Deterioration of skeletal muscle and its neural inputs begins between the ages of 50 and 60 years. Much of deterioration of skeletal muscle observed may actually be a result of inactivity, rather than age itself.
Limitation in the range of motion of different joints is a frequent finding in older persons. This may be a result of joint incongruity, capsular and ligamentous contracture, or loss of elasticity in tendons. All of these processes may affect force development by modifying the leverage component of the length-tension relationship.
Muscle strength appears to be relatively stable through 50 years of age. A 15% loss in muscle strength per decade occurs between the ages of 50 and 70 years of age. From the ages of 70-80 years, a 30% loss in muscle strength has been noted. Clinically, one observes a decreased ability to engage in physical activity in older persons due to decreased muscle strength.
The decline in muscle strength may have important functional consequences. The decline in strength in muscles of the lower extremities may be associated with gait disorders, falls, and hip fractures. A reduction in upper body strength increases the risk of accidents in activities which require lifting, pushing, or pulling maneuvers (such as housekeeping, cooking, and eating). Weakness of low back muscles may be related to problems such as disk herniation and chronic low back pain of soft tissue origin.
The decline in muscle strength with aging may be attributed to the loss of muscle mass (decrease in skeletal muscle fiber number, decrease in muscle fiber size), to some alteration of the muscle's capacity to generate force (recruitment of skeletal muscle fibers, force/cross-bridge), or to a combination of these two mechanisms, thus making older muscle intrinsically weaker. Men, from 24-80 years, lose approximately 40% of total skeletal muscle mass. Total muscle cross-sectional area peaks at the age of 24 years. From 24-50 years of age, 10% of the total muscle cross-sectional area is lost. Thereafter, muscle atrophy is accelerated so that between 50 and 80 years of age, an additional 30% of total muscle cross-sectional area is lost. The decline in muscle mass may account, in part, for the decline in muscle strength seen with aging. Interestingly, postural muscles, such as the quadriceps and soleus, show age-related atrophy to a greater extent than non-postural muscles.
The age-related atrophy of the human vastus lateralis muscle is a function of equivalent losses in the numbers of both Type I and II muscle fibers, and a preferential greater loss of Type II fiber cross-section. No reduction in the mean cross-sectional area of Type I fibers has been observed. This may account, in part, for the slowing of contraction times with aging. Muscle fiber loss is non significant (about 5%) from the ages of 24-52 years. However, a dramatic 35% loss of muscle fibers has been observed in human subjects from ages 52-77 years. Thus, some aging-related atrophy seems to be due to loss of muscle fibers, as well as to the decreased fiber cross-sectional area.
Finally, the prevalence of common chronic diseases in the elderly population is high. Illnesses affecting the musculoskeletal system, the heart and circulation, and the kidneys may be associated with muscle atrophy and weakness. The loss of muscle strength may be a result of the pathological process itself or the accompanying reduction in physical activity. In conclusion, a gradual loss of strength can be attributed to alterations in skeletal muscle mechanical and morphological properties. Further understanding of age-related muscle changes could enable the development of interventions that will slow the decline of muscle function with aging. Older persons who remain physically active have only moderate losses in skeletal muscle mass, but exactly how much of the decrease in muscle is a consequence of aging or a reduction in physical activity or specific types of activity, or both, is currently unknown.
Training has effects on the physical capacity of older persons irrespective of whether they trained in youth. Older adults have much to gain by exercising. Taekwondo instructors can teach their older students how to safe and effective train.
Strength Training Programs
Aging affects almost all of the factors of muscle strength. Since these age-related changes are similar to some of the effects of inactivity, it has been postulated that they are partly due to lack of physical activity and to not the aging process itself. If this is true, it can be suggested that strength training may slow down or reverse some of these alterations.
A program of regular resistive training, carried out over an appropriate period of time, results in a myriad of physiological adaptations. Muscle hypertrophy and increases in strength, alterations in body composition, hormonal and neural adaptations, and changes in cardiovascular capacity have all been documented subsequent to various resistive training protocols. Up to the present, these adaptive responses have most frequently been shown in young men and women.
Brief isometric exercise programs are useful. The most effective isometric training regimen would seem to be one in which maximal contractions are used and in which the product of contraction duration times the number of contractions per day is large. The advantages of this program lie in its brevity, its lack of special equipment needs, and its effectiveness. However, since most ambulation, self care tasks, and athletic events include some dynamic requirements, one should not rely entirely upon isometric exercise to train for these tasks.
An isokinetic program accommodates any torque developed throughout the range of motion. In addition, it offers subjects the advantage of seeing an immediate display of their performance. However, this type of exercise has not been fully investigated for the elderly and there is very little data from which to draw optimum frequency, intensity, duration, and speed of training. A major disadvantage is the requirement of special equipment.
