Exercise library · simple → complex
Levers involved
Educational approximation · static, gravity-only
Why curls feel hard mid-range (lever terms)
The elbow is a 3rd-class lever: the flexors attach only ~3–5 cm from the joint while the dumbbell sits ~27 cm away. That's a mechanical disadvantage (MA ≈ 0.1) — the muscle must pull ~8–12× the load's weight. At ~90° gravity acts perpendicular to the forearm so the load arm is longest; the muscle's own effort arm also peaks there but cannot grow as fast. Net: the biggest force requirement lands mid-range — the sticking point. The trade-off: small muscle shortening produces big, fast hand movement (speed & range advantage).
Grip (forearm position)
Palm up — biceps share highest. Educational approximation.
Convention: 0° = full extension (arm straight), 150° = full flexion. Upper arm held vertical.
Strength curve · torque vs angle
Model (educational approximation): τ = (mDB·g·Lhand + mfa·g·Lcom)·sin θ · peaks near 90°. Cursor follows scrubber. Ignores acceleration & wrist position.
Muscles at this angle
Educational work-share model · responds to angle + grip
Movers · % of work
Stabilizers & antagonists · effort %
Coach notes
Physiology
Concentric vs eccentric
Concentric (lifting, angle ↑): flexors shorten while producing force. Eccentric (lowering, angle ↓): flexors lengthen under load — you can resist ~20–40% more load eccentrically, and it drives more soreness (DOMS) and strong hypertrophy stimulus. Control the lowering: 2–3 s.
Why mid-range feels hardest
Gravity pulls the dumbbell straight down. Torque at the elbow = weight × horizontal distance from elbow to the load. With the upper arm vertical that distance is largest when the forearm is horizontal (~90°). At the bottom (0°) and top (~150°) the load sits almost under/over the elbow, so torque is small. The "sticking point" is near 80–100°.
Length–tension relationship
Sarcomeres generate most active force at an intermediate length where actin–myosin overlap is optimal. Biceps is long (stretched) near full extension and very short near full flexion — near the top it is weaker, but the external torque is also low. Mid-range: good overlap and best muscle moment arm, which is why you can lift there despite peak demand.
Grip changes the share
Supinated (palm up) maximizes biceps contribution because biceps is also a powerful supinator. Hammer (neutral) shifts more work to brachioradialis. Pronated (palm down) reduces biceps leverage and raises brachialis / brachioradialis share. Numbers here are educational approximations, not EMG truth.
Joint actions
Elbow: flexion (concentric) / extension (eccentric) in the sagittal plane about a medio-lateral axis. Radio-ulnar: grip sets the forearm position. Shoulder: keep static — letting the elbow drift forward shortens the moment arm and steals work from the flexors.
Coaching cues
Elbows pinned at your sides · full extension at the bottom (train the stretched position) · squeeze at the top · no torso swing · try incline curls (more stretch) or preacher curls (shifts peak torque lower) to change the resistance profile.