Higher Reps vs. Lower Reps: The Science of Muscle Building (Part 1)
This weeks blog will be split into 2 parts! Part 1 dives into the basics of higher v lower reps and the science behind our muscles. Next week in Part 2 we will specifically discuss Time Under Tension and what this does to our muscles on a cellular level :) Enjoy!…..
When it comes to building muscle, one of the most enduring debates in the fitness world revolves around the question of higher reps vs. lower reps. Fitness enthusiasts, trainers, and athletes alike are often left wondering: should you lift heavy for fewer reps, focusing on raw strength and power? Or is it better to lighten the load and pump out more reps, chasing that muscle-burning fatigue? While both approaches are effective in their own ways, the truth is that neither is inherently better than the other. The choice between high reps and low reps depends on several factors, including your specific goals, the type of muscle fibers you're targeting, and the physiological adaptations you're aiming to achieve.
To understand the debate fully, it's essential to recognize that muscle building, or hypertrophy, doesn’t follow a one-size-fits-all formula. Different rep schemes lead to different outcomes, whether you're looking to maximize strength, increase endurance, or stimulate muscle growth through metabolic stress. The number of reps you perform directly influences not just the physical load your muscles experience but also the underlying biological processes that drive muscle growth and development. In other words, your body's response to high-rep vs. low-rep training goes beyond the obvious difference in weight and repetitions—it touches on how your muscles adapt at a cellular level.
So, should you focus on heavy weights and fewer reps to build muscle, or go lighter and do more reps to achieve that coveted muscle pump? The answer isn’t as simple as choosing one over the other—it depends on your individual goals, the adaptations you're seeking, and how you want to shape your training program for long-term success.
Muscle Fiber Types:
Endurance vs. Power
The Science Behind Muscle Fiber Types
Metabolic Pathways:
Type I (Slow-Twitch) Fibers: These fibers rely predominantly on aerobic respiration, which uses oxygen to generate energy. They contain higher concentrations of mitochondria, the "powerhouses" of the cell, allowing them to produce ATP (the cell’s energy currency) through oxidative phosphorylation. This process is slower but sustainable, making Type I fibers ideal for prolonged, low-intensity activities. They also have greater capillary density, enhancing oxygen delivery to the muscle fibers.
Type II (Fast-Twitch) Fibers: These fibers are designed for anaerobic activity, primarily using glycogen stored in the muscle to generate ATP quickly through glycolysis, a process that doesn’t require oxygen. Type II fibers have fewer mitochondria and capillaries compared to Type I, but they contain a higher concentration of enzymes that facilitate rapid energy production. While this provides the power needed for short, intense bursts of activity, it also leads to quicker fatigue.
Contractile Properties:
Type I Fibers: These fibers contract more slowly and with less force, but their endurance capacity is unmatched. The presence of more myoglobin, a protein that binds oxygen, helps sustain aerobic activity for longer periods.
Type II Fibers: These fibers contract more rapidly and forcefully. Type II fibers are further subdivided into Type IIa (fast-twitch oxidative) and Type IIx (fast-twitch glycolytic). Type IIa fibers have a moderate capacity for aerobic energy production, offering a balance between power and endurance, whereas Type IIx fibers are purely glycolytic, producing the most force but also fatiguing the quickest.
Why There Are Two Fiber Types
The existence of two muscle fiber types is an adaptation to allow humans (and other animals) to meet the varied demands of physical activity. Some activities, like walking or standing, require long-lasting, low-intensity muscle contractions, best handled by Type I fibers. In contrast, other activities, like sprinting or lifting heavy objects, need short, powerful bursts of force, which Type II fibers are optimized for.
From an evolutionary perspective, this division makes sense because it maximizes efficiency. Early humans needed to both sustain activity (for long-distance travel or hunting) and perform quick, powerful movements (to capture prey or defend themselves).
The Role of Genetics in Muscle Fiber Composition
Your genetic makeup plays a significant role in determining the proportion of slow-twitch versus fast-twitch fibers in your muscles. Some individuals are genetically predisposed to have a higher percentage of Type I fibers, making them naturally better suited for endurance sports like marathons or cycling. Others may have a predominance of Type II fibers, giving them an advantage in power-based sports like sprinting or weightlifting.
Heritability of Muscle Fiber Composition: Studies have shown that muscle fiber type distribution is largely inherited, with estimates suggesting that genetics account for up to 50-80% of an individual's fiber type composition. This means that while training can influence muscle adaptation and fiber recruitment, your genetic blueprint sets the foundation.
