The Mystery of Muscle Loss

IDEA PERSONAL Trainer april 2002 Sarcopenia

Motor-Unit Remodeling

Motor neurons are responsible for sending signals from the brain to the muscles to initiate movement. A motor unit consists of the motor neuron and all of the muscle fibers that it connects to or innervates. The number of fibers a motor neuron innervates depends on the function of the specific muscle. For example, a muscle that requires precise movements, such as an eye muscle, will have motor units with a motor neuron innervating a few muscle fibers. Muscles that require less precise movements, such as the quadriceps muscles, will have motor units with a motor neuron innervating hundreds and possibly more than 1,000 muscle fibers.

Age-related changes in the neuromuscular system may play a role in the onset of sarcopenia. With age, the number of spinal cord motor neurons and functioning motor units declines (Roth, Ferrel & Hurley 2000; Roubenoff 2001). This decline is continuous throughout life and is considered irreversible (Roubenoff 2001). Human nerve cells have a predetermined life span and their decline is dependent on their location in the body, a person’s age, and the presence of disease (Vandervoort & Symons 2001). However, nerve cells can remain present in aged individuals yet be impaired because of biochemical changes (Vandervoort & Symons 2001).

Motor neurons will die with age, resulting in a denervation of the muscle fibers within the motor unit. This denervation causes the muscle fibers to atrophy and eventually die, leading to a decrease in muscle mass (Roth, Ferrel & Hurley 2000). When a motor neuron dies, an adjacent motor neuron, usually a slow-twitch (ST) motor neuron, may reinnervate the muscle fibers, preventing atrophy. This process is called motor-unit remodeling.

Compared to FT motor units, ST motor units have slower firing rates, are slower to contract, produce less muscle force and are smaller in size and fiber number. Motor-unit remodeling by ST motor neurons leads to less efficient motor units. The remodeled ST motor unit will have less precise control of movements, less force production and slower muscle mechanics (Roth, Ferrel & Hurley 2000; Roubenoff 2001; Waters, Baumgartner & Garry 2000). This may help explain the loss of balance and speed of movement with age. In addition, denervation rates of FT muscle fibers may exceed reinnervation rates by ST motor neurons, further explaining atrophy of FT muscle fibers in the elderly (Roth, Ferrel & Hurley 2000).

Protein Synthesis and Regeneration

Another factor affecting sarcopenia is the rate of muscle protein synthesis. The quality and quantity of protein in the body is maintained by a continuous repair process, which involves both protein breakdown and synthesis (Nair 1995). The balance of protein synthesis and breakdown determines the protein content in the body. With age, the changes in whole-body protein turnover reflect a decreased synthesis rate rather than an increased catabolic rate (Vandervoort & Symons 2001). Additionally, research has consistently shown that muscle protein synthesis rates are lower in older adults than younger adults (Nair 1995; Yarasheski et al. 1999; Hasten et al. 2000; Roth, Ferrel & Hurley 2000). A decrease in muscle protein synthesis will result in the loss of muscle mass.

In addition, muscles’ ability to regenerate following injury or overload also decreases with age. Muscle regeneration and muscle tissue growth require the assistance of specialized satellite cells, which are located in the basal membrane of the muscle (Roth, Ferrel & Hurley 2000). The number of satellite cells in skeletal muscle decreases as an individual ages, providing a possible mechanism for the loss of muscle mass and strength (Roth, Ferrel & Hurley 2000).

Hormones

Aging is associated with several changes in hormonal levels, including a decrease in the concentrations of growth hormone (GH), testosterone (T) and insulin-like growth factor (IGF-1). The development of sarcopenia may be linked to decreases in these hormones for the following reasons.

A sustained decrease in these hormones is linked to decreased muscle mass and increased body fat (Waters, Baumgartner & Garry 2000). Although these hormones are involved in protein metabolism and maintenance, there is conflicting evidence on whether hormone replacement is effective in maintaining or gaining muscle mass (Roubenoff 2001). Studies examining hormone replacement effects on lean body mass have mainly focused on GH. These studies have reported that GH replacement has not always been effective in increasing muscle mass and strength in older subjects (Roubenoff 2001; Waters, Baumgartner & Garry 2000). Changes in female estrogen levels may play a role in the development of sarcopenia during menopause. However, limited research on this topic exists.

Lifestyle Factors

Sarcopenia is accelerated with a lack of physical activity, especially a lack of overload to the muscle (i.e., resistance exercise). Physically inactive adults will see a faster and greater loss of muscle mass than physically active adults. However, sarcopenia is not completely prevented by exercise, as it is also evident, but to a lesser degree, in physically active individuals. It is possible that if the physical activity is not intense or long enough to recruit FT muscle fibers, the FT fiber might atrophy and sarcopenia will develop (Roubenoff 2001). Therefore, it appears that progressive resistance training should include cycles of high-intensity, low-volume training (relative to the person’s fitness level).

