The average, recreational Crossfitter typically has a good amount of muscle with a low amount of body fat compared to the average person while a professional Crossfit athlete has enough muscle to rival the physique of a bodybuilder.
So, with no specific muscle-specific training splits how do Crossfit athletes continue to build so much muscle while a majority of their workouts are predominantly cardio based with lighter weights?
There are 3 reasons why (backed by science) below. The following data comes from research conducted by @theheatonminded
High reps at sub-maximal weights have been shown to be one of the most important factors for muscle growth.
“Resistance training (RT) is the primary exercise intervention for increasing muscle mass in humans. It is theorized that the volume of training performed in a RT bout—herein determined by the formula: repetitions /×/ sets (1)—plays a significant role in chronic muscular adaptations such as muscle size and strength (2). As compared with single-set routines, acute studies indicate that performing multiple sets augments the phosphorylation of p70S6 kinase and muscle protein synthesis (MPS), suggesting that higher volumes of training are warranted for maximizing the hypertrophic response.“https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6303131/
The average Crossfit workout contains hundreds of reps with bodyweight or with relatively light weight.
Of course Crossfit athletes, recreational and professional, do also follow strength programs and some workouts do contain heavy weights but the majority are high rep, low weight.
Muscle growth occurs when subjects do sets until failure, regardless of weight.
A 2012 study has shown that completing 3 sets to failure performed with either 30% or 80% of your 1RM has lead to more muscle growth than 1 set to failure done with 80% of your 1RM.
Meaning, more sets done to failure leads to more muscle, regardless of weight used.
Performing workout sets to failure—that is, continuing an exercise set until you can’t complete another repetition with proper form—can stimulate muscle growth for several reasons:
Muscle Fiber Recruitment: When you lift weights to the point of failure, your body recruits as many muscle fibers as possible to handle the load. This process targets both slow-twitch (endurance) and fast-twitch (strength and size) muscle fibers, leading to more comprehensive muscle development.
Mechanical Tension: Lifting weights creates mechanical tension in your muscles, one of the primary factors in muscle growth. When you lift to failure, this tension is maximized.
Metabolic Stress: Working out to failure also creates metabolic stress by causing a buildup of byproducts like lactate in your muscles. This “pump” sensation is associated with anabolic (muscle-building) processes.
Muscle Damage: Intense workouts cause microscopic damage to muscle fibers. While it might sound harmful, this damage is actually a catalyst for muscle growth. Your body repairs these damaged fibers through a cellular process where it fuses muscle fibers together to form new muscle protein strands (myofibrils). These repaired myofibrils increase in thickness and number to create muscle hypertrophy (growth).
Hormonal Responses: Training to failure can lead to a greater acute hormonal response, including an increase in testosterone and growth hormone, both of which are conducive to muscle growth.
During a Crossfit workout the typical athlete will push themselves to get the fastest time possible and in pursuit of that goal they will often get very close to failure on a certain movement. Doing this on a consistent basis leads to consistent muscle growth.
3) CONSTANTLY VARIED WORKOUTS, MOVEMENTS, TRAINING DAYS
First, what is the “repeated bout effect“?
Repeated Bout Effect: The repeated bout effect refers to the adaptation whereby a single bout of eccentric exercise protects against muscle damage from subsequent eccentric bouts.
Muscle is built when you have created tears in the muscle (damage) through resistance training and subsequently repair and fill those tears with nutrients in order for them to grow.
When you consistently repeat the same workout with the same amount of weight your body protects against muscle damage as best as it can, limiting your growth.
In Crossfit no day is ever the same. You may do squats 2 days in a row, you may have cardio paired up with heavy strict press followed by swimming the next day, etc.
Because Crossfit training is designed to have you prepared for anything you may encounter your routine will never be the same, and because of this your body will always be forced to adapt to new stimuli.