giving your muscles the best chance of growing bigger and stronger. Of course, discuss any rigorous exercise program with your primary care provider before starting.