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A skeletal muscle is wrapped in epimysium; inside, bundles of fibres (fascicles) are wrapped in perimysium and each fibre in endomysium. A muscle fibre is one long cylindrical cell, up to 100 μm across, with many nuclei, formed when many myoblasts fuse.

The functional unit is the sarcomere, from one Z-disc to the next. Its regular thin (actin) and thick (myosin) filaments make skeletal and cardiac muscle look striped (striated); smooth muscle is not striated.

Sliding filament model: myosin heads pull the actin filaments past the myosin filaments. The Z lines move closer together and the I band gets narrower, while the A band stays the same width: the sarcomere shortens, not the filaments.

Experiment: Fatigue and recovery: does arm position matter?

1. Hypothesis: if muscles rely on a steady oxygen supply for long work, holding the hand above the head (where blood has to be pumped uphill) may make fatigue come sooner. Write your own prediction.
2. Condition A: arm relaxed by your side. Count peg squeezes in six consecutive 20-second intervals with no rest. Record each count.
3. Rest for at least 5 minutes, then do condition B with the same hand, holding the arm straight up above your head.
4. Control variables: same peg, hand, fingers, interval length, counter; decide in advance what counts as one full squeeze. Randomise which pupils do A or B first.
5. Plot squeezes per interval against time for both conditions. Calculate the percentage drop from interval 1 to interval 6. Pool class data, find the mean and range, and judge whether the difference is bigger than the spread.

Interval (20 s)	Squeezes: arm by side	Squeezes: arm above head
1	Predict the percentage drop from interval 1 to 6 for each condition.
2	Predict the percentage drop from interval 1 to 6 for each condition.
3	Predict the percentage drop from interval 1 to 6 for each condition.
4	Predict the percentage drop from interval 1 to 6 for each condition.
5	Predict the percentage drop from interval 1 to 6 for each condition.
6	Predict the percentage drop from interval 1 to 6 for each condition.

Questions

1. Describe the sliding filament model, including what happens to the A band, the I band and the Z lines.

2. Put these in order and explain each: acetylcholine, action potential, Ca²⁺ release, troponin, cross-bridge cycling.

3. Using the textbook values (biceps at 4.00 cm; forearm 2.50 kg at 16.0 cm; book 4.00 kg at 38.0 cm; g = 9.80 N/kg), show that the biceps force is about 470 N.

4. From your fatigue data, state the percentage drop for each condition and say whether the difference is larger than the class range. Give two sources of uncertainty.

Sources for every fact on this sheet: echoesof.science/school/muscles/sources/

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