Muscle Strength

 

There are unavoidable changes associated with aging; degenerative joints or loss of skin elasticity, for example.  We can preserve muscle strength and size.  Our muscles grow when we are developing from childhood to adulthood.  Once we reach adulthood, any further “growth” is referred to as hypertrophy.  We exercise or exert ourselves and any muscular work done against a challenging load leads to increases in muscle mass and cross-sectional area.  This is referred to as muscle hypertrophy.  The increase in dimension is due to an increase in the size (not length) of individual muscle fibers.  Both cardiac (heart) and skeletal muscle adapt to regular, increasing work loads that exceed the preexisting capacity of the muscle fiber. With cardiac muscle, the heart becomes more effective at squeezing blood out of its chambers, whereas skeletal muscle becomes more efficient at transmitting forces through tendinous attachments to bones.

      When we begin a new regimen of exercise the muscles “learn” the new movements and accommodate the new weight loads we impose.  This is a function of neural inputs and physiologically is known as “neural learning”.  For approximately two weeks neural learning serves as the main mechanism for strength building and muscle training in any new exercise routine.  With continued exercise, the muscles’ synthetic contractile protein mechanism becomes upregulated, through stimulation of the family of immediate-early genes, including c-fos, c-jun and myc. These genes appear to dictate the contractile protein gene response and through this response the muscles gain strength (how this occurs is still scientifically poorly defined).    Finally additional contractile proteins become incorporated into the myofibrils resulting in increased muscle fiber size.   The muscle fibers sustain mild trauma from the overload of exercise and this trauma stimulates a component of the muscle fiber, their “satellite” cells, to proliferate.  These cells are located on the outer surfaces of the muscle fibers and are usually dormant..  When these satellite cells proliferate in response to injury, their daughter cells are drawn to the damaged muscle site They then fuse to the existing muscle fiber, donating their nuclei to the fiber, which helps to regenerate the muscle fiber. It is important to emphasize the point that this process is not creating more skeletal muscle fibers (in humans), but increasing the size and number of contractile proteins (actin and myosin) within the muscle fiber.   This injury regeneration process continues for up to 48 hours.  By exercising repeatedly every other day or so, one can keep the process ongoing, maximizing muscle hypertrophy.  But there is a limit to how massive each myofibril will grow.    There are numerous growth factors that have been identified that play some role in muscle hypertrophy including insulin-like growth factor, fibroblast growth factor, hepatocyte growth factor, growth hormone and testosterone.  To date though there are no successful ways to use any of these factors to safely control human muscle hypertrophy.

Keep Cool, Stay Healthy

The human body maintains a fairly constant internal temperature, even though it is being exposed to varying environmental temperatures. To keep internal body temperatures within safe limits, the body must get rid of its excess heat, primarily through varying the rate and amount of blood circulation through the skin and the release of fluid onto the skin by the sweat glands. These automatic responses occur to maintain a temperature of 98.6oF and are controlled by the brain, specifically by the hypothalamus.   In this process of lowering internal body temperature, the heart begins to pump more blood, blood vessels expand or dilate to accommodate the increased flow, and the microscopic blood vessels (capillaries) that thread through the upper layers of the skin begin to fill with blood. The blood circulates closer to the surface of the skin, and the excess heat is lost to the cooler environment. If heat loss from increased blood circulation through the skin is not adequate, the brain continues to sense overheating and signals the sweat glands in the skin to shed large quantities of sweat onto the skin surface. Evaporation of sweat cools the skin, eliminating large quantities of heat from the body. As environmental temperatures approach normal skin temperature, cooling of the body becomes more difficult. If air temperature is as warm as or warmer than the skin, blood brought to the body surface cannot lose its heat. Under these conditions, the heart continues to pump blood to the body surface, the sweat glands pour liquids containing electrolytes onto the surface of the skin and the evaporation of the sweat becomes the principal effective means of maintaining a constant body temperature. Sweating does not cool the body unless the moisture is removed from the skin by evaporation. Under conditions of high humidity, the evaporation of sweat from the skin is decreased and the body's efforts to maintain an acceptable body temperature may be significantly impaired. These conditions adversely affect an individual's ability to function in the hot environment. With so much blood going to the external surface of the body, relatively less goes to the active muscles, the brain, and other internal organs; strength declines; and fatigue occurs sooner than it would otherwise. Alertness and mental capacity also may be affected.

