The Affects Of Aging On Skeletal Muscle In Older Humans Essay, Research Paper

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 the anterior leg, laterally it parallels the sharp anterior margin of the tibia. The tibialis anterior is the prime mover of dorsi flexion; it also inverts the foot and assists in supporting the medial longitudinal arch of the foot.

The Extensor digitorum longus (EDL) is on the anterolateral surface of the leg, lateral to the tibialis anterior muscle. The EDL dorsi flexes the foot and is the prime mover of toe extension.

The Peroneus tertius is a small muscle that is usually continuous and fused with the distal part of the extensor digitorum longus. It dorsi flexes and everts the foot.

The Extensor hallucis is deep to the extensor digitorum longus and tibialis anterior it extends the great toe and dorsi flexes the foot.

The study done by Lexell, et al. (1997) focused on the fatigue rate, endurance level and the relative reduction in muscle force during maximal voluntary contraction (MVC) while performing dynamic knee extensions.

The knee extensors measured were the rectus femoris, vastus lateralis, vastus medialis, vastus intermedius (quadriceps), and the triceps surae.

The rectus femoris is a superficial muscle of the anterior thigh it runs straight down the thigh and is the only one of the quadriceps to cross the hip joint. It extends the knee and flexes the thigh at the hip. The nerve supply is the femoral nerve.

The vastus lateralis forms the lateral aspect of the thigh and extends the knee. The vastus medialis forms the inferomedial aspect of the thigh, it extends the knee and its inferior fibers stabilize the patella. The vastus intermedius is obscured by the rectus femoris, and lies between the vastus lateralis and vastus medialis on the anterior of the thigh. Like the above muscles it extends the knee and its nerve supply is the femoral nerve. This group, along with the rectus femoris, forms the quadriceps.

The Triceps surae refers to the muscle pair that shapes the posterior calf and inserts by a common tendon into the calcaneus of the heel (Gastrocnemius and soleus).

Research Review

Skeletal Muscle Fatigue and Endurance in Young and Old Men and Women

Britta Lindstrom, Jan Lexell, Bjorn Gerdle, and David Downham

Lindstrom et al (1997) used 38 physically healthy individuals, 22 young and 16 old to test the fatigue rate, the endurance level, and the relative reduction in muscle force. There were 14 men and 8 women 28 years old that made up the young group. The older group consisted of 8 men and women 73 years old. None of the 38 volunteers were elite athletes, but all of them participated regularly I low intensity aerobic exercise (walking, cycling, etc.). (Lindstrom et al.1997)

The term fatigue is defined as failure to maintain force or power output, in contrast to weakness, which is failure to generate force. The method used to assess muscle fatigue has been used for over 10 years and measured the reduction in muscle force during 100 ? 200 repeated contractions. It also allows the researchers to estimate indirectly the maximal voluntary contraction and to determine muscle endurance. (Lindstrom et al.1997)

In this study, muscle fatigue and endurance were assessed in the knee extensors, previous studies assessed muscles in the thigh, these muscles have different actions and different muscle fiber type compositions, and the results are not always comparable. A common well known problem studying human muscles is the difficulty controlling factors such as the individual?s nutritional status, level of physical activity, etc. (Lindstrom et al.1997)

It was found that the rate at which muscle force was lost during the fatigue test was unaffected by increasing age. The only noticeable difference between younger and older individuals was the larger variability in fatigue rate among both older men and older women compared to younger men and younger women. This increased variability in fatigue rate in older individuals could be due to age-related alterations in fiber-type composition. (Lindstrom et al.1997)

The results in this study imply that increasing age does not markedly alter the ability of the quadriceps muscle to maintain force throughout repeated dynamic contractions, and is in agreement with previous studies. (Lindstrom et al.1997)

Slowed Muscle Contractile Properties are not Associated with a Decreased EMG/Force Relationship in Older Humans

Alexander V. Ng and Jane A. Kent-Braun

Ng and Kent-Braun (1999) tested the hypothesis that as a result of slower muscle contractile properties, the EMG force relationship decreased during voluntary contractions in older compared to young humans. The group consisted of 12 men and 10 women aged 25 ? 44 and 9 men and 11 women aged 65 ? 82. The volunteers were healthy nonsmoking individuals that had no more than two regular exercise sessions per week for the previous three months. (Ng & Kent-Braun, 1999)

To measure force/frequency relationship the peak muscle force during supramaximal, electrically stimulated contractions of 1 s duration at 1, 5, 10, 20, and 50 Hz. The measurements were obtained after the twitch and CMAP (compound muscle action potential) measurements were taken. (Ng & Kent-Braun, 1999)

