How Strength Training Recruits Fast and Slow Twitch Fibres
Myth Explained
Forget what you thought you knew about "fast" and "slow" twitch fibres– the real story is about how becoming "weaker" during a workout can actually make you stronger.
The intensity of a muscle contraction is directly proportional to the number of muscle fibers activated. Fewer fibers result in a lighter contraction, while more fibers yield a stronger one. Your brain orchestrates this recruitment process, signalling through the central nervous system via motor nerves, and crucially, it follows a fixed, orderly sequence.
There are 4 primary types of muscle fibres:
Slow-Oxidative (SO) Fibres: These are your endurance fibres, highly efficient for sustained, low-intensity activities. They require minimal energy to activate.
Fast-Oxidative (FO) Fibers: Offering more speed than SO fibers, they possess a relatively poorer endurance profile.
Fast-Oxidative Glycolytic (FOG) Fibers: These are intermediate fibers, exhibiting characteristics of both anaerobic and aerobic systems, balancing endurance, power, and speed.
Fast-Glycolytic (FG) Fibers: These are built for explosive power, delivering maximal force but with very limited endurance. They demand the most energy for activation.
By strengthening and increasing the proportion of FG fibres, individuals can improve their body composition, reduce the risk of obesity and metabolic disorders, and enhance their ability to perform activities requiring strength and power. This is because FG fibres increases the organism's tolerance to excess adipose tissue such that it is able to maintain metabolic parameters with a normal range. It’s important to note that your individual muscle fiber composition is largely genetically predetermined. While training can enhance their capabilities, it cannot fundamentally alter their inherent type. When performing a light to moderate lift, your brain first engages the SO fibres. As greater force is required, it progressively calls upon the FO fibres and then the FOG fibres. Only when the load is sufficiently heavy, demanding maximal effort, will the powerful FG fibres be activated.
The load you select for your exercise profoundly influences fiber recruitment:
Too Light: This primarily recruits slow-twitch fibers. Due to their slow fatigue rate, they can recover and re-engage, preventing the deeper stimulation of higher-order muscle fibers. This limits the overall stimulus to your musculature.
Too Heavy: If you can only perform one or two repetitions, you will simultaneously recruit all available motor units. While this generates maximal force, the set will terminate quickly as the fastest-twitch units fatigue. This often means you haven't sufficiently fatigued your lower-order motor units, missing out on a significant portion of the exercise's productive capacity.
Therefore, a moderately heavy weight is often most effective. This is precisely where proper strength trainingdistinguishes itself as a superior and safer modality. It offers unique advantages:
Enhanced Safety at Peak Effort: As you progress through a strength training set, fatiguing your lower-order fibers, you effectively become "weaker." By the time you engage those powerful fast-twitch units, your body is in a fatigued state, inherently reducing the risk of self-inflicted harm from excessive force. The resistance (weight) remains constant, but your diminishing force output as you near fatigue makes the end of the set paradoxically safer than explosive activities.
Comprehensive Muscle Stimulation: This orderly recruitment pattern ensures a thorough stimulation of all fiber types, leading to a far more complete physiological and metabolic engagement. No muscle fiber is left unstimulated.