Muscles and strength training are always "in". If you are ripped you are hip. However, there are many more reasons for building muscle mass other than physical appearance. Research has shown that muscle strength and size declines significantly with age. (Muscle mass typically starts to decline at around age 30, while muscle strength typically starts to decline at around age 50.) Increasing one's muscle strength and size is one of the most effective ways to preserve health and slow the aging process. Building muscles will also make you feel more energetic, less stressed, more confident, and it will increase your body's immune system.
The action of muscle growth, scientifically known as "hypertrophy", can be broken down very simply. When you exercise under stress you are creating small micro tears in your muscles. Once you are at rest, your body will start to repair these small tears and will re-build your muscles bigger than they were before. The more stress (heavy weight) you can place on your body the deeper the tears in the muscle fiber and the bigger you muscles will grow during recovery.
Abrupt starts and sudden exertions cause tremendous distress and danger to your body. ALL workouts should begin with warm ups. Warm ups introduce the body to gradual excitement of muscular activities and forces blood to flow to the muscles that are to be worked. The more blood your muscles recieve prior to strenuous work the more nutrients they'll have and the faster they'll be able to recover and grow.
Strength training is stressful not only on your muscles but your central nervous system, your connective tissues, and your immune system. While these stresses are all positive, too much weight lifting will have negative consequences. Ideally, you want to train three days per week and no more than four. Overtraining is probably the single biggest mistake people make when they start a weight training program.
Intensity is the amount of effort put forth by your muscles on each given strength exercise. Progression is consistently increasing the amount of weight lifted or the number of repetitions performed. Your strength training success will very much depend on the principles of intensity and progression. You must train with enough intensity and effort in order to trigger the growth response and you must progressively increase the workload from week to week by increasing the weight lifted or the number of repetitions performed.
Recovering from a strength training session is absolutely crucial! If you start a weight training regimen you must have good sleep habits and you must give your body the nutrition it needs to build muscle. Proper rest and proper nutrition will literally make or break your results if you're looking to pack on lean muscles.