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Muscle capability and generation declines in individuals as they age. Studies show that muscle strength is maintained between the ages of 30-50 but after the age of 50, there is about a 30% drop in muscle strength (Lambert). This cause of this decline in strength is due to a reduction in the size and number of type II (fast twitch muscle fibers) (Lambert). Loss in muscle mass, to a large degree, is accounted for by the age-associated decreases inactivity levels, muscle strength, and metabolic rate, which is the cause of the decrease in energy requirements of the elderly. However, loss of muscle mass may not be the only cause of decreased strength. Reduced muscle activation and increased activation, as well as reduced muscle quality, may be involved. The decline in muscle strength relates to decreased functional ability. There has been reports of a strong relationship between the reduction in muscle mass and the reduction in resting oxygen consumption. This lower energy expenditure may contribute to an imbalance between energy intake and energy expenditure and an accumulation of body fat. Resistance exercise training has shown to increase strength, muscle mass, and resting energy expenditure in the elderly and improve performance of functional tasks. This research mainly focuses on muscle strength, which brings up other topics that relate to muscle strength. Other topics of research that relate to muscle strength include, muscle fiber size, contractile function, muscle activation, and the effect of resistance training. 

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Multiple experiments were conducted in order to explore this topic. Both humans and animals were used in experimental trials. In one experiment, men and women between the ages of 60 and 90, were placed in an endurance training program in order to study their muscle capacity and capabilities (Frontera). The training was been based on progressive resistance exercise with a progressive change in the training load to adjust for the gained muscle capability made during the previous sessions. Multiple training sessions were held every week. The purpose of this experiment is to maintain the training stimulus and keep it constant. Also, it simulates the constant physical activity over time. CT scan (computer tomography scans) and MRIs (magnetic resonance imaging) were used to examine the skeletal muscles and their cross sections, specifically to look for hypertrophy, enlargement of an organ or tissue, within the muscles.  In another experiment, individuals were assigned an exercise resistance regimen and the test subjects observed and reported any correlations between muscle capability and their form of exercise. The age group that was focused on in this experiment were the elderly. Then, data analysts organized the reports in order to make conclusions and support the test subject findings (Lambert). Other experiments did not use humans in their experimental trials. One experiment used rats in a laboratory setting to simulate the exercise that humans typically do (CDC). Rats were assigned to either a program of treadmill exercise, or inactive conditions. The rats were euthanized, and muscles were removed and frozen. Longitudinal sections of the muscles were fluorescently stained to visualize both slow- and fast-twitch fibers. Images were collected with microscopes and quantified. Muscle cross-sections were chemically stained to assess muscle fiber size and type. 

Overall, the reduction in muscle mass is the primary reason for reduced muscle strength in the elderly. It was found that the most important factor in the loss of force generating capacity with increasing age is the reduction in muscle CSA. The reports showed a strong negative relationship between peak muscle capability and age in men and women age 20 to 84. Based on recorded data, and those of others, there appears to be a strong relationship between muscle CSA and muscle capability. Another discovery that was made was that muscle fiber size changes with age. The number and size of Type II muscle fibers are extremely important to muscle force production. Type II fibers produce more force than Type I fibers. Further, at the whole muscle level, Type II fiber percentage is positively related to increased maximal muscle capability. It has been reported that in some muscles, increasing age leads to a reduction in the number and size of type II fibers than type I fibers. Due to the greater torque production for Type II fibers observed at both the single fiber and whole muscle level, the reduction in Type II fiber number and fiber size would appear to play an important role in the loss of force producing capabilities with age. Muscle quality and aging, measured by the skeletal muscle using CT analysis, was significantly related to muscle capability. Although no data on young individuals was presented, it would appear that muscle quality, as measured by the skeletal muscle, does not play a major role in the reduction of strength with aging. Effects of age on muscle activation involves the ability of the muscles to produce force is first initiated in the nervous system. The effect of age on muscle activation has not been extensively studied but there are unreplicated data on many muscle groups. Muscle protein synthesis in young and old muscle mass is primarily determined by the balance of the rate between muscle protein synthesis and degradation. Data regarding muscle protein degradation, with aging, has received considerable attention. Recent reports show that mixed muscle protein synthesis was similar in older and younger individuals when measured. There was an increase in muscle mass has been consistently reported based on measurements using imaging techniques (CT scan or MRI). Individual muscle fibers, type I and II, also show hypertrophy in the CSA of biopsy specimens histochemically stained. It seems that the process that leads to strength gains and hypertrophy includes responding to the stimulus provided by exercise which causes the synthesis of new muscle fibers. Lastly, strength training has been shown to preserve bone density while improving muscle mass, strength, and balance. 

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