Monday, August 3, 2015

Understanding Muscle Physiology

The job of a strength and conditioning coach is to improve on-field performance which means improving movement skill.

Producing movement is a result of the unique ability of muscle to contract and shorten/lengthen its tissues and cause skeletal movement. Movements come from the ability of the body to generate tension and force. It is dependent on muscles, tendons, and the nervous system. The nervous system communicates to the muscles how and when they should contract. The muscular system contracts to generate tension and transmits this tension (along with the help of tendons) to the skeletal system to create movement.

The amount, speed, and frequency of force and consequently movement depend on the certain factors outlined below.

Motor Unit Recruitment

The recruitment of motor units is the basis for producing force. Motor unit recruitment relates to the number of motor neurons innervated during a muscle contraction. The ability to recruit more motor units equates to higher forces generated. Athletic movements occur as a result of skeletal muscles acting upon the skeletal system and using them as levers. These muscular contractions occur as response to signals sent from the nervous system.

Size Principle

The recruitment size and area of a motor neuron is directly related to the size of its axon; the larger the axon, the greater the amount of stimulation required. This principle is fundamental to the understanding of power because the size of the soma is indirect in proportion to the size of the motor unit or the number of the muscle fibers innervated by the alpha motor neuron.

Smaller motor units innervate less muscle fiber and produce smaller amounts of force which causes them to be recruited first.  Larger motor units on the other hand produce greater amounts of force.  As each muscle possesses a wide variety of motor units ranging in size, motor units are recruited in ascending order of size in what is referred to as “Henneman’s size principle”. The amount of load and how fast the weights or implement is lifted play a major role in determining the number of motor units and muscle fibers get stimulated. This is why heavy weights and/or power movements are essential to athletic output and used primarily in training.

Rate Coding

Rate coding is the term used to describe the frequency of signaling from the central nervous system to the motor unit. Basically it is how quick that signal is received from the nervous system until it results in a muscle contraction. Obviously increasing signal frequency can result in greater power production because of an increase in the firing rate of motor units. It has been shown that increased rate coding leads to higher rates of force development.

Rate coding is a quality that will improve over time with repetitions and neuromuscular adaptations. But if the signal frequency reaches too high of speed that the muscle fibers cannot completely relax between the bouts of signals, rate coding will be undermined and contraction efficiency and power will be reduced.


Synchronization is how well the nervous system manages to fire the muscle fibers in concert with one another, with more synchronized firing patterns leading to higher levels of force generated. The more motor units that can simultaneously activate can lead to performance gains. Motor unit synchronization may not directly enhance maximal force output, but it may be most related to performance in rate of force development and movements that require coactivation of multiple muscles at the same time.

Stretch-Shortening Cycle

The stretch-shortening cycle takes advantage of the elastic properties of the muscle-tendon units. Tendons don’t actually contract but act as a bridge between muscle and bone to aid and add extra force to contractions. Tendons are especially useful during stretch-shortening actions and their high elastic properties. Essentially tendons act similar to springs - they stretch and store energy during eccentric contractions and snap back during the concentric portion.

Power and speed can increase through this stretch-shortening cycle and can easily be seen in the difference between a depth jump and a static vertical jump. The depth jump takes advantage of the SSC while the static vertical jump doesn’t utilize these key elastic components.

This process works due to specific mechanoreceptors, such as muscle spindle fibers, responding to the muscle being stretched and send signals to the central nervous system. This communication causes a signaling of the muscle fibers to contract to prevent potential overstretching. This involuntary contraction allows for higher power and faster speeds of movement.

Sarcomeres in Series

The amount of force output within a muscle group is dependent on the number of
sarcomeres arranged in series (more sarcomeres running in series allows for each sarcomere to contract over a smaller range of motion to produce a given change in overall muscle length.  Multiple studies have also shown that faster athletes consistently exhibit longer muscle fascicles than their slower peers. This is why we train, dynamic resistance training has been shown to increase fascicle length, and conversely speed.

Motor Units in Muscle Groups

Every muscle in the body is different in terms of the number of motor units and muscle fibers it innervates due to its action, role, and need. Smaller, finer muscle groups and areas will contain less muscle fiber per motor unit due to its need for small, precise movements. These can be seen in your eyes and fingers. Larger muscle groups that need to be able to produce large amounts of strength and power will contain 1,000 or more muscle fibers per motor unit.

All or None Principle

When a motor unit is recruited all of the muscle fibers it innervates will contract. There is no half-way or partial contraction. If a signal is sent to stimulate that motor unit, every single fiber it innervates will contract; this is the all or none principle.

Force-Velocity Curve

The classic force-velocity curve demonstrates that as the velocity of movement increases the force produced is decreased. Basically it states something that we all know, as the weight gets heavier; we can’t move it as fast. One thing that is important about the force-velocity curve is that we train on all spectrum's of the curve, to get a maximum training benefit. Effectively we want the line to move up and the right. By doing so are able to produce more force at higher speeds, which effectively increases power and speed, and this is the goal for almost every sport.

Training only one end of the spectrum will help improve that specific quality, but may not affect the whole spectrum. Basically working on strength will give a bigger ceiling for speed development, and working on speed will benefit strength and RFD. One without the other will not produce maximal outcomes, and this curve can be a reminder of the interplay between physical qualities. The force-velocity curve is also a good guideline for coaches to structure their training. It leads into periodization, sport specificity, player position specificity, and loading schemes. Is your training and progressions attacking the whole curve, or are you just living in one area? There should be dedicated time to improving all of these qualities, to what degree will depend on the sport, athlete, and your coaching philosophy.
“The Goal Is To Move The Curve Up And To The Right”

Understanding the basics of muscle physiology allows for better programming and better outcomes for athletes.  Improving sports performance comes down to understanding physiology and how the nervous system operate under the demands of sports.

Go Get 'Em!

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