Muscle Function: Effects of Aging
Original Editor - Wendy Walker
- 1 Introduction
- 2 Age-related Changes in Muscle Structure
- 3 Effects of Endocrine Changes on Muscle
- 4 Conditions Associated with Impairment of Skeletal Function
- 5 Physiotherapy Interventions to Minimise or Reverse Sarcopenia
- 6 Dietary Advice
- 7 Resources
- 8 References
- The age-related loss of muscle function is known as Sarcopenia, derived from the Greek words for flesh (sarcos) and loss (penia) and its definition includes loss of muscle strength and power, as well as reduced function. It occurs with increasing age, and is a major component in the development of frailty.
- The loss of muscle mass during the aging process is important clinically as it reduces strength and exercise capacity, both which are needed to perform activities of daily living. The video below gives a good summary of the changes and effects on performance and health.
Sarcopenia - consequences
Sarcopenia is not a disease but rather refers specifically to the universal, involuntary decline in lean body mass that occurs with age, primarily due to the loss of skeletal muscle. Systematic review and meta-analyses among Japanese community-dwelling older adults suggest the prevalence of sarcopenia (9.9% overall: 9.8% among men, and 10.1% among women), providing valuable information in addressing sarcopenia prevention in the older community. A narrative review published in International Journal of Molecular Sciences (2020) provides new evidence regarding the mechanisms, evaluation and detection methods, and spinal sarcopenia treatment modalities.
Sarcopenia has important consequences.
- The loss of lean body mass reduces function, and loss of approximately 40% of lean body mass is fatal.
- It has been attributed to a reduction of muscle size as well as a reduction in satellite cells (a stem cell that lies adjacent to a skeletal muscle fibre and plays a role in muscle growth, repair, and regeneration), mitochondrial numbers, and elasticity.
- Sarcopenia is seen in increasing numbers with advancing age but is not universal.
- Sarcopenia varies in degree of physical activity, gender, and race.
- Sarcopenia has a marked effect on function in all activities of daily living, contributing (along with reduction in balance) to reduced gait speed, falls, and fractures. The combination of osteoporosis and sarcopenia results in the frailty which frequently occurs in the elderly population.
Reduced muscle mass (replaced by increased fat mass)
Reduction in lower limb muscle cross-sectional area have been observed to begin in early adulthood and accelerate beyond 50 years of age.This reduction in muscle cross-sectional area associated with decreases in contractile structures accompanied by increases in non contractile structures such as fat and connective tissue. A cross-sectional study suggested that the older inpatient showed an increase in the intramuscular quadricep muscle adipose tissue approx 1.7 times that of the healthy older individuals. Also, the study observed increased intramuscular adipose tissue with older inpatients who were unable to walk independently as compared to older inpatients who were able to walk freely.
Reduced muscle strength
The total number of muscle fibers is significantly reduced with age, beginning at about 25 years and progressing at an accelerated rate thereafter The decline in muscle cross-sectional area is most likely due to decreases in total fiber number, especially type II fast-twitch glycolytic fibers. This results in reduced muscle power. A study examining 1-year changes in the physical functioning of older people using the ICF framework showed a significant decrease in muscle strength (both hip abductors and knee extensors) walking capacity, speed, mobility, sit-to-stand performance, upper extremity function, and balance performance at the end of 1 year.
Muscle Fibre changes:
- Changes in Muscle Fiber Size
Elderly individuals often fall because of poor muscle strength and reduced balancing ability related to muscle aging. Types IIA and IIB muscle fibers decrease with age in the area percentage, fiber number percentage, and mean fiber area, whereas Type I fibers increase in area and number but not in size. Morphologically, Type II fibers appeared smaller and flatter. Investigations suggest deterioration in muscle quality and balancing coordination in elderly patients. A research done provided data to help determine treatments for reversing muscle fiber changes and reducing the number of falls and related fractures in patients.The reduction in number of muscle fibers contributes more to the decrease of whole muscle cross-sectional area than does the reduction in area of individual fibers. The individual fast-twitch type II fibers decrease in cross-sectional area suggest that the relative contribution of fast-twitch type II fibers to force generation is less in the older adult.
2. Motor Unit Number and Size
In the aged motor unit there are decreased in the number of functional motor units associated with enlargement of the cross sectional area of the remaining units. This motor unit remodelling is achieved by selective denervation of muscle fibers, (especially type 2 b fibres) followed by re-ennervation by axonal spouting from juxtaposed innervation units.