Gene Variants:
ACTN3 Gene: One of the most well-known genes linked to muscle performance is the ACTN3 gene, which codes for a protein found in fast-twitch muscle fibers. Variants of this gene can determine whether an individual is better suited for strength and power activities (fast-twitch dominant) or endurance activities (slow-twitch dominant). Individuals with a functional version of the ACTN3 gene (common among sprinters and strength athletes) may have an advantage in activities requiring explosive power. In contrast, those with a non-functional version may have greater endurance capabilities.
Training and Muscle Plasticity: While genetics set the stage, training also plays a key role. Training can lead to adaptations within muscle fibers, such as increasing mitochondrial density in Type I fibers or enhancing glycolytic capacity in Type II fibers. It's also possible to slightly shift fibers within their subtype. For example, training can promote a shift from Type IIx to Type IIa fibers, increasing their endurance capacity without compromising too much power.
Implications for Training
Understanding the science behind muscle fiber types and genetic predisposition helps explain why some individuals naturally excel in certain physical activities and why training programs must be tailored to individual needs. For example, an athlete with a higher proportion of Type I fibers may need to emphasize strength and power work to activate their fast-twitch fibers, while a Type II dominant athlete might benefit from endurance training to balance their explosive capabilities with sustained performance.
Mechanical Tension vs. Metabolic Stress: Two Paths to Muscle Growth
Building muscle relies on two main stimuli: mechanical tension and metabolic stress. Each is engaged differently depending on whether you’re doing higher reps or lower reps.
Mechanical Tension: When you lift heavier weights for fewer reps, you're creating more mechanical tension across your muscle fibers. This tension is a key factor in promoting hypertrophy, especially in Type II fibers. It’s what makes strength training such a powerful tool for building muscle. Think of low-rep, heavy-weight exercises like squats or deadlifts. These movements load your muscles with serious tension, stimulating growth through increased force.
Metabolic Stress: Higher rep ranges (with lighter weights) don’t necessarily create as much mechanical tension, but they do cause something called metabolic stress. You know that burning sensation you feel in your muscles toward the end of a high-rep set? That’s metabolic stress at work. As your body accumulates byproducts like lactate and hydrogen ions, your muscle cells experience increased stress, leading to growth over time.
Both approaches—mechanical tension and metabolic stress—are valid paths to muscle hypertrophy. However, they affect your muscles in different ways, which is why varying your rep ranges can lead to more balanced muscle development.
1. Mechanical Tension Workout (Low Reps, Heavy Weights)
Workout Example:
Exercise: Deadlift
Sets/Reps: 4 sets of 5 reps
Rest: 2–3 minutes between sets
Weight: Heavy enough to reach near failure by the fifth rep.
Exercise: Barbell Bench Press
Sets/Reps: 4 sets of 6 reps
Rest: 2–3 minutes
Weight: Heavy (aiming for 85%–90% of 1RM).
Exercise: Weighted Pull-Ups
Sets/Reps: 4 sets of 5 reps
Rest: 2–3 minutes
Weight: Add weight using a belt or vest to make 5 reps challenging.
Who Should Do This Workout:
Strength athletes (e.g., powerlifters, weightlifters) or anyone looking to increase raw strength and muscle density.
Advanced lifters who want to improve their Type II muscle fibers, which are more responsible for strength and power.
People focusing on big compound movements and looking to increase their 1RM on major lifts like squats, deadlifts, and presses.
This style of training places more mechanical tension on muscles, leading to greater recruitment of fast-twitch fibers, which contribute to building muscle size and strength. The heavier loads stimulate muscle growth through increased force production.
2. Metabolic Stress Workout (High Reps, Lighter Weights)
Workout Example:
Exercise: Dumbbell Goblet Squat
Sets/Reps: 4 sets of 15–20 reps
Rest: 45–60 seconds
Weight: Moderate (you should feel a burn in the last few reps).
Exercise: Push-Ups
Sets/Reps: 4 sets of 15–20 reps
Rest: 45–60 seconds
Weight: Bodyweight, adding a resistance band if needed.
Exercise: Dumbbell Bicep Curl
Sets/Reps: 4 sets of 12–15 reps
Rest: 45–60 seconds
Weight: Moderate (light enough for high reps but enough to induce fatigue).
Who Should Do This Workout:
Beginners or those new to resistance training who are building endurance and working up to heavier weights.
Bodybuilders or anyone focusing on muscle size and aesthetics (hypertrophy), as this approach helps create muscle fullness and the pump associated with metabolic stress.
Athletes in sports like boxing or swimming that require endurance and muscular stamina.
This workout induces metabolic stress, which leads to muscle swelling and builds endurance in the muscle tissue. The higher rep ranges with shorter rest periods increase the accumulation of metabolic byproducts like lactate, signaling muscle growth without the need for extremely heavy loads.
