Muscles and levers.

Step 1 of 6

Muscles only pull

A muscle can do one thing: shorten, and pull on whatever its ends are tied to. Muscles are tied to bones by tendons, and each one crosses a joint. To put a joint back you need a second muscle on the other side, so muscles work in antagonistic pairs. At the elbow the biceps, at the front of the upper arm, bends it, and the triceps, behind, straightens it. The working muscle glows.

Step 2 of 6

A losing lever

The forearm is a lever, and the elbow is its pivot. What turns a lever is the moment of a force: the force times its distance from the pivot. When the forearm stays still, the moments on each side of the elbow balance:

$F_{\text{muscle}} \times d_{\text{muscle}} = F_{\text{load}} \times d_{\text{load}}$$F_{\text{muscle}} \times d_{\text{muscle}} =$$F_{\text{load}} \times d_{\text{load}}$⁠

With the Elbow angle at a right angle, the kettlebell hangs 33 cm from the elbow, and the biceps tendon joins the bone only 4.53 cm from it. The forearm itself, about 1.6 kg, has weight too: it adds a moment of 2.35 N·m.

Step 3 of 6

Why so close?

Every lever trades force for movement. A biceps pulling eight times the weight sounds like poor design; think what is gained in return.

Step 4 of 6

The limit

A muscle can pull only so hard, and the chart shows that limit as the blue line; the amber line is the pull the load needs. As they come together, almost every fibre in the muscle is pulling, and the arm starts to shake.

Step 5 of 6

Sliding filaments

Zoom into the muscle is at Filaments: one sarcomere, the repeating unit of a muscle fibre, from one Z disc to the next. Thin filaments of actin are anchored in the discs, and thick filaments of myosin sit in the middle. Myosin heads grab the actin, swing and drag it towards the middle, and let go; each cycle uses one molecule of ATP. Drag Elbow angle from 45° to 160°, nearly straight, and watch the filaments’ length.

Step 6 of 6

Strongest in the middle

How hard a muscle can pull depends on how much its filaments overlap: only a myosin head that reaches actin can pull. Load is locked at 16 kg, and the biceps only just holds it with the elbow at a right angle, 90°.