Mental acuity, comprehension and retention of information are often lowered.  Increased body temperature and physical discomfort promote irritability, anger, and other emotional states that impair judgment and function.  Time spent in a hot environment may lead to heat-induced disorders; transient heat fatigue, heat rash, fainting, heat cramps, heat exhaustion, and heat stroke.  Heat stroke, the most serious of the conditions, occurs when the body's temperature regulatory system fails and sweating becomes inadequate.  A heat stroke victim's skin is hot, usually dry, red or spotted. Body temperature is usually 105oF or higher, and the victim is mentally confused, delirious, perhaps in convulsions, or unconscious.  Heat exhaustion is caused by the loss of large amounts of fluid by sweating, sometimes with excessive loss of salt.  A person suffering from heat exhaustion still sweats but experiences extreme weakness or fatigue, giddiness, nausea, or headache. In more serious cases, the person may vomit or lose consciousness. The skin is clammy and moist, the complexion is pale or flushed, and the body temperature is normal or only slightly elevated.   Heat cramps are painful spasms of the muscles that occur among those who sweat profusely in heat, drink large quantities of water, but do not adequately replace the body's salt loss. The drinking of large quantities of water tends to dilute the body's fluids, while the body continues to lose salt. Shortly thereafter, the low salt level in the muscles causes painful cramps. The affected muscles may be part of the arms, legs, or abdomen, but tired muscles are usually the ones most susceptible to cramps. Cramps may occur during or after heat exposure.  Standing erect and immobile in a hot environment for a prolonged period may cause fainting.  With enlarged blood vessels in the skin and in the lower part of the body due to the body's attempts to control internal temperature, blood may pool in the legs rather than return to the heart to be pumped to the brain. If the brain senses that it is not getting enough blood, it causes a series of reactions that will lead to dizziness and fainting.   Heat rash, also known as prickly heat, is likely to occur in hot, humid environments where sweat is not easily removed from the surface of the skin by evaporation and the skin remains wet most of the time. The sweat ducts become plugged, and a skin rash soon appears. When the rash is extensive or when it is complicated by infection, prickly heat can

be very uncomfortable.  Transient heat fatigue refers to the temporary state of discomfort and mental or psychologic strain arising from prolonged heat exposure. People unaccustomed to the heat are particularly susceptible and can suffer, to varying degrees, a decline in task performance, coordination, alertness, and vigilance. The severity of transient heat fatigue will be lessened by a period of gradual adjustment to the hot environment (heat acclimatization).  Humans are, to a large extent, capable of adjusting to the heat. This adjustment to heat, under normal circumstances, usually takes about 5 to 7 days, during which time the body will undergo a series of changes that will make continued exposure to heat more endurable. Treatment involves moving to a cool environment, resting and replacing lost fluids and salts.  For the more severe conditions, medical attention may be required, as the effects of heat stroke can become severe very quickly and in extreme cases, can be fatal.  People with heart problems or those on a low sodium diet who spend an excessive amount of time in hot environments require special consideration.  The efficacy of certain medications, or our body’s metabolism of medications may also be affected by elevated temperatures.  One should speak to their doctors about the risks of heat affecting their treatments or active diseases.  Clothing inhibits the transfer of heat between the body and the surrounding environment so wearing appropriate fabrics can reduce adverse heat effects.  The effects of heat can truly be extreme and everyone must take the time to recognize and acknowledge their individual needs in a hot environment.  It is foolish to insist that one is strong enough to withstand a hot day.  Learn to recognize the signs and symptoms of heat-induced body changes and tend to them as quickly as possible.‚