Strength conditioning will result in an increase in muscle size and this increase in size is largely the result of increased contractile proteins. The mechanisms by which the mechanical events stimulate an increase in ribonucleic acid synthesis and subsequent protein synthesis are not well understood. Lifting weight requires that a muscle shorten as it produces force (concentric contraction). Lowering the weight, however, forces the muscle to lengthen as it produces force (eccentric contraction). These lengthening muscle contractions have been shown to produce ultrastructural damage (microscopic tears in contractile proteins muscle cells) that may stimulate increased muscle protein turnover. This muscle damage produces a cascade of metabolic events that is similar to an acute phase response and includes complement activation, mobilization of neutrophils, increased circulation of skeletal muscle interleukin-1, macrophage accumulation in muscle, and an increase in muscle protein synthesis and degradation. Although endurance exercise increases the oxidation of essential amino acids and increases the requirement for dietary protein, resistance exercise results in a decrease in nitrogen excretion, lowering dietary protein needs. This increased efficiency of protein use may be important for wasting diseases such as human immunodeficiency virus infection and cancer and particularly in elderly people suffering from sarcopenia. Research has indicated that increased dietary protein intake (as much as 1.6 g protein × kg-1 × day-1) may enhance the hypertrophic response to resistance exercise. It also has been shown that in very old men and women the use of a protein-calorie supplement was associated with greater strength and muscle mass gains than the use of placebo.

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