The number of muscle fibers and type (fast twitch or slow twitch) in your body was determined during the 2nd trimester of your mother's pregnancy. Each of your muscle fibers is composed of 75 percent water, 20 percent protein, 5 percent phosphates, calcium, magnesium, sodium, potassium, chloride, fats, carbohydrates, and amino acids. You have 430 voluntary muscles which represents 40 to 50 percent of your body weight. Skeletal muscle is the largest single tissue in your body.
You have two basic types of muscle fibers. Your postural muscles are Type I, endurance, red, and are considered slow twitch muscles. These muscles hold you in an erect position all day long. Type I fibers are recruited first during your strength and speed work and are capable of less force but can help you perform more repetitions (reps.) and run longer and slower than Type II fibers. Type I fibers utilize oxygen which means they are aerobic in nature. They are smaller and contain less glycogen than Type II fibers but their myoglobin content is high. They contain capillaries and provide endurance for long distance activities. Some studies show more Type I fibers in females, but other investigations demonstrated no apparent difference in fiber distribution between men and women.
Type II, fast twitch fibers are recruited for fast, powerful movements. There are two subclasses of Type II fibers. Type II a-intermediate fibers are somewhat oxidative. They use a combination of the aerobic and glycogen systems. These are recruited after Type I fibers. Type II a-intermediate fibers are fast twitch with moderate myoglobin content, capillary density, force production and endurance. If you performed 8 hard reps. on the bench press, the first several reps. utilized primarily Type I fibers. Next Type II a-intermediate fibers assisted. And finally, when you were pushing out that last rep., Type II-b non-oxidative fibers were employed.
Type II-b fibers are non-oxidative (not aerobic). They are stronger and provide more force, but they fatigue quickly. Type II-b fibers are anaerobic with a high glycogen content and fast twitch rate. They have few capillaries and low endurance but a high power output. Your muscles need glycogen, ATP, and innervation to become active. A stimulus to a motor unit contracts your muscles on an all-or-none principle. A muscle fiber will contract all the way, or not at all. One motor neuron may innervate 1000 muscle fibers in your calf, while another motor neuron may activate only 10 muscle fibers around your eye.
Muscle takes up less space than fat. One pound of fat bulges 18 percent more than a pound of muscle. Fat occupies 1.1 liters per pound while muscle requires just .9 liters per pound. Studies show that women who train with weights offset any gain in circumference by losing fat. That is, if they do not increase their fat stores by wolfing down extra calories. Men are stronger than women because they have more muscle fibers. Women are only 68-71 percent of the cross sectional area of that of men. Women can improve their strength in a similar proportion to men, however. In the lower body it is thought that women can match men in strength when compared pound for pound. In the upper body however, even when comparing women and men pound for pound, women are only 70 percent as strong as men. This is probably because women tend to use their upper body's less than men for activities of daily living that require brute strength such as lifting heavy boxes, digging, moving heavy furniture, etc.
Two women may derive different results on the same weight training program. Body types, play a major role in how women develop. A mesomorphic body type is one with well developed and defined muscles on the trunk and limbs. These women are broader in the shoulders and hips and narrower at the waist. They have a high muscle to fat ratio and may look fit even without exercise. Mesomorphs who train with weights notice a dramatic increase in strength and muscle mass.
An endomorphic body type is rounder, softer, and pear shaped. There is more fat surrounding their gluteals and quadriceps (hips and thighs). Their muscles are not well defined and they have a higher fat to muscle ratio on their trunks and limbs. Exercise helps with fat loss but it is slow, especially in their hips and thighs. Strength training should focus on upper body development to balance larger hip proportions.
An ectomorphic body type is long and rectangular, flat chested, slender in the hips, with no defined waist. These women have poor muscle development on their trunks and limbs with relatively low body weight. Strength training should be of a higher intensity since this body type has difficulty retaining muscle. One of the most wonderful inventions in the last decade with regard to strength training is the PowerBlock. And when you are traveling, you can’t beat the SportCord for convenience and versatility.