Trained muscle

Efforts to decrease the risk of falling include balance training, muscle strengthening, removal of hazards in the home, installation of fall-reduction measures such as handrails in the home, and discontinuation of predisposing medications.1,2,10 Additionally, the use of hip protectors is an effective way to prevent hip fractures, although adherence to this measure is poor, and some patients may not be amenable to wearing them at all.10... 

Cellular Responses In Trained Muscle. Recent evidence, obtained from appropriate measurements of metabolites within the cell during contractions, suggests that skeletal muscle of trained individuals Is better able to adjust, as compared to skeletal muscle of untrained Individuals, to the energy demands of a submaxlmal contraction effort. This Is apparent since metabolic conditions altered by contractions within trained muscle change less than in untrained muscle from that found at rest. For example, the PCr content of... 

Endurance training also has a noteworthy effect on fat metabolism. In sedentary individuals, especially those who have undergone calorie-restricted diets, adipocytes are resistant to the stimulation by the sympathetic nervous system that accompanies physical exercise. For reasons that are still poorly understood, endurance training results in an increased sensitivity of adipocytes to these hormones. This is an important point, because the amount of fatty acids transported into muscle cells and used to drive muscle contraction is directly related to their concentration in blood. Trained muscle can generate more energy by oxidizing ketone bodies. 

In addition to its internal blood flow operation, the heart has its own system of blood vessels to keep the muscle wall of the heart, the myocardium, supphed with oxygenated blood (Fig. 3a). The coronary arteries, which branch from the aorta to the right and left sides of the heart, are vital to maintaining that supply. The heart is an extraordinary electromechanical muscle that can be trained to increase blood flow to the body sixfold. It can range from 5 to 30 L /min during exertion. 

One of the more intriguing cardiovascular developments is cardiomyoplasty where implantable technologies are blended with another part of the body to take over for a diseased heart. One company, Medtronic, in close collaboration with surgeons, has developed a cardiomyoplasty system to accompany a technique of wrapping back muscle around a diseased heart which can no longer adequately pump. A combination pacemaker and neurological device senses the electrical activity of the heart and correspondingly trains and stimulates the dorsal muscle to cause the defective heart to contract and pump blood. Over 50 implants have been performed to date. 

Figure 2. Muscle stimulation, a) a single nerve impulse (stimulus) causes a single contraction (a twitch). There is a small delay following the stimulus before force rises called the latent period, b) A train of stimuli at a low frequency causes an unfused tetanus. Force increases after each progressive stimulus towards a maximum, as calcium levels in the myofibrillar space increase. But there is enough time between each stimulus for calcium to be partially taken back up into the sarcoplasmic reticulum allowing partial relaxation before the next stimulus occurs, c) A train of stimuli at a higher frequency causes a fused tetanus, and force is maximum. There is not enough time for force to relax between stimuli. In the contractions shown here, the ends of the muscle are held fixed the contractions are isometric.

Notes Assuming a moderately active 70 kg man with 40% of body weight as muscle (28 kg), a liver weight of 1.8 kg, a plasma volume of 3 L, and 9 kg of adipose tissue. Endurance trained individuals store -125-150 mmol/kg wet muscle of glycogen in muscle and 400-700 mmol/kg wet tissue of glycogen in liver,... 

There is no single underlying cause for the myotonia seen in the muscles of myotonic patients. The typical myotonic response is a train of action potentials generated in a muscle fiber in response to a single stimulus. Experimental work has shown that such a response can be generated in normal muscle fibers in which chloride conductance is suppressed, and this may be the cause of the myotonia of Thomsen s disease (see Barchi, 1988 for examples). It is almost certainly not the cause of myotonia in myotonic dystrophy in which there is an associated fall in... 

Angel is 12 and a member of the local swimming club.She is a good swimmer and has won a number of medals for backstroke and butterfly stroke events. The staff at the club tell her and her parents that she could make it to international or even Olympic level if she is prepared to train very hard. One day her personal coach at the club suggests to her that she take some special tablets that will help her build up her muscles. He tells her that they are only food supplement tablets but also tells her that she should not tell anyone else about them, not even her parents. 

Repeated periods of exercise reduce the likelihood of damage to skeletal muscle during subsequent bouts of the same form of exercise and this appears to be associated with an increase in the activity of muscle SOD (Higuchi et al. 1985), a reduced level of lipid peroxidation products during exercise in trained rats (Alessio and Goldfarb, 1988), and a modification of the concentration of antioxidants and activity of antioxidant enzymes in trained humans (Robertson etal., 1991). Packer and colleagues (Quintanilha etui., 1983 Packer, 1984) have also examined the exercise endurance of animals of modified antioxidant capacity and found that vitamin E-deficient rats have a reduced endurance capacity, while Amelink (1990) has reported that vitamin E-deficient rats have an increased amount of injury following treadmill exercise. 

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