A few of the benefits gained from resistance exercise are:
When done appropriately there are many biological and physiological effects of resistance exercise that make it the optimal mode of exercise for weight management and improved metabolism. Many people say they don't want to lift weights because they want to lose weight so they run instead. Running can lead to weight loss, but sometimes also through a loss of muscle mass, which is not the goal. Muscle is a very metabolically expensive tissue to have around, in other words, muscle requires a lot of energy to function so naturally, if you have more muscle you will burn more calories because your body has to burn more fuel to support all the muscle it has around. This is one of the reasons muscle is lost when you don't exercise regularly, your body doesn't keep muscle around unless it absolutely has to. When you contract your muscles you induce the transport of glucose into the cells without the requirement for insulin. Therefore, if someone is diabetic they can still clear glucose from their bloodstream via muscle contraction. High intensity training also depletes the muscle stores of glycogen, increasing glucose turnover and moving glucose from the blood to be stored in the muscle. Over time this will decrease the body's need to release insulin and allow the cells to become more insulin sensitive. Insulin is an anabolic hormone and it helps build muscle and store fat. In order to burn fat, insulin levels must be able to be reduced to release the enzymes that allow for fat mobilization. This is one of the reasons it can be difficult for someone who is diabetic to lose fat, because they live in a state of chronically elevated insulin to try and force the insulin signal, but since their cells are resistant they don't hear it. Resistance training aids in weight loss by decreasing insulin levels which allows for fat mobilization while increasing muscle mass so you lose fat but not muscle.
Many people believe to improve their cardiac function they need to do classic "cardio" like a long run or a spin class. Some don't think that resistance exercise can improve cardiac function and some even believe it is detrimental to the heart. Studies have shown improvements in cardiac function after resistance exercise in people with congestive heart failure. The American Heart Association even includes resistance exercise protocols in its recommendations for preventing heart disease. It was believed in the past that muscle contractions can increase peripheral vascular resistance and trap blood in the veins, but just the opposite is actually true. Resistance exercise increases vasodilation which decreases vascular resistance allowing increased blood flow to the muscles to support the high intensity activity. These muscular contractions then help milk the blood in the veins back to the right side of the heart which directly stimulates increased blood being pumped out of the left side of the heart, to supply blood to the muscles and the rest of the body. Your heart doesn't just beat when you're running or are on a stationary bike, it beats all the time and resistance exercise can help improve this process.
Bone Density, Muscle mass and function
These next two positive benefits are probably the main reasons I personally believe strength training is the best form of exercise for health and longevity. Decreased bone density, also referred to as osteoporosis and loss of muscle mass and function with aging, also known as sarcopenia are two conditions thought to just be part of normal aging. Both of these conditions are multi-faceted and have complicated mechanisms that are incompletely understood and while there's still lots of research to be done it does seem that resistance training could help treat or even prevent both of these conditions. It has been shown that athletes and individuals who regularly perform weight bearing exercises like squats, deadlifts, running and jumping are able to increase their bone density. Nutritional approaches to osteoporosis have had mixed results and do not seem as effective and consistent as resistance exercise. Similarly, there has been a lot of research showing the safety and effectiveness of strength training for preventing sarcopenia. It is important to mention that sarcopenia is the loss of both muscle mass and function. This is why nutrition alone and hormonal approaches to increase muscle mass aren't totally effective treatments for sarcopenia. Just increasing muscle size doesn't solve the problem of muscle quality, that's why proper nutrition must be combined with resistance exercise. The improvements in muscle mass and function and bone density improve the quality of life for the elderly significantly because they can move around and perform daily tasks much easier. They are also less prone to falling and if they do fall are less likely to be injured since they have more muscle and bone density to break their fall. It was old wisdom that said elderly people shouldn't lift heavy things because they're weak and frail, but the truth is the exact opposite: elderly people are weak and frail because they don't lift heavy things.
Stay healthy San Diego,
Much of my inspiration and the information for this post came from the book Body by Science by Doug McGuff, M.D. and John Little. Their website is: http://www.bodybyscience.net/home.html/?page_id=18