Chances are that if you’re starting a workout program, ‘toning up’ is probably one of the most important outcomes that you want to achieve from your time spent sweating. Did you know that the term tone comes from ‘tonus’, which is the technical term for a state of contraction of a muscle fiber? If all of the fibers within a muscle stay in a state of semi-contraction it results in the shape we commonly associate with a well-defined muscle.

The primary purpose of muscle fibers is to control force moving through the body. Muscle shortening actions generate force to move an external resistance. Muscle lengthening actions happen to decelerate a force such as when the quadriceps and glutes lengthen to control the motion of the body as it drops to a seated position. If you are interested in an exercise program that produces results it is important to first know how muscle fibers work and respond to exercise. Here are 10 important things to know about muscle fibers and how they respond to exercise:

  1. Muscle fibers adapt to the specific types of exercises used during training. Mechanical stress is the physical stress applied during resistance training which causes micro-trauma to muscle fibers. Once fibers are damaged they signal the biochemical reaction to produce new satellite cells responsible for repairing the mechanical structures of the muscle cell as well as building new muscle proteins.
  2. Muscle fibers are activated by a motor neuron, which is the connection between the central nervous system and a specific unit of muscle. A muscle motor unit is the motor neuron and the attached muscle fibers. Think of a motor unit as a light switch for the muscle, as a muscle is required to generate a force the motor units will ‘light up’ to stimulate the fibers to shorten in order to produce that force. There are different types of motor units often organized into three general categories: type I, type IIA and type IIB
  3. According to the all-or-none theory, a motor unit is either active or inactive; when it is signaled to be active it contracts ALL of its attached muscle fibers. Slow-twitch motor units have a low threshold for activation and low conduction velocities, and are attached to type I muscle fibers. Fast-twitch motor units have a higher activation threshold, are attached to type II muscle fibers and are capable of conducting signals at higher velocities resulting in greater amounts of muscle force.
  4. Type I fibers are known as aerobic fibers. These fibers have a higher density of mitochondria which are efficient at aerobic metabolism, the process of creating energy to fuel muscle activity with oxygen. The mitochondria give the cell a darker color and are the reason why these are known as red muscle fibers. Type I fibers use oxygen to create energy for lower-intensity, long term endurance-oriented activities like walking, running, swimming, cycling or standing for extended periods of time.
  5. Type IIB fibers are known as anaerobic muscle fibers. Type IIB fibers store energy that is released for short, explosive extremely high-intensity activities. Type IIB fibers do not have mitochondria and have a colorless appearance so they are known as white fibers. Type IIB fibers are used for strength and power activities requiring a high amount of force in a short period of time. Because they have a limited supply of stored energy, type llB fibers fatigue quickly.
  6. Type IIA fibers have mitochondria so they can be involved in aerobic activities but can also be used to produce force rapidly during activities requiring a high amount of strength or power. Fast-twitch muscle fibers also have a greater diameter than type I fibers and play a more significant role in hypertrophy. Recruiting and innervating type II muscle fibers requires creating enough mechanical overload to fatigue the involved muscle by the end of the set.
  7. Muscle fibers work according to the size principle. As a muscle requires force it will start by activating the smaller type I motor units, and when they cannot provide the necessary force (or fatigue), the larger type II motor units and muscle fibers are recruited to perform the work. A muscle has a finite number of motor units and the higher-threshold type II motor units are not “turned on” unless a high level of force is needed. The most common way to increase motor unit activation is by lifting heavier weights; an increased load placed on a muscle will cause a greater number of motor units to activate more fibers to generate the force necessary to overcome the resistance. This is why your muscles shake when you try to lift a heavy weight for the first time, muscle motor units not previously used are being “woken up” and called into action.
  8. Muscle fibers experience two specific types of hypertrophy (the technical term for muscle growth). Myofibrillar hypertrophy refers to the increase in size or thickness of individual actin and myosin protein filaments, which can improve the force production capacity of individual fibers. Myofibrillar hypertrophy does not lead to larger muscles; rather, it results in thicker muscle fibers capable of generating more force. Sarcoplasmic hypertrophy is an increase in the volume of the semifluid inter-fibrillar substance surrounding an individual muscle fiber. This fluid contains the proteins used to promote tissue repair and growth. The muscle “pump” that body-builders work to achieve is actually sarcoplasmic hypertrophy—the cross-section of muscle fibers will increase, but most of the enhanced muscle size is due to an increased volume of the sarcoplasm and non-contractile proteins not directly involved with force production.
  9. One of the long-term adaptations of muscle to resistance training is an increase in muscle fiber cross-width. As the cross-sectional area increases in size, the fibers have more surface tension and become capable of generating higher amounts of force. Muscles with a larger cross-sectional area of individual muscle fibers are capable of producing greater amounts of force. In addition to being responsible for producing the force necessary for dynamic movements, type II muscle fibers have a greater diameter (cross-width) than type I fibers and are responsible for the hypertrophy, or increased size, of a particular muscle.
  10. Type IIA and IIB muscle fibers are responsible for generating movement as well as muscle size and definition. Both classifications of type II muscle fibers create higher levels of force to produce human movement and are known as phasic muscles. Type I fibers are responsible for maintaining postural control along with joint stability and can be categorized as tonic muscles.

Here is an example of why it is important to know about muscle fiber physiology:

If you want to improve muscle definition or increase your existing levels of strength the only way to achieve those results are by activating the type II motor units and muscle fibers. The variable resistance offered by an ActivMotion Bar can help you activate all of the fibers within a specific muscle. Using light weights for high repetitions can improve definition when the exercise is performed to fatigue; the shifting mass of the ActivMotion Bar will cause the involved muscles to work harder to maintain control during the exercise leading to fatigue in a shorter period of time.

Knowledge is power but knowing how to apply the knowledge can help you to achieve specific fitness goals along with determining the most effective course of action for reaching them.

– Pete McCall, MS, CSCS, ActivMotion Bar Master Trainer

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