THE ARTICLE
Many of us have read the research or had instructions from a personal trainer that it is not a good idea to work the same muscle group every day, so you shouldn’t lift weights with the same muscles two days in a row. In fact the American College of Sports Medicine and the USDA Dietary Guidelines both recommend moderate to vigorous physical activity most days of the week, but resistance training less frequently. Since cardio activity uses the same leg muscles every day, sometimes intensely, is it safe to do cardio every day? What is the difference between working the gluteals and the thighs while doing squats and lunges and working them while riding a bike or using a stair-climbing machine?
The main difference in these activities is the intensity of the muscle contractions used in cardio exercise and those used in resistance exercise. This difference affects how hard the muscles are worked and which kind of muscle fibres are activated. Aerobic exercise, such as running, biking, swimming, and aerobic dance, uses many muscle groups over an extended period of time, activating mainly what are known as slow-twitch muscle fibres. Slow-twitch muscle fibres use fat as their main fuel and because the activity is relatively less intense than that of resistance training, you can maintain the activity for up to several hours, depending on your fitness level. The fact that slow twitch fibres use fat for fuel is also why, in general, moderate intensity aerobic activities are considered essential for weight loss and maintenance.
Weight lifting and other resistance exercises (such as pushups) activate fast-twitch muscle fibres, which are the fibres that perform more intense and short-term activities. Fast twitch fibres are needed to provide greater force and they use glycogen as their main fuel source. Because fast-twitch fibres aren’t designed for endurance and because they use quickly depleting glycogen instead of fat for fuel, you can’t sustain resistance exercises for more than a few minutes. During these exercises, your muscles are overloaded (if you lift enough weight to challenge them) and result in microscopic breakdowns in the muscle fibres. Over the next couple of days, your body repairs the breakdowns and makes the fibres more resilient for next time, and this is how you get stronger. You need the 48-hour (plus, depending on intensity) period to allow your body to build up the fibres, otherwise you’ll keep breaking them down without giving them a chance to get stronger. You also need to maintain the resistance-training schedule, or your fibres will weaken and return to their pre-training level. A well-designed resistance training programme complements aerobic activity by helping promote a healthy body composition, develop strong bones and joints, and maintain bone density into old age.
Of course, it is possible to exercise aerobically at such intensity that you can involve many more of your fast-twitch muscle fibres along with the slow-twitch fibres. Depending on your fitness level, a long-distance run, hiking up steep hills, or simply doing an extended aerobic activity you are not used to performing may challenge you to the point of depleting your glycogen stores and breaking down your muscle fibres. This kind of activity will result in the need to rest for 48 hours or more, before working them again. In this case, you will know by listening to your body (and feeling the soreness!) that you can’t exercise at that intensity every day.
Most of us do not work that hard every day, and we also tend to use different machinery or different exercises so our muscles don’t get overworked to the point of not being able to work out the next day. In fact, it is recommended that you perform cardio activities almost every day, to increase calorie expenditure, combat stress, and build up overall cardiovascular health. Cross training, or the use of a wide variety of different aerobic and strength training exercises, is the perfect way to develop overall muscular and cardiovascular fitness, without overdoing. You can exercise this way nearly every day, if you choose, although one rest day a week is always a good idea.
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