We often hear the saying “a tired dog is a good dog” but does that mean we should be exercising our dogs to the point of tiredness on a regular basis? In this article, I discuss the definition of muscle fatigue, how muscle fatigue increases the likelihood of injury, and how to prevent fatigue?

What is muscle fatigue?

Muscle fatigue can be defined as exercise induced, temporary decline in skeletal muscles’ ability to generate force during voluntary muscle contractions. Fatigue develops gradually over the course of sustained activity and is characterised by a decrease in muscle force capacity and motor behaviours causing a decline in performance.

Why does fatigue occur?

There is no single cause of fatigue. It is gradual, involves several machanisms, and depends on the task being performed.
During the performance of a task, one or more processes that enable contractile proteins in the muscles to generate force are impaired.

Some of the mechanisms involved in muscle fatigue are:

• Central fatigue – Central fatigue is associated with the development of abnormalities in the central nervous system during exercise. It is characterised by an impairment in the activation of motor neurons that drive muscle fibres. This impairment is caused by decreased output from high motor centres and increased synaptic inhibition directed to the motor neurons. Over the time course of a muscle contraction, motor neurons adapt to these changes and become less responsive to synaptic excitation.

• Peripheral fatigue – Peripheral fatigue occurs in the peripheral nervous system and is associated with changes in the transmission of muscle action potentials located at the nerve endings and neuromuscular junction. These changes occur due to an accumulation of metabolites and alterations in other chemical concentrations in the muscle.

• Skeletal muscle disease or disorders – Some diseases that affect the neuromuscular system can cause abnormalities in the processes involved in muscle contractions.

How does fatigue affect the muscle?

Motor units are the basic functional elements of the dog’s nervous system that are responsible for producing movement.
A motor unit comprises a motor neuron in the ventral horn of the spinal cord, an axon, and muscle fibre that is innervated or “powered” by an axon. The dog’s nervous system controls muscle force by modifying the activity of the motor units in the muscles.

Motor units in muscles have a range of contractile properties including force capability, speed of contraction and resistance to fatigue. There is a direct link between a motor unit’s force and fatigability. The stronger the motor unit’s force, the quicker it is likely to fatigue. Once recruited, a motor unit increases its firing rate with increased synaptic excitation until it reaches its maximum firing rate.

During a maximal voluntary muscle contraction, when the whole force of the muscle is applied, the torque is greatest at the start of the contraction and gradually declines through the sustained contraction. For a muscle to produce maximum force, the maximum number of motor units are recruited at the start of the contraction and firing rates gradually decline through the contraction. That is, the discharge rates of motor units recruited at the start of the muscle contraction declines over the course of the contraction as they reach their maximum firing rate. As the firing rate decreases and ultimately ceases, the animal’s ability to sustain the muscle contraction also declines.

On the other hand, during a sub-optimal contraction, the number of motor units recruited at the beginning of the contraction depends on the force required. The number of units recruited gradually increases over time and then slowly declines. The motor units recruited at the start of the contraction fire repetitively however over time their firing rates slow and eventually cease. They are replaced by motor units with slower firing rates. The increase in motor unit recruitment through the course of sub-optimal contraction are evidence of the central nervous system processes compensating for the decrease in force caused by the slowing of firing rates of the motor neurons recruited earlier in the contraction.

How long does it take for a muscle to fatigue?

It is difficult to predict the time course for muscle fatigue as it depends on a range of variables. Firstly, the rate at which motor units fatigue occurs at different speeds depending on the type and properties of the motor unit. Secondly, the rate at which muscle activation declines varies among individuals and even day to day in the same individual. Finally, the rate of muscle fatigue is task dependent and influenced by factors such as exercising in the heat and the individual’s energy reserves.

How can muscle fatigue result in a soft tissue injury?

For humans and dogs to move normally and perform everyday functions, they need effective control of their limbs and optimal muscle force. Without adequate control of their limbs and muscle force, the animal’s gait and movement will change. The animal’s ability to respond appropriately to sudden and unexpected changes in the environment such as uneven flooring will decline and the likelihood of the animal making extreme movements such as overstretching a limb will increase. These alterations in movement increase the likelihood of a slip, collision, fall or uneven distribution of load. All of which can cause a soft tissue (muscle, tendon or ligament) strain or tear.

Changes in movement is not just caused by the muscle’s inability to generate force but also the effect of fatigue on the animal’s proprioception. Proprioception refers to the signals from receptors called mechanoreceptors in the animal’s surface cells that are sensitive to changes inside the animal independent of stimuli produced internally. Based on this definition, proprioception refers to

• Regulation of posture (postural equilibrium)
• Regulation segmental posture (joint stability)
• Initiation of conscious peripheral sensation (muscle senses)

Muscle senses operate in four ways:

• Posture – the animal’s sense of the position of its joints
• Passive movement – the animal’s awareness of position and passive movement of body parts.
• Active movement – the animal’s awareness of position and active movement of body parts.
• Resistance movement – sensation of heaviness

Mechanoreceptors located in the joint capsule, muscle, tendon, ligaments, skin and fascia that convey proprioceptive signals are referred to as proprioceptors and include the following:

• Pacinian corpuscle-like receptors are sensitive to changes in pressure. They are stimulated at the initial and terminal stage of joint range of motion, with rapid change in speed or direction or when tissue is under pressure.

