Muscle Fibre Types and Contraction

Muscles generate heat and force for movement, help us breathe, and keep our bodies upright. Skeletal muscle tissue is composed of two fibres, actin (thin fibres) and myosin (thick fibres). These two fibres give the muscle a striated appearance. In order for muscle to contract nerves called motor neurones must first stimulate it. A single motor neurone and the muscle fibres stimulated by it are called a motor unit. The recruiting motor units play a large part in the force of the muscle during contraction. The more motor units (muscle fibres) recruited the stronger the force of contraction.

Muscle fibres are classified as Type I, Type IIa and Type IIb fibres. "Fast" and "slow" twitches are also two other classifications for muscle fibres. Type I fibres (slow twitch) fibres are more resistant to fatigue than Type IIa or IIb fibres and have a high capacity for aerobic metabolism, fatigue faster and are mainly anaerobic.

Slow twitch fibres are mainly for endurance while fast twitch is for speed and performance. A muscle will generally have an equal amount of both fast and slow twitch muscle fibres. In regards to hypertrophy (muscle growth), fast twitch fibres grow faster and larger than slow twitch. Within the fast twitch muscle fibres, type IIa fibres are considered intermediate between fast and slow twitch fibres in relation to speed and contraction. For example, Type IIa fibres can become more glycolytic or aerobic depending on the type of training an athlete performs. If an endurance runner were to stop running and start weight lifting, then his or her Type IIa fibres would become more glycolytic in order to handle the stress of the activity.

Muscle growth and endurance is an adaptation to stress. For example, a sprinter will develop large quadriceps and hamstrings in order to adapt to the stress, while an endurance runner will develop more endurance to efficiently handle the stress. Type I muscle fibres respond to stress by becoming more efficient and stronger with slight hypertrophy, rather than the extreme hypertrophy seen with Type IIa and IIb muscle fibres. This is the premise behind trainers recommending 6 reps for pure strength/muscle gain and why 10-15 reps are recommended to "tone" a muscle.

Finally, there are four different actions a muscle can perform; isometric, eccentric, concentric, and isotonic. An example of an isometric contraction would be pushing against a wall. Lifting a dumbbell during a bicep curl is considered the concentric portion while lowering of the weight is called the eccentric portion of the exercise. There are also called the positive and negative portions respectively. And finally, isotonic contractions are those that involve full body actions such as skating or running.

Diabetes and Exercise

There are two main types of diabetes, type I and type II. Type I diabetes is characterized by the pancreas making too little or no insulin. An individual with diabetes type I will have to inject insulin throughout the day in order to control glucose levels. Type II diabetes, also known as adult onset diabetes, is characterized by the pancreas not producing enough insulin to control glucose levels or the cells not responding to insulin. When a cell does not respond to insulin, it is known as insulin resistance. When a subject is diagnosed with type II diabetes, exercise and weight control are prescribed as measures to help with insulin resistance. If this does not control glucose levels, then medication is prescribed. The risk factors for type II diabetes include: inactivity, high cholesterol, obesity, and hypertension. Inactivity alone is a very strong risk factor that has been proven to lead to diabetes type II. Exercise will have a positive effect on diabetes type II while improving insulin sensitivity while type I cannot be controlled be an exercise program. Over 90% of individuals with diabetes have type II.

Exercise causes the body to process glucose faster, which lowers blood sugar. The more intense the exercise, the faster the body will utilize glucose. Therefore it is important to understand the differences in training with type I and type II diabetes. It is important for an individual who has diabetes to check with a physician before beginning an exercise program. When training with a diabetic, it is important to understand the dangers of injecting insulin immediately prior to exercise. An individual with type I diabetes injecting their normal amount of insulin for a sedentary situation can pose the risk of hypoglycemia or insulin shock during exercise. General exercise guidelines for type I are as follows: allow adequate rest during exercise sessions to prevent high blood pressure, use low impact exercises and avoid heavy weight lifting, and always have a supply of carbohydrates nearby. If blood sugar levels get too low, the individual may feel shaky, disoriented, hungry, anxious, become irritable or experience trembling. Consuming a carbohydrate snack or beverage will alleviate these symptoms in a matter of minutes.

Before engaging in exercise, it is important for blood sugar levels to be tested to make sure that they are not below 80 to 100 mg/dl range and not above 250 mg/dl. Glucose levels should also be tested before, during, after and three to five hours after exercise. During this recovery period (3-5 hours after exercise), it is important for diabetics to consume ample carbohydrates in order to prevent hypoglycemia.

Exercise will greatly benefit an individual with type II diabetes because of its positive effects on insulin sensitivity. Proper exercise and nutrition are the best forms of prevention for type II diabetics. It is important for training protocols to be repeated almost daily to help with sustaining insulin sensitivity. To prevent hypoglycemia, progressively work up to strenuous activity.

As with individuals with type I diabetes, carbohydrates should also be present during training to assist in raising blood sugar levels if the individual becomes low.