I know because I do that, and our members do that. One thing that's helped me think twice about skipping workouts is finding out just how fast and to what extent not working out changes your body for the worse.
Within the first two days of not working out, your levels of feel-good and alertness chemicals (such as beta-endorphin and adrenaline) drop. Your mood and energy levels decline (the people around you notice before you do). The energy drop is ironic because people (me) usually skip workouts because they're too busy or too tired. With the drop in these chemicals your sleep quality goes down, and so does energy.
The groundwork is also being laid for some nice fat gain in the near future because those chemicals also have a natural appetite-suppressing effect.
By day three your muscles are really stiffening up, and your aerobic capacity has already dropped 5 percent. (Your heart and lungs are already 5 percent less fit.)
Within a week of skipping workouts, your metabolism has already started to slow down, and your fat stores are creeping up. A big reason why your metabolism slows is because your body's ability to use oxygen has dropped 10 percent, and your body needs oxygen to burn calories -- 4.7 calories burned up per liter of oxygen you can use. The oxygen your body can use is not the same as what you simply inhale.
Within two weeks your body starts shedding muscle and replacing it with fat while your metabolic rate continues to drop. In addition, your heart is now 15 percent less fit.
By the time week three rolls around, your body's ability to use oxygen has dropped another 10 percent, which is now 20 percent in total. This makes it harder for your body to create energy (you're more tired) and this slows your metabolism even more. In addition, your body has become much worse at keeping you cool when it's hot. Meaning it spends more energy to try to cool you down, meaning you have less energy for everything else in your life. You're uncomfortable and tired.
Within 25 days you've lost 10 to 15 percent of your muscle mass, and it has likely been replaced by fat. And by day 29, your strength has dropped by as much as 30 percent. What a difference a month can make.
Don't let the above information make you feel bad for skipping a workout, let it help to motivate you to be more consistent. One of the biggest secrets to being more consistent in the gym is that when time or energy is short, do less. For example, let's say your current plan calls for you to do three sets of eight repetitions of a handful of different exercises for your resistance training, and work has been kicking your butt, so energy and time are short.
The best thing you could do is show up to the gym and only do one or two sets of the exercises instead of the full three sets. Yes, one set.
People think, "Just do one set? I won't get anything out of one set. If I can't do it all, I won't do anything at all." Precisely the opposite is true. You will actually get the same post-workout, 72-hour metabolic boost from one set of resistance training compared to doing the full three sets. That's not to say that three sets is completely equal to one set, but the post-workout metabolic boost is equal, and that one set is infinitely better than none. You don't need to be perfect, you just need to be consistent.
Josef Brandenburg is a Washington, D.C.-area certified fitness expert with 11 years of experience. In 2004, he started The Body You Want personal training fitness program.