Energy Expenditure and Muscle Adaptation to Exercise

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Types of Muscle Fibers

tension will increase in a graded manner over time in a stepwise fashion known as treppe. In this process, the muscle contractions become more efficient. The phenomenon of treppe results from a higher concentration of calcium ions in the sarcoplasm that can only occur when there is enough ATP in the muscle cell.

There is rarely a time when the muscles are completely relaxed. There will always be some amount of tension necessary to maintain its contractile proteins. This is called muscle tone. This requires a complex interaction between the nervous system and skeletal muscle so that few motor units get activated at a time, which allows some motor units to recover while others are active.

A lack of low-level contractions is referred to as hypotonia, which may occur when the CNS is damaged in some way. It can also happen if there is a loss of innervation to skeletal muscle. Hypotonic muscles will appear flaccid and will have weak reflexes. Excessive muscle tone is referred to as hypertonia, which is often accompanied by hyperreflexia. Hypertonia can be displayed as spasticity or muscle rigidity as is seen in Parkinson’s disease.

ENERGY EXPENDITURE AND MUSCLE ADAPTATION TO EXERCISE

Physical training alters the appearance of skeletal muscle, changing muscle performance. In the same way, a lack of muscle use can result in decreased performance and a change in muscle appearance. Hypertrophy involves an addition of structural proteins to a muscle fiber, while increasing the cell diameter. The reverse occurs when structural proteins are decreased as well as the muscle mass; this is called atrophy. Agerelated atrophy of the muscles is called sarcopenia.

Slow fibers are predominant in endurance exercises that require little force but will require numerous repetitions. These fibers will involve aerobic metabolism that allows for the maintenance of contractions over long periods of time. What endurance training does is modify these slow fibers to make them even more efficient by allowing for increased numbers of mitochondria and myoglobin in the muscle cell.

Aerobic training will also result in an increase in capillaries around the muscle fiber, which is referred to as angiogenesis. These capillaries supply oxygen and remove

metabolic waste, supplying nutrients to deep portions of the muscle. It does not greatly increase the mass of the muscle and allow the muscle to sustain itself for longer periods of time. The muscles that have a high proportion of slow oxidation muscles will benefit most from endurance activity.

Resistance training requires a large number of fast glycolytic fibers to produce short, more powerful movements. There is a high rate of ATP hydrolysis and strong crossbridges that result in more powerful muscle contractions. Resistance trainers will have a high ratio of FG fibers to SO fibers in keeping with the force of contractions required. The muscles will thicken and hypertrophy as a result of this type of training. Interestingly, there isn’t the increase in mitochondria or capillary density seen with endurance training. Connective tissue and tendon strength will increase as this resistance training proceeds.

For effective strength training, the intensity of the exercise must be increased on a continual basis. The weight lifter must lift increased weights in order to increase muscle size. This is called “increasing the load.” If done improperly, resistance training can lead to overuse injuries of the tendons, muscles, or bone. These injuries can happen if the load is too heavy or if the muscles aren’t rested enough to recover between workouts.

Muscles will increase their strength and size when they become forced to contract at near-maximal tensions. Muscles need to overload in order to hypertrophy and become stronger. There needs to be increased protein synthesis and decreased protein degradation in order to have muscle hypertrophy. Fast fibers will increase the rate of protein synthesis, while slow fibers will decrease the rate of protein degradation. Amino acid uptake occurs to a greater degree when muscles are at their highest level of contraction.

Proper rest intervals are necessary for maximizing muscle tension between exercises and during training sessions. Insufficient rest leads to poor recovery and decreased muscle capacity. Most athletes will do strength training three to four times a week; however, large muscle exercises are done no more often than twice a week. This is completely empiric but appears to be enough to have adequate recovery between sessions.

There will be specificity between the nature of the exercise and the adaptation of the muscle. For example, if an athlete decides to strength-train the leg muscles, the leg muscles will hypertrophy and not any other muscles. The different muscle types have characteristic contractile properties as you have seen. The low-threshold muscles are the slow twitch fibers, used for jogging and the basic activities of daily living. High-intensity activities and high-speed activities all require recruitment of fast twitch motor units.

As mentioned, there are individual differences between the fast twitch to slow twitch ratio in athletes that excel in certain areas of athletics. Endurance athletes will have more slow-twitch, SO muscle fibers, while sprinters and weight trainers will have more fast-twitch, FG muscle fibers. This is genetically determined. On the other hand, there are benefits from training irrespective of a person’s genetics such that good training can help make up for some differences in an athlete’s genetic makeup.