• Ruffini end organ-like receptors are sensitive to changes in speed and direction. They are sensitive to stretch stimuli and therefore are excited at the end of joint range of motion.

• Golgi tendon organ-like receptors, along with Ruffini end organ-like receptors, convey information about the position of muscle relative to each other and joints.

• Free nerve endings respond to forceful joint rotations and noxious stimuli producing a sensation of pain

When muscle fatigue occurs, then muscle activity declines and inhibits proprioceptive function. Fatigue impairs the animal’s neuromuscular control and adversely alters the dog’s joint proprioception. Joint proprioception is critical for maintaining joint stability. When the animal’s muscles are fatigued, the accuracy of motor control and thereby voluntary muscle stabilising activity is disrupted and impairs the animal’s ability to resist joint forces. Any loss of joint stability risks ligament damage.

In a human study investigating the effects of fatigue on proprioception, before the performance of a fatiguing exercise, the test subjects demonstrated the proprioceptive ability to actively reposition their knee joint to avoid over rotation. However, after the fatiguing exercise, this proprioceptive ability was diminished. This study concluded that fatigue affects the muscle’s contractile ability and the function of mechanoreceptors. The fatigue protocol used involved a joint at the end of its range of motion. The study found that mechanoreceptors sensitive to muscle tension became desensitised with fatigue and therefore the subjects were unable to reposition their limbs accurately.

As fatigue impairs muscle strength, reaction time and proprioception, it affects the dog’s ability to respond to sudden changes in the environment and requires them to alter their movement accordingly. In human studies, fatigue in different muscle groups, across different joints affects movement differently. When fatigue is localised, it increases changes in muscle co-ordination, speed, and force compared with more generalised muscle fatigue. Likewise, in animal studies, when fatigue varies the animal’s pattern of movement to a small degree, so there are only slight changes in their gait, then this impacts the dog’s ability to respond to environmental changes and causes tissues to be stressed repeatedly. In doing so, the risk of overload type soft tissue injury increases. On the other hand, when movement patterns are altered in a more significant way, then stresses are distributed to different areas of the animal’s body which increases the risk of errors and injury by increasing the likelihood of extreme movements.

How to prevent fatigue in dogs?

By the time the effects of fatigue are obvious, damage to soft tissue and joints could already have occurred. If you are planning to introduce your dog to a new exercise routine here are some tips for preventing fatigue and its potentially damaging effects on muscles and joints.

• Increasing exercise duration – When increasing duration, consider reducing the intensity of the exercise while you slowly (over the course of weeks) increase duration. Ways to reduce intensity may include: exercising when it is cooler, slowing the pace, and reducing effort such as inclines, reducing the number of dogs your dog is playing with etc

• Increasing exercise intensity – So you want more of a cardiovascular workout for your best mate, then consider reducing duration and effort while gradually increasing intensity. For example, if you are building to your dog to jogging or riding next to your bike, then perhaps do some run / walk intervals for several weeks and eliminate any inclines. If you want your dog to play with some doggy friends, depending on the intensity of the play, then after 1 – 2 minutes of high intensity running and romping, put the dogs on lead and let them have a break.

• Introducing a new activity – You may want to add swimming, bush walking, or off lead play with another dog to your dog’s exercise routine. When introducing a new activity, start with short bursts of activity followed by rest. For example, you may swim the dog for 1 – 2 minutes and then come out of the water for a 5 minute rest. Before returning to swimming, check the dog’s demeanour and gait for signs of fatigue.

• Changing exercise time – You have a change in work routine and now need to exercise your dog at different times of the day. If you are exercising when it is hotter than normal, then consider reducing the duration and intensity of exercise until your dog acclimates to the change over several weeks.

• Changing environment – Have you found a new park or beach to run your dog? Consider any changes in the effort to exercise in this place and adjust the duration and intensity accordingly. For example, if you dog has only run off lead in a flat, mowed park and you are now allowing them to run on soft sand, then the effort is greater due to change of uneven surface.

If you dog has experienced some soft tissue injury, Full Stride offers remedial massage treatments to relieve pain and help restore normal joint function.

Until next time, enjoy your dogs.

Sources:
Abd-Elfattah, H. M., Abdelazeim, F. H., & Elshennawy, S. (2015). Physical and cognitive consequences of fatigue: A review. Journal of advanced research, 6(3), 351-358.

Cowley, J. C., & Gates, D. H. (2017). Proximal and distal muscle fatigue differentially affect movement coordination. PloS one, 12(2), e0172835.

Enoka, R. M., & Duchateau, J. (2008). Muscle fatigue: what, why and how it influences muscle function. The Journal of physiology, 586(1), 11-23.

Ortega-Auriol, P. A., Besier, T. F., Byblow, W. D., & McMorland, A. J. (2018). Fatigue influences the recruitment, but not structure, of muscle synergies. Frontiers in human neuroscience, 12, 217.

Potvin, J. R., & Fuglevand, A. J. (2017). A motor unit-based model of muscle fatigue. PLoS computational biology, 13(6), e1005581.

Voight, M. L., Hardin, J. A., Blackburn, T. A., Tippett, S., & Canner, G. C. (1996). The effects of muscle fatigue on and the relationship of arm dominance to shoulder proprioception. Journal of Orthopaedic & Sports Physical Therapy, 23(6), 348-352.

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