Resistance bands increase tension as they're stretched.
Strength training, whether you do it with resistance bands or free weights, is effective for building strength or size in your muscles because of the stress it places upon your muscles. The type of equipment that is best for you depends on your training habits. Because each strength training implement has specific benefits, it would be suitable to use both types in your fitness regimen.
Both resistance bands and free weights provide a resistance that your muscles have to overcome. This stress eventually elicits gains in muscle strength and size. Free weights provide resistance, because of the pull of gravity on them. Bands offer resistance, because their tension increases as they're stretched. The resistance levels of bands and free weights can be increased or decreased according to a person's strength. However, advanced lifters with high strength levels may find it difficult to find a band that provides enough of a resistance that helps them further develop strength gains.
Differences in Effectiveness
According to Dr. Jim Stoppani of BodyElastics.com, resistance bands have been found to produce similar muscle recruitment activity levels and peak load levels as free weights. The same percentage of muscle fibers recruited is similar and the amount of force that the muscle fibers have to produce is also similar. In addition, a 2006 study published in the "Journal of Strength and Conditioning" found that athletes who incorporated resistance band exercises into their workouts along with free-weight exercises saw significantly greater developments in leg power than when they only used free weights.
Training with resistance bands is safer and likely a better choice when you're working out alone. When lifting with free weights, you increase your risk of injury because you're lifting the weights over your body and head. If you lose control of the weights, a serious injury could result. Thus, a spotter, who stands by ready to assist if you do lose control, is recommended when lifting free weights. Care also needs to be taken when training with resistance bands, however, as they can snap or break in the middle of an exercise. Double check that each band isn't showing any signs of wear before you use them and ensure that they're securely attached to any stationary objects you use when exercising.
Convenience and Affordability
If you're strength training at home or on the road, resistance bands are likely the way to go. Bands are easy to store in a drawer or in a suitcase, because they don't take up much space. Traveling with a set of free weights is not convenient; using resistance bands instead means you don't have to find a hotel that features a fitness center. According to the American College of Sports Medicine, resistance bands are more affordable than free weights and cost just six to 20 dollars each.