An additional factor in the development of sarcopenia may be an inadequate energy intake. Many older individuals may not be taking in enough calories and/or protein to sustain their muscle mass (Evans 1995).

Preventing Sarcopenia
With Resistance Training

Resistance training has proven a powerful intervention in the prevention and treatment of sarcopenia (Roth, Ferrel & Hurley 2000). Research has shown that it positively influences the neuromuscular system, hormone concentrations and protein synthesis rates. According to Roubenoff (2001) and Roth, Ferrel and Hurley (2000), a properly designed resistance training program may increase motor neuron firing rates, improve muscle fiber recruitment and create a more efficient motor unit. An increased motor neuron firing rate combined with increased recruitment of muscle fibers would lead to faster muscle contractions and greater force production.

Although protein synthesis rates decrease with age, research has shown that progressive resistance training can increase protein synthesis rates in as little as two weeks. Hasten et al. (2000) reported that following a two-week supervised resistance training program, muscle protein synthesis rates increased by up to 182 percent from baseline in seven, 78 to 84 year olds. Yarasheski et al. (1993) also found that muscle protein synthesis rates in older adults (63-66 years) increased significantly in response to two weeks of resistance training. In addition, Yarasheski and others (1999) reported that three months of supervised progressive resistance training increased the rate of muscle protein synthesis by approximately 50 percent in 17 frail 76 to 92-year-old men and women. These findings suggest that older men and women retain the ability to increase the rate of muscle protein synthesis in response to acute and long-term progressive resistance training. Furthermore, acute and long-term resistance training increases the number of satellite cells trained in the muscle, leading to faster muscle regeneration (Roth, Ferrel & Hurley 2000).

Theory Into Practice

It is well established that age produces a progressive loss of muscle mass and muscle strength. Physically inactive people older than 30 years lose approximately 3 to 5 percent of muscle mass per decade and experience a parallel decline in muscle strength (Nair 1995). The development of sarcopenia is multifaceted and many of the causative factors are uncontrollable; however, the easiest and possibly the most effective treatment within our control is progressive resistance training. The following are general resistance training guidelines from the American College of Sports Medicine (ACSM) (Balady et al. 2000).

The exercise contraindications for older and frail individuals are similar to those for younger adults (Porter 2000). Resistance training can be progressively introduced to individuals with cardiovascular disease, diabetes, dementia, pulmonary disease, chronic renal failure, peripheral vascular disease and arthritis. Clients who have uncontrolled conditions, such as hypertension, chest pain, metabolic disturbances and acute illnesses should be medically assessed before starting a strength training program. If any of the health-related illnesses are rapidly deteriorating, a health professional needs to be consulted immediately as this is most likely a contraindication to exercise.

Complete eight to 10 exercises, working all of the major muscle groups (e.g., pectorals, latissimus dorsi, deltoids, abdominals, gluteals, quadriceps and hamstrings). Multijoint exercises (e.g., bench press, lat pull-down, shoulder press and lunges in place) are recommended over single-joint movements because multijoint exercises utilize more muscle mass and place less stress on the client’s heart, while enhancing
the relevant muscles groups involved in balance and muscle coordination.

Perform 10 to 15 repetitions that elicit a “somewhat hard” perceived exertion rating (12 to 13 on a Borg rating of perceived exertion [RPE] scale, a copy of which can be found in the November-December 2001 issue of IDEA Personal Trainer, p. 40). Allow for a progressive adaptation to the intensity. As the mature adult gains strength, first increase number of repetitions, and later increase resistance (Balady 2000). The use of RPE in resistance exercise is contemporary, and worthy of more research from scientists and practical feedback from personal trainers in the field. Also, as indicated from this review of literature on sarcopenia, it appears that periodizing (systematic changing) workout cycles to include higher-intensity sessions with moderate-intensity sessions is advisable.

ACSM recommends performing at least one set per major muscle group. However, in most studies with older adults, the subjects have done two to three sets (Porter 2000). It is possible that multiple-set programs provide greater strength gains when conducted over a longer time span. However, for attaining the majority of the health benefits from resistance training, one set may be sufficient for most elderly client goals.

More research is needed to determine the optimal weight training frequency, which may depend upon the muscles being trained, for the mature and frail adult (Porter 2000). However, previous research has shown that strength maintenance in seniors has been achieved with as little as one workout day per week.

Spread the Word

Individuals without access to a fitness facility with weights can use therapy bands, therapy balls and body weight as resistance. Strength training is a wonderful tool for preventing and partially reversing sarcopenia. Personal trainers are in a position to spread the word and motivate clients to engage in a progressive resistance training program for a strong and healthy life. l

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