To create a program that meets our clients' needs and to help them achieve their goals we need to have an intimate knowledge of muscle tissue—all the way down to the fibers that create each muscle spindle.
Slow twitch muscle fibers are predominantly used during aerobic exercise, such as long-distance running. These fibers contract slowly and have a very high aerobic capacity. As a result, they produce ATP through aerobic glycolysis. These fibers are often referred to as type I. Type I fibers rely heavily on oxidative phosphorylation rather than glycolysis for ATP production. They have a high resistance to fatigue and a high oxidative capacity.
Fast twitch muscle fibers are mainly recruited during anaerobic exercise. When a muscle is placed under high demand or heavy load, these fibers take control. Activities like sprinting and weightlifting rely on fast twitch muscle fibers due to high levels of intensity and shorter work periods. These fibers contract faster, allowing them to produce greater amounts of force, power, and strength, but they fatigue faster.
Fast twitch muscle fibers include type IIa and type IIb fibers. Type IIa fibers have a fast shortening speed and transfer energy from aerobic and anaerobic sources. Type IIb have a faster shortening speed and greater anaerobic potential. These fibers have a high number of glycolytic enzymes, low resistance to fatigue, and low oxidative capacity compared to type IIa.
If a client's main goal is to improve muscular endurance you must design a program that will target type I muscle fibers. To target type I muscle fibers, you need to train at lower intensities, but perform higher repetitions. For each exercise, aim to complete 3 sets of 12 or more repetitions. The training intensity should be individualized based on fitness level, but always 55% to 65% of 1RM to stimulate type I muscle fibers. Training at a lower or higher intensity will not activate type I muscle fibers, instead higher intensities will recruit type II muscle fibers.
If the main focus is increasing strength, the repetition range will be lower with a higher intensity to target type II muscle fibers. The goal for each exercise regarding strength training is to complete 6 or less repetitions and 2-6 sets. The training intensity for each set should be 85% or higher to promote the best results.
To focus more on power, aim to complete 1-2 repetitions with 3-5 working sets at 85% to 95% of 1RM. Since training for power involves explosive movement, motor units do not have time to activate accordingly. This leads to disruption of hormonal adaptations that would normally occur during hypertrophy training. This limits muscle growth but enhances force production.
Hypertrophy training is most effective when exercise intensity is 60% to 85% of 1RM. Aiming to complete 6-12 repetitions for 3-6 sets per exercise at this intensity will recruit more muscle fibers. By recruiting more muscle fibers, you create more tissue damage, which in turn improves growth. Heavier weights activate type IIb muscle fibers because of the size principle. According to the size principle motor units of a muscle are recruited in order of threshold and firing rate. In order to recruit the most motor units they have to be activated in order, which hypertrophy training does.
Different types of activities will result in separate muscle contractions. Slow twitch muscle fibers are exactly what the name says, they contract slowly. These fibers are most commonly active during endurance training. This is because endurance athletes commonly train with high reps and lower intensities. Aerobic training has its benefits including helping mitochondria make energy more efficient. Mitochondria is responsible for producing ATP molecules for energy using oxygen as fuel.
If you have a client perform an exercise at a low or moderate intensity you will use more oxygen compared to if they perform the exercise with heavy weight or at a high intensity. The more oxygen you pump to your muscle cells and their mitochondria through aerobic workouts, the faster and better they work to produce energy. This leads to optimal performance for your clients who need to specifically train slow twitch fibers.
Higher intensity weight training will recruit different types of muscle fibers compared to performing higher reps and lower intensity workouts. Resistance exercise with heavy weights will create faster impulse signals that travel through motor neurons. This is what causes your client’s muscle contraction to be faster.
For example: Sprinters create more muscle tension and resistance through explosive short duration movement. This leads to workouts that are fast twitch dominant.
It is not the muscle group your client is targeting that determines the fiber types used. If you increase the weight on a bench press exercise you will recruit more of the fast twitch muscle fibers versus if you lower the weight creating a lower intensity. Fast twitch fibers have a larger cross-sectional area than slow twitch fibers, no matter the muscle group.
As a result of this, slow twitch muscle fibers are not as efficient in force production and muscle growth or size. This alludes to: Does combining cardio and weight training work against one another? Resistance training and steady-state aerobic training both supply fantastic benefits, but the muscular changes caused by resistance training can be in the opposite direction of those caused by aerobic training and vice-versa.
In the end, a client's goal will tell you the type of training you should use. The type of training you use should target the muscle fibers responsible for producing the client’s desired adaptations and results. To target specific muscle fibers, you must consider training intensity, repetition range, working sets, and rest intervals.
If you find that most of your clientele training revolves more around hypertrophy and bodybuilding workouts, be sure to explore ISSA’s Bodybuilding Specialist course. This course covers training, recovery, motivation, and nutritional strategies to prepare you, as a fitness professional, to work with bodybuilders leading them to proper prescribed training intensities.