Your skin stretches and grows as you do. You appreciate this elasticity as you gain height when you grow up from childhood. But, you may not appreciate it as much when you put on pounds and then take them off. The skin that accommodated a bigger you tends to remain long after you shrink.
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Skin doesn't shrink or tighten directly in response to exercise. What exercise can do is build up muscle to stand in place of any fat that you lost. But, if you did drop a significant amount of weight -- there's only so much space you can replace with muscle. You'll have to wait for the skin to shrink back as best it can, but it may never look as taut as you want it to.
If you have loose skin at your back due to weight loss, do take some measures to tighten it up.
Why Skin Gets Loose
Your skin expands as you do. When you gain weight and then lose it quickly, the elastic components in your skin don't always have time to adapt. In fact, 70 percent of people who undergo bariatric surgery develop loose skin, says research published in Obesity Surgery in 2013. So, the fat may be gone, but the skin remains.
Read More: How to Tighten Loose Abdominal Skin
Poor nutrition, dehydration, smoking and too much time in the sun can also affect your skin's ability to bounce back. Aging also contributes to the breakdown of collagen -- a protein that contributes to elasticity -- making skin look looser and more droopy, especially if you subject it to drastic weight fluctuations.
Remember that your skin is a living organism, so it will eventually spring back somewhat over time -- sometimes several months or years. In extreme cases, you might need a medical intervention to correct all the loose skin, depending on where it's located on your back.
What Exercise Can Do
Exercise can help to strengthen the muscle that exists underneath the skin, to fill it out a bit and make it look less saggy. A comprehensive approach to building lean tissue, or muscle, on your back and elsewhere, helps fill you out. You can't expect exercise to work overnight, nor for it to correct extreme flaps of loose skin that hang from your shoulder blades or lower back of your waist.
A comprehensive plan involves working every major muscle group three to four times per week. Use heavy weights that bring you to fatigue in eight to 12 reps, and work up to three to five sets total. If you're just starting out, your body weight is enough. Build up to weighted moves such as chest presses, rows, squats, lunges, shoulder presses, biceps curls and triceps extensions over several months.
Since your back is the area of most concern, target it with multiple resistance exercises. For example, rear deltoid flyes get at the muscles of the upper back -- behind the shoulder blades; lat pull-downs and rows target the large lat muscles at the backs of the ribs; and back extensions target the lower region of the back.