As our dogs age, we notice they are not quite so willing to jump onto the lounge, they may lack confidence on the stairs and they are a little stiff when getting out of bed. These are all signs of the normal effect of aging on skeletal muscles. Human and animal studies confirm that age negatively impacts muscle function in the following ways:

• Muscle strength
• Locomotion efficiency
• Recovery from inactivity

This article discusses how aging effects muscles and how we can slow the effect in our dogs.

Muscle strength

Muscle strength is linked in part to muscle mass, that is, the size of the muscle. As we age, humans and animals experience a gradual reduction in the number of muscle fibres (between 30 – 50%). The size of muscle fibres also decreases by 10 – 40%. A possible cause of loss of muscle mass is oxidative stress. Reactive oxygen species (ROS) are produced when the dog performs physical activity. The production of ROS activates a number of metabolic processes including protein synthesis and ROS detoxification, and as such plays a crucial role in skeletal muscle’s adaptation to exercise. Age downregulates several processes that affects the cellular response to oxidants. As a result, reactive oxygen species accumulate in the muscle tissue causing oxidative stress which impairs muscle function. In this state, protein breakdown exceeds the rate of protein synthesis which is seen in loss of muscle mass and a decline in muscle performance.

Muscle strength is also impacted by the muscle’s contractile abilities. Muscle fibres’ contractile capabilities are characterised by the speed at which they contract and when the fibres are recruited to perform particular tasks. The speed of a muscle contraction and recruitment pattern is determined by the type of muscle fibres that comprise the muscle bundles. As we age, the proportion of muscle fibre types change. The number and size of “fast twitch” fibres, Type IIa and IIx decrease and are replaced by Type I fibres which contract more slowly. The change in the type and number of muscle fibres can be seen as impairment to co-ordination such as stumbling over a step. The loss of function can also be seen in the time it takes to perform a task.

In addition to the change in muscle fibre type, aging causes changes in the nervous system including atrophy of the motor cortex, changes in neurotransmission and a decline in motor cortical excitability and plasticity. Further, as humans and animals age, reinnervation of muscle fibres slows leading to the accumulation of denervated fibres and the muscle atrophies. Age related changes in the nervous system contribute to reduced motor coordination and strength. In older dogs, we see these changes as a reduction in the size of muscles and the dog being less steady on their feet.

The combination of the decrease in muscle mass, changes in muscle fibre types and changes in the nervous system results in a decline in muscle strength and contractile speed, so performance of activities requiring explosive muscle contractions such as jumping onto the couch is noticeably impaired. Likewise, activities like walking up the stairs which require muscle strength are also impacted with age.

Locomotion efficiency

An animal’s locomotory systems rely on the skeletal muscles and connective tissues including the fascia and tendons, working together to facilitate correct movement. The fascia and tendons connect with muscle bundles within a muscle to form a force generating structure which is connected with bone to produce movement. To maintain correct movement, the health status of both the muscles and connective tissues needs to be maintained.

The fascia comprises structural proteins such as collagen, growth factors, and water. It has plastic and elastic properties which play a key role in an animal’s locomotion. With age, tissue changes are observed in fascia including increases in collagen crosslinking causing thickening of the fascia, decrease in elasticity due to dehydration, and changes in the composition of the tissue. These changes cause the fascia to be thicker, and more rigid.

Aging also effects tendons. As animals and humans age, there is a decrease in the number of stem cells and fibroblasts in the tendons, water and collagen content is reduced, and cross linking between tendon fibrils increases. These changes in tendons combined with increased stiffness and rigidity in the fascia result in reduced joint flexibility and mobility. There is also an increased risk for ligaments to be damaged from mechanical stresses.

Finally, age effects the physical connections between muscle, fascia and connective tissues. These connections weaken with age thus impacting force transmission capabilities of both the muscles and connective tissue.

Recovery from inactivity

Aging has a negative effect on muscle plasticity. When muscles are young, they are able to adapt and modify their structure in response to environmental changes, namely regrowth following a period of inactivity such as crate rest. With age, muscles’ regenerative capacity is reduced. Satellite cells lose their potential to regenerate muscle tissue. The loss of muscle plasticity with age can be seen when older dogs have a period of immobility and are not able to re-build muscle strength.

Ways to slow the effect of age on dog’s muscles

Physical activity

The most effective way of slowing the effect of aging on dog’s muscles is to maintain a regular regiment of physical activity. Exercise stimulates muscle hypertrophy (increase in muscle mass) as it increases the muscle’s oxidative capacity resulting in increase in muscle mass and strength. Physical activity also delays muscle impairment by slowing the rate of decline of mitochondria and proteins involved in muscle contractions.

For more information about the benefits of physical activity to slow the effects of aging, please see:
https://www.fullstride.com.au/blog/how-to-keep-you-dog-healthy-so-they-live-longer-exercise-for-healthy-ageing-in-dogs

Therapies

Massage has been shown to be effective in enhancing regrowth of muscle mass after a period of disuse (such as injury or surgery). The mechanical forces of massage can emulate the intramuscular pressure of moderate muscle contractions experienced during exercise. Such pressure has been shown to modulate macrophage recruitment and in so doing alleviate inflammation.

Further, massage can positively affect signalling pathways associated with muscle remodelling and elevate the rate of protein synthesis in myofibrillar and cytosolic proteins to increase muscle size.

Finally, massage has been shown to increase blood flow and in so doing can be effective in reducing adhesions in connective tissue, lubricating joints and addressing muscle stiffness.

Aging in all animals effects the performance of skeletal muscles and soft tissues, however by maintaining physical activity and complementary therapies such as remedial massage, muscle performance and mobility can be sustained. Full Stride provide remedial massage treatments to help maintain dogs’ activity levels and quality of life.

Until next time, enjoy your dogs.

Sources:

Miller, B. F., Hamilton, K. L., Majeed, Z. R., Abshire, S. M., Confides, A. L., Hayek, A. M., … & Dupont‐Versteegden, E. E. (2018). “Enhanced skeletal muscle regrowth and remodelling in massaged and contralateral non‐massaged hindlimb.” The Journal of physiology, 596(1), 83-103.

Saitou, K., Tokunaga, M., Yoshino, D., Sakitani, N., Maekawa, T., Ryu, Y., … & Suzuki, K. (2018). Local cyclical compression modulates macrophage function in situ and alleviates immobilization-induced muscle atrophy. Clinical Science, 132(19), 2147-2161.

Waters-Banker, Christine (2013) “Immunomodulatory effects of massage in skeletal muscle”, Theses and Dissertations – Rehabilitation Sciences, Paper 18