Built to Last: Why Muscle Matters More Than You Think

When we talk about muscle in the human body, we’re referring to three main types: skeletal, cardiac, and smooth muscle. Cardiac muscle is found only in the heart and contracts involuntarily to pump blood. Smooth muscle lines the walls of organs like the intestines and blood vessels, also working involuntarily.

Skeletal muscle, on the other hand, is the only type we can voluntarily control, and it’s the one most associated with movement, strength, and physical appearance. But beyond aesthetics or performance, skeletal muscle plays a vital role in metabolic health, mobility, and longevity.

Research has consistently shown that maintaining and building skeletal muscle improves health outcomes. Low muscle mass is associated with increased risk of falls, frailty, insulin resistance, and early mortality. A 2014 study published in The American Journal of Medicine found that muscle mass index was a better predictor of longevity than body mass index (BMI) in older adults (Srikanthan & Karlamangla, 2014). Skeletal muscle serves as a metabolic reservoir for glucose and amino acids and helps regulate inflammation, immunity, and even brain health. It's not just about strength—skeletal muscle is an organ of longevity.

It’s important to distinguish muscle from strength. Muscle refers to the size and mass of the tissue, while strength is the ability to produce force. You can gain strength without much increase in muscle size—especially in beginners—thanks to improved neuromuscular coordination. Conversely, hypertrophy training increases the size of muscle fibers but doesn’t always lead to maximal strength gains. While they often go hand in hand, the pathways to building strength and muscle diverge, especially as training becomes more advanced.

When it comes to muscle growth (hypertrophy), current understanding suggests there are three primary mechanisms: mechanical tension, muscle damage, and metabolic stress.

• Mechanical tension is created when you lift progressively heavier loads through a full range of motion. It’s the most direct and essential trigger for hypertrophy.

• Muscle damage occurs when the muscle fibers are strained under load, especially during eccentric contractions. This damage signals the body to repair and reinforce the tissue.

• Metabolic stress, often associated with the “pump” you feel during high-rep sets, results from the buildup of lactate, hydrogen ions, and cellular swelling, which also contributes to muscle signaling pathways.

Of these, mechanical tension appears to be the most consistently powerful driver of hypertrophy, especially when applied progressively over time (Schoenfeld, 2010).

This is where resistance training shines. No other form of exercise applies mechanical tension to the muscles with as much control, consistency, and progressive load. Whether it’s body-weight, dumbbells, barbells, machines, or resistance bands, resistance training allows you to apply the three key hypertrophy mechanisms in a measurable and adjustable way.

To optimize muscle growth, you need to consider training volume, intensity, frequency, and load (sets and reps).

Volume—typically measured as total sets × reps × weight—is the most important variable for hypertrophy (Schoenfeld et al., 2016). You can increase volume by training more frequently, adding more sets, or lifting heavier weights. But volume must be balanced with recovery and managed progressively.

Frequency (how often you train a muscle group) spreads volume across the week. Training a muscle 2-3 times per week appears more effective than once weekly, especially for natural lifters.

Intensity (percentage of 1-rep max) affects how heavy you go, while reps determine the fatigue and metabolic stress side of the equation. Most hypertrophy happens in the 6-20 rep range, assuming sets are taken close to failure.

Not all muscle-building workouts need to look the same. One common misconception is that you must lift very heavy weights for low reps to build muscle, or that high-rep training with light weights is ineffective. In reality, both approaches can stimulate hypertrophy—as long as you train with sufficient volume and intensity.

You can achieve similar muscle-building outcomes by doing fewer reps with heavier weights (e.g., 4–6 reps at 80–90% of your 1-rep max) or more reps with lighter weights (e.g., 12–20+ reps at 40–65% of your 1-rep max)—if you train close enough to failure. Studies have shown that as long as you're pushing the muscle hard and accumulating enough overall volume, both methods can work effectively for hypertrophy (Schoenfeld et al., 2015). What matters most is the effort and proximity to muscular failure, not just the weight on the bar.

This is where the concept of "Reps in Reserve" (RIR) becomes useful. RIR is a way to gauge how close you are to muscular failure during a set.

• An RIR of 0 means you reached failure—you couldn't do another rep with good form.

• An RIR of 1 means you likely had one good rep left in the tank.

• An RIR of 2 means you could have done two more reps, and so on.

For optimal muscle growth, most research suggests staying within 0–3 RIR, depending on the movement and your training goals.

• For compound lifts like squats or deadlifts, training to 1–3 RIR is often ideal to minimize injury risk while still providing enough stimulus.

• For isolation exercises or machine-based movements (like curls or leg extensions), going closer to failure (0–1 RIR) is safer and may be more effective at fully fatiguing the target muscle.

  
  

Training to complete failure every set isn’t necessary and may actually increase recovery time without providing extra benefit. Instead, training close to failure ensures you're hitting the intensity needed for growth while allowing consistent performance and progress over time.

Training Style	Rep Range	Load (% 1RM)	Recommended RIR	Effort Description	Best For
Heavy Load (Low Reps)	4–6 reps	80–90%	1–3	Leave 1–3 reps in the tank; focus on form and control	Strength-focused hypertrophy, compound lifts
Moderate Load (Moderate Reps)	6–12 reps	65–80%	1–2	Push close to failure, typically 1–2 reps left	Balanced muscle growth, general hypertrophy training
Light Load (High Reps)	12–20+ reps	40–65%	0–1	Train very close to failure; may occasionally reach failure safely	Metabolic stress, isolation exercises, limited equipment

By manipulating reps, load, and effort (RIR), you can adjust your training to fit your equipment, preferences, and recovery ability—without sacrificing muscle-building potential. Whether you're lifting heavy for 5 reps or lighter for 15, what matters is that you're applying enough mechanical tension, accumulating enough volume, and getting close enough to muscular failure.

In summary, skeletal muscle isn’t just for athletes or bodybuilders—it’s a key driver of healthspan and quality of life. Resistance training remains the most effective method for building muscle, primarily through applying mechanical tension at sufficient volume. And while the pump may feel good and soreness might signal you worked hard, consistent progressive overload and adequate volume are what really grow muscle and support a healthier, longer life.