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Sarcopenia and Ageing
Sarcopenia and Aging
Abstract
Sarcopenia is referred to as the steady decrease in mass and quality of muscle with increasing age. There is rising support which connects sarcopenia to functional disability, falls, reduced density of bones, glucose intolerance, and reduced tolerance to heat and cold in older adults. Decline in physical movement, increase in undernourishment, increased activity of cytokine, oxidative stress, and irregularities in growth hormone and sex steroids are some of the factors which are accountable for increases instances of sarcopenia. Currently, progressive resistance guidance is the best method shown which can slow down or overturn this situation. Initial studies confirm that the employment of some trophic factors, particularly testosterone and DHEA (Dehydroepiandrosterone), may have a helpful effect on mass of muscle and muscle strength in grown-up adults. However, before making any specific conclusion regarding the clinical importance of these substances in the management of sarcopenia more research is essential.
Introduction
It is a well-known fact that the process of aging is associated with wide number of changes in the human body. One of the most important anatomical changes in the human body related to age is decrease in the skeletal muscle mass and strength.
Figure 1. Normal Muscle Anatomy (structures in which function decreases with age is marked with circle) (www.agen.ufl.edu)
The term ‘sarcopenia’ is used to describe degenerative loss of skeletal muscle mass (about 0.5 – 1% loss per year after 25 years of age). It is derived from the Greek Word “sarx” and “penia” which means flesh and loss respectively. Sarcopenia is now regarded as an important geriatric syndrome and is considered one of the characteristic features of process of aging. Class I sarcopenia is regarded as having ratio of skeletal body mass to total body mass below 1, but 2 standard deviations (s.d.) below the sex specific mean for young adults between the age 18-39 years. Class II sarcopenia is the ratio of greater than 2 s.d. below the mean (Hastings G.E., 2005). With the increase in age, lean mass of the body decreases and fat mass increases (Roubenhoff R., 2000). This occurs due to natural changes in metabolism and increase in inactive life. There is a reduction in mass of muscle by about 12-15% per decade starting from 45-55 years of age upto 80 years (Kamel & Hosam, 2003). Bone strength also decreases with aging. There is also a decrease in synthesis of proteins by about 28% (Kamel & Hosam, 2003). With increasing age, every person undergoes reduction in homeostatic reserves and incapability to act in response to metabolic challenges as well as nutritional stress. This in turn leads to the higher rate of disease and disability in the older population.
Causes of Sarcopenia
There are a number of mechanisms that may be concerned with the beginning and development of sarcopenia. This can be shown in Figure 2.

Figure 2. Mechanisms of Sarcopenia
Synthesis of proteins, proteolysis, neuromuscular integrity and fat content of muscle can also be incorporated in the mechanism of sarcopenia. Many different mechanisms are involved in causing sarcopenia and their comparative role may differ over time. If these mechanisms are identified and their fundamental causes are also identified, then it will help to propose intervention trials which will aim at one or more fundamental mechanisms. The primary biological mechanisms that are accountable for the progress of age-associated sarcopenia are possibly difficult and not comprehensively recognized. It has been seen that even champions of athletes develop sarcopenia to some extent and, consequently, biological mechanisms, which are inherent to aging and are not dependent of behaviour and environment, should be forwarded.
Reactive oxygen species (ROS) are produced within mitochondria of muscle cells. Large amount of energy rich compound ATP is also produced in mitochondria with the help of electron transport chain (ETC). It causes a large amount of damage to muscle cells. There is no direct evidence which suggests that with the increase of age the production of ROS and the oxidative stress rate also increases (Ji L.L., 2001). Nevertheless, it has been proved that with the increase of age, there is an increase in concentrations of antioxidant scavengers, especially superoxide-dismutase (SOD) in the tissues as well as in the serum. This observable fact is thought to be reactive to increase in oxidative stress, although recent studies do not verify this theory and suggest alternative explanations (Ji L.L, 2001). In normal physiological conditions, presence of Reactive Oxygen Species in small amounts is helpful and good because it helps to increase antioxidants production. It also increases the metabolic yield, and continuously leads to the repair and replacement of fibres which have been damaged. Young individuals can make sufficient quantities of antioxidants when large number of ROS are produced but older people seem to gradually lose the capability to produce antioxidants. Thus, in most probability in older adults although the activities of antioxidants might be greater such activity may be insufficient to protect muscles from harmful effects of ROS. Amazingly, it has been verified that the increase in antioxidant actions of molecules such as SOD is not related to a similar increase in the mRNA for SOD. This signifies that the increase in activity of antioxidant is not caused by improved expression of gene, and some still unidentified post-translational mechanisms have to be hypothesized.
Altered level of Cytokines
With increasing age, the innate immunity of body decreases and adipose tissue increases. It has also been proposed that the level of pro-inflammatory cytokines such as TNF-ɑ and IL-6 also increases (Lutz CT., Quinn LBS., 2012) also increases with aging. Cytokines which are obtained from adipose tissues are known as adipokines (leptin, adiponectin, resistin, IL-6, IL-1, etc.) and those obtained from skeletal muscle cells are known as myokines (Nampt, Il-6, Il-8, TNF-ɑ, leukemia inhibiting factor, etc). With aging, expression of adipokine increases and myokines decreases. These altered levels of cytokines in turn affect Natural Killer Cells (NK) which impart immunity to the system. Thus it is important to study about levels of cytokine to understand the mechanism of sarcopenia and to cure it.
Catabolic effect of chronic inflammation might be responsible for sarcopenia. Many researches have proposed the catabolic outcome of pro-inflammatory cytokines such as TNF-α, IL-1, and IL-6 on muscle fibres (Pedersen BK, Ostrowski K, Rohde T, Bruunsgaard H., 1998). These pro-inflammatory cytokines...