Exercise is a stress to your body that elicits changes to your physiology. These changes make it possible for your body to perform the activities you ask of it more effectively and efficiently. However, these changes take time and will be lost if you do not maintain your activity. Long-term aerobic and resistance exercise elicits different changes in your muscles that occur over a significant period.
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Mitochondria inside your muscle cells are responsible for aerobic metabolism to produce energy during exercise. If you have more mitochondria inside your muscles, your muscles can produce more energy any time. This is exactly what occurs inside your muscle in response to regular endurance training. Your body increases the number of mitochondria producing energy, which increases your fitness and endurance.
Regular, long-term resistance training induces a change in the size of your muscle fibers. Resistance training causes your muscles to hypertrophy, meaning muscle fibers inside your muscle increase in size. Your muscle fibers can increase in cross-sectional size between 20 and 45 percent. However, it will take at least 16 regular exercise sessions before you begin to see any change in muscle growth, according to Len Kravitz, PhD, of the University of New Mexico.
Capillary density refers to the number of capillaries that innervate a muscle. Capillaries are the smallest blood vessels that supply a working muscle with oxygen-rich blood and remove harmful carbon dioxide. With regular exercise, your muscles can increase capillary density; an increase in 5 to 20 percent may appear within 12 weeks of regular exercise, according to "Exercise Physiology.” Greater adaptation will be evident in the long-term. This adaptation allows for greater endurance in working muscles.
Regular resistance training with overload, or a progressive increase in weight or resistance, can cause your muscles to gain strength. Your muscles will adapt to whatever types of strength you need. For example, athletes like shot putters may need speed strength to produce a large amount of force in a short period. Whatever overload stimulus you give your muscles -- as long as you train specifically for your sport or activity -- will result in necessary strength gains.