- The majority of the literature indicates that muscle fiber loss is due to a loss in motor neurons. There is consistent denervation and reinervation of the muscle fiber throughout one’s lifespan, but in the aged, denervation appears to outpace reinveration.
- Data indicate that a 60-year-old has approximately 25-50% fewer motor neurons than a 20-year-old, with the greatest losses in distal fast twitch motor neurons.
- With the loss of the motor neuron, the denervated fast twitch muscle fibers that were attached to it are either permanently denervated and undergo apoptosis, or are reinverated with a different motor neuron most likely that of a slow-twitch neuron, potentially making the fiber take on slow twitch characteristics
Effects of Endocrine Changes on Muscle
With increased age, the following changes in endocrine function result in sarcopenia:
- Increased insulin resistance
- Decreased growth hormone
- Reduction in oestrogen and testosterone
- Vitamin D deficiency
- Increased parathyroid hormone
Conditions Associated with Impairment of Skeletal Function
- Metabolic syndrome
- Chronic Obstructive Airways Disease (COPD)
- Congestive Cardiac Failure
- Chronic Kidney Disease
Physiotherapy Interventions to Minimise or Reverse Sarcopenia
Outcomes Measure: SARC-F: A Simple Questionnaire to Rapidly Diagnose Sarcopenia
Resistance exercise training:
The effects of resisted exercise on ageing muscles are the same as for young muscles:
- improved muscle strength
- increased muscle power - power is a product of both strength and speed. Optimal power reflects how quickly you can exert force to produce the desired movement
- improved muscle composition
Resistance or weight training has been demonstrated to produce increases in muscle strength and power, and also mobility function, in older people living in the following settings:
- independently in the community
- in nursing homes
- hospitalised elderly people
A Systematic Review of Randomized Controlled Trials suggested that a low dose of creatine monohydrate along with resisted exercises may improve upper and lower extremities strength in healthy older adults.
A randomized control trial in 72 prefrail adults (65 yrs and above) with mild-to-moderate fall risk found significant improvement in fall risk, proprioception, muscle strength, reaction time, postural sway and health-related quality of life with the Multi-system Physical Exercise (MPE) which consisted of proprioceptive, muscle strengthening, reaction time, and balance training exercises.
Increased muscle quality from resistance training is a common finding in older adults, and in men there appears to be no difference in young versus old, but there is a study that suggests that older women have a blunted response relative to younger women.
Frequency of resistance training
Studies have demonstrated that resistance training regimes performed once, twice or even three times a week all result in strength improvements.
Length of training programme
There are many studies which clearly demonstrate that older people who participate in resistance training programs lasting at least 6 to 12 weeks will show increase in both strength and mobility function.
The authors collated the results from 121 RCTs examining the effects of resistance strength training exercises, and came to the following conclusions:
"This review provides evidence that PRT (Progressive resistance strength training) is an effective intervention for improving physical functioning in older people, including improving strength and the performance of some simple and complex activities. However, some caution is needed with transferring these exercises for use with clinical populations because adverse events are not adequately reported."
The Society for Sarcopenia, Cachexia, and Wasting convened an expert panel to develop nutritional recommendations for sarcopenia prevention and management.This panel concluded that for preventing and treating this condition key components are
- protein and energy intake
- both resistance and aerobic exercise
The greatest effects are observed when resistance training and high protein diets are combined and appear to act synergistically.
- Specifically, consuming 20-35 grams of protein per meal is advised, as such amounts provide sufficient amino acid content to maximize MPS, thus minimizing age-related muscle loss. eg Chicken Breast: 23.1 g Protein Per 100 g, Canned Tuna: 23.6 g Protein Per 100 g, Cocoa: 20 g Protein Per 100 g, Cheddar Cheese: 24.9 g Protein Per 100 g.Beef Jerky: 33.2 g Protein Per 100 g.
- Additionally, patients with sarcopenia are recommended to consume 1.0 - 1.2 g/kg (body weight)/day
Is there a role for supplements?
There is some evidence suggesting that additional supplementation with the amino acid Leucine (or its metabolite HMB) could potentially increase the effects of resistance training to combat sarcopenia. A randomised double-blind study has found that supplementation with l-leucine can be used in the treatment of sarcopenia in older individuals.
- Aging in Motion (AIM) is a coalition of organizations working to advance research and treatment of sarcopenia and age-related functional decline: aginginmotion.org/
- The American Physical Therapy Association, APTA, www.apta.org/ have a useful document Physical Therapist's Guide to Sarcopenia and Frailty: www.moveforwardpt.com/symptomsconditionsdetail.aspx
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