Eccentric exercise may be a more adequate stimulus for increasing strength because of the force-velocity relationship of skeletal muscle. The disadvantage of eccentric training is that muscle soreness in the early part of the program does not allow rapid and progressive increases in resistance.
No matter which training program is used, several training elements need to be considered. To obtain a training effect in muscle, an exercise overload must be applied. Adaptations of muscles to training are related to the type, intensity, and duration of the training and to the type of muscle being exercised. The extent of training is, at least in part, a function of the pre-training state of the muscles being used in the exercise.
Skeletal Muscle Adaptations to Training
Skeletal muscle adaptations to exercise training and the improvement in muscle force production can be related to changes which occur within the muscle and/or the nervous organization of muscle contraction. Specifically, improvements in muscular strength are the result of structural changes in muscle and are due to neural adaptations that typically account for strength gains during the early part of training. Increases in skeletal contractile proteins and anaerobic capacity are examples of muscle structural changes that occur with resistive training.
Training Responses of Older Individuals
Early studies involving the ability of older people to adapt to resistive exercise indicated that 12 to 26 weeks of resistive exercise training elicited only minimal improvements in muscle strength of men and women between the ages of 60 and 75 years. Recently, muscle rehabilitation programs for well, older populations have shown significant increases in muscle strength, muscle volume, and other parameters of muscle structure and function. Studies have documented that, given an adequate training stimulus, older men and women show similar or greater strength gains compared to young individuals after resistive training.
It has been suggested that lack of muscle loading could contribute to muscle atrophy in the aged since individuals aged 60-90 years can increase/maintain muscle mass with resistance training. Men, who are 70 years of age who have resistance-trained since 50 years of age, had muscle cross-sectional area and strength comparable to a group of 28-year-old sedentary subjects. These findings were also compared with three groups of 70-year-old subjects (sedentary controls, swimming trained, or running trained) who all had a 20-24% decrease in muscle cross-sectional area and strength.
Recommendations for a Therapeutic Approach for Improving Force Generation in Older Muscle
The first step in beginning an exercise program for older adults is to consult with a healthcare professional (physical therapist, nurse, and physician). To be successful, any exercise program must be effective, safe, and have some motivational appeal for the participant. To be effective and to achieve the physiologic benefits, an exercise routine must be of appropriate mode, duration, frequency, and intensity.
The resistance training protocol (according to the literature reviewed) that produced the greatest increases in muscle strength and attenuated atrophy to the greatest extent in older adults was three sets of eight repetitions of exercise performed at an intensity of 80% of a muscle's 1 RM, three days a week for 12 weeks. Training of older muscle can be initiated at an intensity of 50% to 60% of the muscle's 1 RM for one week and then increased to 80% of the muscle's 1 RM for the remainder of the training period. Throughout the training period, maximum training loads need to be reestablished on a regular basis.
In addition to the weight training, older adults should he instructed to include at least five to ten minutes of warm-up and cool-down exercises in their routine. Programs should be individually tailored to the needs and interests of participants. Any exercise routine which includes adequate warm-up and cool down periods, proper stretching exercises, and is designed to progress slowly in intensity is less likely to result in injuries. Older adults also should be instructed to exercise in well-lighted and un-crowded areas without noisy distractions.
Lastly, an exercise routine must have some motivational appeal if seniors are to adhere to it long enough to achieve the desired results. A program with incremental, achievable goals and a mechanism to measure progress is likely to encourage participation. Successful classes often use wall charts, individual records, or even computer programs to track progress. Perhaps of even greater importance is the ongoing assessment of the participant's response to exercise, including monitoring for changes in balance, strength, and flexibility, as well as any symptoms of adverse responses like pain, dyspnea, dizziness, or irregular heartbeats.
Exercise programs for older individuals must be safe as well as effective. Simple instructions by therapists can help seniors avoid injury while beginning an exercise program. Proper shoes with adequate cushioning and non stick soles are essential for most activities.
During high-intensity resistance training, older persons should be continuously monitored for heart rate, blood pressure, and any signs or symptoms of distress. Older individuals should also be cautioned to avoid performing Valsalva maneuvers during exercise periods, as this maneuver may place strain on the individual's cardiovascular system.
A decline in muscle strength can no longer be considered an inevitable consequence of the aging process. Strength training induces changes in muscle and neural control mechanisms. The nature of the training response in the elderly depends on intensity and type of training. The benefits of maintaining or improving strength in the elderly may include correction of gait disturbances and safer ambulation, prevention of falls, reduction in bone fractures, improved mobility and stamina, improved performance of activities of daily living and increased capacity for independent living. As the number of elderly persons increases in our society, it becomes more important to develop strategies for preventing these people from becoming "frail elderly."
Tackett, C. (2003). Factors Affecting Strength.