Resistance training is an essential part of everyone's physical fitness, but it isn't a one-size-fits-all venture.
Not only do personal goals play a sizable role in any form of exercise, but health, fitness level and experience should also drive your workouts. Lighter weights with higher reps may do wonders for you, while heavier weights with fewer reps may be ideal for the person sitting next to you. Talk to a doctor before starting any exercise routine.
Resistance Training: As long as the chosen weight is exerting some level of resistance, you'll experience improvements in muscle strength. Choosing to use lighter weights with higher repetitions can produce somewhat similar results to lifting heavier weights with lower repetitions -- at least when you're new to resistance training. The key is to bring your muscles to fatigue.
Muscular Fatigue: Reaching "muscular fatigue" simply means a temporary decrease in a muscle's ability to perform a given task. As you lift a weight, it should become increasingly more difficult to complete each rep. If this doesn't occur, you're not forcing the muscle to work under enough resistance to provide any gains.
Muscle Growth: When a muscle works under resistance, the stress essentially damages its fibers. As with any trauma, the body responds by rebuilding the damaged tissue. But instead of simply repairing the tissue, your body increases the size and number of proteins within the affected muscles to help meet the demands of the resistance. The repair increases muscle mass and, in turn, increases strength.
Weight Progression: The increase in mass and strength encourages the muscles to work more efficiently under your chosen weight and number of reps. With each workout, these muscles eventually begin to meet the demands you're placing on them and gains in strength inevitably cease. Increasing the demands by increasing the weight continues to tax or overload your muscles, ensuring that improvements continue.
Recommendations: If you're hoping to gain strength, it's fine to start off with lighter weights and higher repetitions. In fact, the lighter weights can give your body the opportunity to adapt to these new demands. Plus, you're not overtaxing the muscles too soon, which can go a long way in the prevention of injuries. For long-term success, however, the lighter weights aren't likely to get you to your goal. As your strength and experience improves, high-intensity training is often better. This means choosing slightly heavier weights and doing fewer repetitions. Talk to a personal trainer to establish the best workout and starting weight to best meet your needs and goals.