The maximal voluntary isometric contraction (MVC) force was obtained after the data for the stimulated force-frequency relationship. Three MVC?s were obtained, each during a voluntary 3-5 s maximal dorsi flexion. One minute of rest separated each MVC measurement. (Ng & Kent-Braun, 1999)

To study possible changes in the EMG force relationship with age, the subjects performed graded, non-fatiguing isometric contractions from 10% to 100% MCV in 10% increments. Contractions were 10 s in duration and separated by one minutes rest. (Ng & Kent-Braun, 1999)

The results of this study showed that in addition to slower dorsi flexion contraction properties, the older compared to the young subjects had an increase in relative force production during low frequency stimuli. Despite this relative increase in force production, the surface EMG at low voluntary force levels was increased, not decreased, compared to younger adults. Thus, slowed muscle contractile properties in older compared to younger adults did not lead to a decreased EMG force relationship. Therefore, slower muscle contractile properties in older adults do not result in increased neural efficiency during voluntary contraction. (Ng & Kent-Braun, 1999)

Gender Differences in Isometric Contractile Properties and Fatigability in Elderly Human Muscle

Audrey L. Hicks and Neil McCartney

This paper compares the isometric contractile characteristics and fatigability of the elbow flexors and ankle dorsi flexors in healthy males and females between 60 and 80 years, and examines the effects of 22 months of resistance training on these variables. (Hicks, A. L. & McCartney, N. 1996)

Resistance training took place twice a week, on alternate days. The training program was designed to involve several muscle groups, including the elbow flexors and ankle dorsi flexors. The exercises were completed using a circuit set system, with 2-minute rests between sets; each set consisted of either 10 (arms) or 12 (legs) repetitions. The training progressed from 2 sets of each exercise at 50% 1RM (one rep maximum) over the course of the study. The 1RM was measured every 6 weeks, and the training loads were adjusted accordingly. The muscle contractile properties were measured at three time points– baseline, 10 months, and 22 months. (Hicks, A. L. & McCartney, N. 1996)

When compared to younger adults, the muscles of older individuals are smaller, weaker, and slower to contract. In this study the isometric and contractile speed of both the elbow flexors and ankle dorsi flexors were weaker and slower in older adults than in younger adults. Although the older subjects had smaller evoked Pts for the elbow flexors compared to that in younger adult, the Pts for the ankle dorsi flexors were not unlike those reported in the younger subjects. (Hicks, A. L. & McCartney, N. 1996)

The measurement of muscle fatigue during voluntary isometric effort revealed a notable gender difference in fatigue resistance in the population, with female possessing a significantly greater endurance than males. Two years of twice weekly dynamic resistance training resulted in virtually no changes in the isometric contractile properties or fatigability, despite very significant gains in dynamic strength. (Hicks, A. L. & McCartney, N. 1996)

Summary

The studies reviewed did not find definite results that aging affects specific strength, contractile properties, decreased EMG/force relationship, or endurance in older human muscle. Ng and Kent-Braun did however suggest during the study that testosterone might play a role in differentiating the strength of young men from older men. They also suggested that relative force-frequency relationship was likely the consequence of the slower muscle contractile properties and could be considered adaptive or compensatory in nature.

Hicks and McCartney suggested the change in contractile properties are muscle specific, and the degree and direction so change may depend on both the type and duration of training, as well as the amount of daily usage.

Further investigations of the differences between muscle performances measured in the lab environment and in functional everyday situations, are a topic of interest that requires further research. Such studies could provide information useful to physicians and older humans on what to expect with aging and how to adapt to the changes that will enable older adults to continue to live their lives independently.

Bibliography

References

Adams, K., O?Shea, P., & O?Shea, K. (1999). Aging: its effect on strength, power, flexibility, and bone density. National Strength & Conditioning Association, 21(2), 65-77.

Hicks, A. L. & McCartney, N. (1996). Gender differences in isometric contractile properties and fatigability in elderly human muscle. Canadian Journal of Applied Physiology, 21 (6), 451-454.

Kent-Braun, J. A. & Ng, A. V. (1999). Specific strength and voluntary muscle activation in young and elderly women and men. Journal of American Physiology [On-Line], 87 (1) 22-29. Available: http//www.jap.physiology.org/cgi/content/full/87/1/2

Lindstrom, B., Lexell, J., Gerdle, B., & Downham, D. (1997). Skeletal muscle fatigue and endurance in young and old men and women. Journal of Gerontology: Biological Sciences, 54 (1), B59-B66.

Marieb, E. N. (1998). Human Anatomy & Physiology. (4th Ed. ) Don Mills: Addison Wesley Longman.

Ng, A. V. & Kent-Braun, J. A. (1999). Slowed muscle contractile properties are not associated with a decreased EMG/Force relationship in older humans. Journal of Gerontology: Biological Sciences, 54A (10), B452-B458.