There are 3 physiological processes through which your dog's behavior response gets stronger:
1.)Improved neuromuscular efficiency
2.)Muscle fiber hypertrophy
3.)Muscle Cell Hyperplasia
The first process is through more efficient recruitment of the existing muscle fibers. The second process actually results in an increase in the cross-sectional size of the muscle fibers. Both processes occur in all dogs when properly stimulated to do so, but the first process- increased efficiency of the existing muscle fibers usually happens first.
Physiologic response #1: increased recruitment in the neuromuscular system
The first physiological response consists of the brain calling upon more motor units to participate in a movement. Each muscle has multiple motor units in it, although the exact number varies. The more motor units that are activated the stronger the muscle contraction is.
How does this work? For every action and movement you or your dog performs, the central nervous system controls it. The brain will decide which muscles to contract, how many muscle fibers to contract, with what intensity to contract at and how often to contract. Muscle fibers are activated via motor units. A motor unit is a single motor nerve and its associated muscle fibers. Each muscle is made up of many motor units.
In essence, the brain decides how many motor units in the muscles to activate in order to accomplish the task at hand. This is a learned response over time. If I am trying to lift a gallon jug of milk for example, my brain will determine whether the job will take x number or y number of motor units in my biceps muscle. Ideally, the nervous system tries to recruit just enough to do the job without wasting energy by contracting too many muscle fibers. Most of the time the nervous system does an accurate job but sometimes we are tricked into thinking something is heavy when it is not or vice versa. (Have you ever thought something was heavy and put a lot of effort into moving it only to realize it was actually quite light and then you nearly flung it across the room?) But too little recruitment and we are not able to do the task.
When your dog first begins a formal strength training program, you will see improvements in strength within 3-5 days mainly because the brain is activating more muscle fibers to participate in the exercises. This is called increased neural recruitment. Eventually the nervous system doesn’t have any more motor units to recruit however and gains in strength will have to come from muscle cell hypertrophy, method two.
Physiologic response #2: muscle cell hypertrophy
Additional strengthening past neural recruitment relies on muscle cell hypertrophy in which the muscle cells actually become larger. How does this happen?
It is a complicated process but it starts with a workout. The physiologic stress of resistance training causes micro damage to the contractile proteins in the muscle fibers. Satellite cells that are hanging out near the membranes of the muscle cells are called into action. These satellite cells multiply and fuse into new protein strands or repair the damaged protein strands. The nucleus in these satellite cells then becomes part of the muscle fiber and can now make additional muscle proteins (which are needed for muscle contraction). Production of more contractile proteins within the muscle cells enlarges the overall size of the cell and the increased ability to contract means more strength.
Essentially, microtrauma is repaired and protein production is upregulated resulting in a bigger, stronger cell. Additionally, because of the stress of the workout, increased blood flow to the muscles occurs. This happens by the body actually creating new capillaries to the tissue to support the bigger muscle cells’ needs.
Note that all this repair and rebuild occurs in the recovery phase AFTER the workout. Many factors affect the ability of an individual’s muscles to hypertrophy, including genetics, hormones, and nutrition. But the basic process is the same in everyone.
Muscle cell hypertrophy takes time. It will typically take weeks to a few weeks to see and be able to measure this muscle enlargement. If your dog’s training program is appropriate your dog should experience both forms of strengthening. I recommend taking pictures daily or every other day at the same time of the day in the same light at the same place. This way you can measure accurately and the glycogen from their food will not throw off the results.
Physiologic response #3: muscle cell hyperplasia
The last physiological way is very similar to the second but instead of becoming a part of an existing muscle fiber, satellite cells can fuse together when there is a release of Insulin Growth Factor 1(IGF-1) and Mechano Growth Factor(MGF) to make an entirely new muscle fiber. What’s interesting enough is that some dogs have a genome for this while others have a gene mutation where it causes an overload of muscle fibers. If you’ve ever seen the blue bull or blue greyhound that is the mutation. The mutation can also be a deficiency of myostatin which eats away at muscle. IGF-1 and MGF can lower levels of myostatin which is partly how the hyperplasia happens.
Rather than growing in size, new muscle cells and muscle fibers grow and stack on top of one another like laying bricks and connect to the motor unit at the muscular junction so that they can respond to the stimuli when the other muscle fibers and muscle cells do. This process when possible whether through having the genome for this to happen or using supplementation and nutrition to create a greater release of HGH, IGF-1 and MGF to create the new muscle fibers and muscle cells