According to Howard G. Knuttgen, PhD., a senior lecturer at Harvard University, a decline in performance and muscle size happens after as little as 1-2 weeks of not exercising. Muscle strength will continue to decrease with no stimulation, and body composition will be altered with more body fat and less muscle. Even two days of inactivity can negatively impact insulin sensitivity as shown in a 2004 study from the Journal of the Physiological Society.
After 2-3 weeks of no exercise, heart strength will also be affected. No matter how physically active you were when younger, the benefits of physical activity do not last. This was shown in a 2010 study in the British Journal of Sports Medicine when researchers looked at former elite male athletes now in their 50's. Men who did not continue any physical activity as they grew older had much greater risks for insulin resistance and other negative metabolic consequences. Basically, the desirable health outcomes from regular exercise do not last and exercise has to be consistently maintained to keep the positive benefits.
There is a silver lining to this. When you can't maintain your normal exercise routine, even doing a little bit is a lot better than doing nothing at all. According to a 2010 study in Medicine and Science in Sports and Exercise, even lifting one time per week can keep the muscular gains from a regular weight lifting routine. Researchers split a group of people ranging in age from mid 20-70's to complete a workout 3 times per week of a weight lifting routine for 4 months. Then, participants were split into a group of no exercising, lifting one time per week, and the final group lifted one time per week but only a fraction of their normal weights. The group who maintained weight lifting one time per week maintained or gained further muscle strength. What these results suggest is if you have baseline fitness, even if you have to cut way back on workouts, this minimal exercise could possibly maintain your fitness.
However, more research needs to be done to tease out confounding factors such as age, time, gender and fitness level. The bottom line is doing a little bit of exercise can pay off when, for whatever reason, your exercise routine drops off. So, even if you are only able to go to the gym for one time the whole week, GO. It will not be a waste. Then, get back into your regular exercise routine as soon as possible. Even if you can not make it to the gym or have enough time for your typical run, go for a walk.
Holly Klamer is a Registered Dietitian and personal trainer in Colorado. She received her undergraduate degree with a double major in Dietetics and Health Fitness from Central Michigan University. She then went to Colorado State University for her Master's degree in Human Nutrition emphasizing in Exercise Science. There she completed her dietetic internship to be a Registered Dietitian and was a teaching assistant in the nutrition department. Holly loves to travel, be outside, run, road bike and hike. She ran cross country and track in college and still enjoys competing in long distance running. Her passions are in sports nutrition, disordered eating, teaching others how to eat healthy on a limited budget, worksite wellness, weight loss and food allergies. She enjoys public speaking for various nutrition topics especially to young athletes, writing nutrition education material, and individual counseling. Holly has a passion to help people reach their goals of health and improve athletic performance. She currently works as a personal trainer, sports dietitian and free lance writer for various health websites. To contact Holly, email her at firstname.lastname@example.org.