Question

When a person performs a bicep curl, several forms of energy are at play. Chemical energy is converted into mechanical energy. The chemical energy comes from the food we eat, which fuels the muscle contractions. Mechanical energy is the energy of motion, which is what we see when the forearm moves up and down.

Several factors affect the number of reps a person can do. These include the weight being lifted, the person's muscle strength and endurance, the person's overall fitness level, and the person's technique.

Answer 1

Your paragraph is basically correct, but I’ll tighten up a few points and add important details.

Short corrected explanation
- The chemical energy that directly powers contractions is ATP (adenosine triphosphate). ATP is produced from the food you eat via cellular pathways (ATP‑PC/creatine phosphate, glycolysis, and oxidative phosphorylation).
- Muscles convert chemical energy (ATP) into mechanical work (moving the forearm) and heat. Mechanical energy during a curl appears mainly as kinetic energy (movement) and gravitational potential energy (raising the weight). During the lowering phase (eccentric), muscles absorb energy; tendons can store/release elastic energy. Much of the metabolic energy becomes heat — muscular efficiency is fairly low (on the order of ~20–25% or less for many movements).
- Different energy systems contribute depending on intensity and duration: ATP stored in muscle lasts a few seconds, creatine phosphate supplies up to ~10–15 s of high power, anaerobic glycolysis supplies energy for tens of seconds to a couple of minutes (producing lactate), and oxidative phosphorylation supplies longer-duration, lower‑power work.

Factors that affect how many reps someone can do
- Load relative to the person’s 1RM (one‑rep max): heavier loads → fewer reps.
- Muscle strength and cross‑sectional area: bigger/stronger muscles produce more force.
- Muscle fiber composition: Type I (slow) fibers resist fatigue and support higher reps; Type II (fast) fibers generate high force but fatigue faster.
- Local muscular endurance and metabolic capacity (glycogen stores, mitochondrial density, buffering capacity).
- Neuromuscular factors: motor unit recruitment and firing rates, coordination.
- Technique and range of motion: poor technique or a longer ROM can reduce reps; better technique can allow more efficient force production.
- Biomechanics (lever arms, joint angles): a longer moment arm of the weight about the elbow increases torque demand, reducing reps for the same mass.
- Contraction type and tempo: eccentric contractions are mechanically efficient and can handle higher loads; slow tempo increases time under tension and tends to reduce max reps.
- Rest between sets, prior fatigue, and overall training status.
- Nutrition and hydration (glycogen, electrolytes), sleep, and recovery.
- Cardiovascular fitness (affects recovery between reps/sets and clearing metabolites).
- Age, sex, hormonal status, motivation, and pain/comfort.

Simple numerical illustration
- Work per concentric rep (approx): lifting a 10 kg dumbbell 0.4 m → W = m·g·h ≈ 10·9.81·0.4 ≈ 39 J per rep. Ten reps ≈ 390 J of mechanical work. Because muscles are not fully efficient, the body expends several times that amount of metabolic energy to do the work and produce heat.

If you want, I can:
- Explain the three energy systems in more detail, or
- Show how to estimate reps from load (e.g., % of 1RM guidelines), or
- Compare concentric vs eccentric energy costs. Which would you like?