Want Results from Exercise?
If you’re into exercise then you want results from your workouts. Knowing a little about how your body responds to exercise can help you identify the best workouts to deliver the results you want. As a fitness educator, I get paid to travel around the world (both in real life and virtually) to teach exercise science to personal trainers and am now using this blog to share that info with you so that you can get the results you want when you exercise.
First, if your goals include ‘toning up’ it’s important to know exactly what that means and how to do it. Tone comes from the term ‘tonus,’ which is the technical term for the contraction of a muscle fiber; when all of the fibers within a muscle maintain a state of semi-contraction that is what creates the shape we commonly associate with a well-defined muscle.
The Role of Muscle Fibers
The primary purpose of muscle fibers is to control the physical forces moving through your body. Muscles shorten to generate the force to create movement; for example when moving from a seated to a standing position the quadriceps and gluteus maximus shorten to help the body stand up against gravity.
When a muscle lengthens it controls and decelerates a force such as when the quadriceps and glutes lengthen to control the motion of the body as it returns to a seated position (being accelerated downwards. by gravity). If you want workouts that produce results it is important to have an understanding of how muscle fibers respond to exercise; here are a few important things to know about your muscle fibers and how they respond to exercise.
Exercise damages muscle fibers
Muscle fibers adapt to the specific type of exercise stimulus imposed during training. Mechanical stress is the physical stress applied during resistance training which causes micro-trauma to muscle fibers. When fibers are damaged from exercise they signal the biochemical reaction to produce the satellite cells responsible for repairing the mechanical structure of the muscle cell which results in thicker fibers capable of generating higher levels of force.
Muscle Fibers are Controlled by the Nervous System
Muscle fibers are activated by a motor neuron which is the connection between the central nervous system and the muscular system. 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 a number of different muscle unit types that are often organized into three general categories: type I, type IIA and type IIB.
How Muscle Fibers Work
According to the all-or-none theory, a motor unit is either active or inactive, when it is signaled to contract it activates 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. This link takes you to my blog on exercise hacks for getting results from your workouts: read it, know it, live it!
Learn exactly HOW your muscles work along with the best exercises that can make them stronger. You will learn about the difference between your muscles and fascia and the different types of workouts that can make them stronger. The course includes examples of workouts that can help you to reach all of your fitness goals. This online course is written for everyone; fitness professionals, personal trainers and group fitness instructors earn continuing education credits, CECs, for taking this course: 0.2 ACE; 0.2 NAMS; and 2 AFAA
Type I Muscle Fibers
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 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.
Type IIb Muscle Fibers
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 they fatigue quickly.
Type IIA Muscle Fibers
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
The Size Principle
Muscle fibers work according to the size principle; as a muscle needs to produce a greater level of force it will start by activating the smaller type I motor units. When type Is can not provide the necessary force, the larger type II motor units and muscle fibers are recruited to perform the work. A muscle has a limited 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.
These 2 e-books review how your muscles work and provide a number of different exercise solutions to help you develop the body that you want.
2 Types of 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. Here’s an example of a workout that combines strength training with functional movements to engage ALL fibers in the glutes:
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.
Each Fiber Type Performs a Specific Function
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.
Why you need to know how your muscle fibers work:
If your goals include improving muscle definition or increasing levels of strength the only way to achieve this is by activating the type II motor units and muscle fibers. Follow this link to my blog that can help you understand the different types of strength training.
There is a common gym myth which mistakenly believes that developing muscle tone (“toning up”) is best achieved by using lighter weights for higher repetitions. It can be possible to achieve definition with lower weights but it requires doing a lot of repetitions until the muscle fatigues and can no longer contract. Unless the exercise is performed to fatigue which occurs only after recruiting the type II fibers, it will not improve definition. However, if time is an issue using a heavier weight for fewer repetitions is an effective technique for stimulating growth and definition from the type II fibers.
Originally written in 2019; this blog was updated in the spring of 2021.
Knowledge is power but knowing how to apply the knowledge can help you reach your goals quicker and make the most of your time in the gym. To learn more about how to design exercise programs to reach YOUR goal, pick up a copy of my book
Or listen to this episode of the All About Fitness podcast with Dr. Brad Schoenfeld, the author of The Science and Development of Muscle Hypertrophy