By alternating between both types of workouts, you can maximize muscle hypertrophy by targeting different mechanisms for growth—mechanical tension for strength and metabolic stress for endurance and size.
Strength vs. Hypertrophy: Matching Your Goals to Your Reps
Here’s where your goals come into play. If you’re focused on building strength, you’ll want to spend more time in the lower rep ranges (1-6 reps), where the emphasis is on lifting heavier weights. This rep range doesn’t just build muscle—it trains your nervous system to recruit more muscle fibers and improve your ability to generate force.
On the other hand, if your primary goal is hypertrophy (muscle growth), higher reps (8-12 or more) are your best friend. These reps build volume and tap into the metabolic stress mechanisms that help your muscles grow. Bodybuilders often use this rep range to maximize muscle size while maintaining a moderate weight.
Volume and Progressive Overload: The Key to Long-Term Gains
At the end of the day, whether you're lifting heavy for fewer reps or light for more reps, what matters most is volume and progressive overload.
Volume refers to the total amount of work you do during a workout, typically calculated as:
Volume=sets×reps×weight lifted
Volume is a critical factor in building muscle because it reflects the cumulative stress placed on the muscles over time. Whether you're lifting heavy weights for fewer reps or lighter weights for more reps, you can still achieve muscle growth as long as your total volume is sufficient.
Progressive Overload means continuously challenging your muscles over time by gradually increasing the weight, reps, or sets. This principle is the cornerstone of muscle growth—whether you're lifting for strength or size.
Progressive overload is the principle of continuously increasing the demands on your muscles to keep them growing. Without progressively challenging your muscles, they will adapt to the workload, and growth will plateau.
Progressive overload can be achieved by:
Increasing the weight lifted.
Adding more reps to each set.
Increasing the number of sets performed.
Shortening rest times, which increases the intensity.
The Difference in Look: Lifting Heavy vs. Lifting Lighter with More Volume
Lifting Heavy: Strength and Dense Muscle
When you lift heavy weights (typically 3-6 reps), you primarily target fast-twitch muscle fibers. These fibers are larger and more capable of producing force, which leads to strength gains and muscle density. Here’s the typical look you achieve from heavy lifting:
Denser, harder muscle appearance: Heavier lifting builds thicker, stronger muscles, which can give you a more compact, dense look.
Lower body fat levels: Heavy lifting often requires more effort in fewer reps, which can lead to higher calorie expenditure. Over time, this contributes to a leaner, more defined physique.
Strength and power focus: Lifting heavy weights maximizes mechanical tension, which leads to greater strength gains. This can give your muscles a powerful, sculpted look without necessarily achieving extreme size.
However, with heavy weights and low reps, overall volume tends to be lower unless you perform many sets.
Lifting Lighter with More Volume: Size and Muscle Endurance
When you lift lighter weights for more reps (8-20+ reps), you focus more on increasing muscle volume through metabolic stress and time under tension. The look you can achieve from lifting lighter with more volume is slightly different:
Fuller muscle appearance: Higher volume training tends to increase the overall size of your muscles (hypertrophy), particularly because it recruits more slow-twitch fibers and enhances the pump effect.
More sarcoplasmic hypertrophy: Lifting with higher reps can increase the fluid within your muscle cells (sarcoplasm), leading to fuller, rounder-looking muscles.
Muscle endurance: Lifting lighter weights builds endurance, helping your muscles sustain contractions for longer periods. This doesn't contribute as much to raw strength but does help with overall muscle size.
Summary: Lifting Heavy vs. Lighter with Volume
Lifting Heavy (Low Reps): Builds dense, compact muscles and strength. It leads to a harder, more defined look, with a focus on fast-twitch muscle fibers and mechanical tension.
Lifting Lighter with Volume (High Reps): Builds fuller, larger muscles with a focus on muscle endurance and sarcoplasmic hypertrophy. This leads to a rounder, more pumped look due to metabolic stress and time under tension.
Ultimately, both approaches can build muscle effectively. A combination of heavy and lighter lifting, incorporating progressive overload and sufficient total volume, leads to well-rounded hypertrophy and strength gains.
The Bottom Line: Mix It Up for Best Results
The science shows that both higher reps and lower reps have unique benefits when it comes to muscle building. Lower reps are best for developing strength and power, while higher reps are great for building endurance and muscle size through metabolic stress.
For a well-rounded approach to muscle growth, it’s best to incorporate both rep ranges into your training program. A mix of heavy, low-rep sets and lighter, high-rep sets will target both fast-twitch and slow-twitch fibers, allowing you to build strength, endurance, and size all at once.
Hope that helps!
Happy Exercising,
Robyn