The Affects of Aging on Skeletal Muscle in Older Humans



Introduction

With the onset of 2000, the average North American’s life span has been extended by three years. The predictable consequences are detrimental changes in body composition, including loss of lean body mass, strength, flexibility, and bone density, along with the increase in body weight and body fat. Inactivity with aging is the primary factor in these changes, because physical activity levels are one of the most important factors affecting body composition from childhood through old age. (Adams, K., O’Shea, P., & O’Shea, K. 1999)
Our knowledge of the affects of aging on fatigability, endurance, the ability to maintain force and power output is limited, and the few studies that have been performed are inconclusive. It is therefore important to assess these areas to give a more detailed account of muscle fatigue, endurance, and contractibility of aging humans. The results of the studies could prove beneficial in helping to prepare older humans to overcome and enhance his or her ability to live an independent lifestyle.
With advancing age, muscle volume is reduced, and the aging atrophy, referred to as “sarcopenia” is accompanied by a decrease in muscle strength. The reduction in muscle strength seems to be equal for both sexes, but women are generally weaker than men throughout all ages. (Lindstrom, B., Lexell, J., Gerdle, B., & Downham, D. 1997)
Since gait pattern also changes with age, especially in women, older individuals have an increased risk of falls and hip fractures. However, both arm and leg muscles in aging men and women can adapt successfully to increased use, in particular following periods of heavy resistance training. Physical exercise is therefore, considered beneficial in reducing the risk of muscle atrophy among older humans. (Linstrom, et al., 1997)
It has been suggested that once strength declines below certain threshold levels required for activities of daily living, significant functional impairment begins to happen. Along with a change in strength is a change in muscle contractile properties, the peak evoked twitch torque may decline and contractile speed becomes typically slowed in aging humans (Hicks, A. L. & McCartney, N. 1996). The slowing is indicated by prolonged contraction and relaxation times during stimulated contractions. One reason for slowing is thought to be a loss of motor units leading to a loss of type II muscle fibers and a shift toward a slower muscle fiber type. It has been suggested that the slowing of contractile muscle with age can result in a fusion of muscle force at lower motor unit firing rate. Such early ending of force may cause force to be produced at lower frequency of stimulation, this has been speculated to be advantageous during voluntary contraction, resulting in an increase in neural efficiency or a decreased motor drive necessary to produce desired force (Ng, A. V. & Kent-Braun, J. A., 1999).
Anatomy & Physiology
The study by Hicks and McCartney (1996) purpose was to compare the isometric contractile characteristics and fatigability in the elbow flexors and ankle dorsi flexors in older males and females to determine the affects of almost two years, twice per week weightlifting training.
The elbow flexors consist of the biceps brachii, pronator teres (weak flexor), and flexor carpi radialis (synergist); the nerve supply is the median nerve.
The Biceps brachii is a two-headed fusiform muscle; the bellies unite as it reaches the insertion point, the tendon of the long head helps to stabilize the shoulder joint. The biceps brachii flexes elbow joint and supinates the forearm; these actions usually occur at the same time (ex. When you open a bottle of wine, it turns the corkscrew and pulls the cork).
The Pronator teres is a two-headed muscle that can be seen in superficial view between the proximal margins of brachioradialis and the flexor carpi radialis. This muscle pronates the forearm and is a weak flexor of the elbow.
The Flexor carpi radialis runs diagonally across the forearm; midway its fleshy belly is replaced by a flat tendon that becomes cordlike at the wrist; it is a powerful flexor of the wrist, it abducts the hand and is a synergist of elbow flexion.

The ankle dorsi flexor muscles consist of the tibialis anterior, extensor digitorum longus, peroneus tertius, and the extensor hallucis longus the nerve supply is the deep peroneal nerve.
The Tibialis anterior muscle is superficial of