Muscle contraction, energy and forces
From muscle to sarcomere, the sliding filament model, calcium and ATP, the ATP sources and fatigue, and why the biceps force is many times the load. With a fatigue and recovery investigation.
Read
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.[3]
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.[3][2]
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.[4]
A motor neuron releases acetylcholine, starting an action potential in the fibre. This releases Ca²⁺ from the sarcoplasmic reticulum; Ca²⁺ binds troponin, which moves tropomyosin off the binding sites on actin. Each cross-bridge cycle needs ATP: without it the myosin head cannot detach. To relax, ATP-driven pumps return Ca²⁺ to the sarcoplasmic reticulum.[4]
Stored ATP lasts only a few seconds of contraction. Creatine phosphate gives about 15 s more, glycolysis about 1 minute, and aerobic respiration is the most efficient but needs a steady oxygen supply. Without enough oxygen, lactic acid forms, which may contribute to fatigue.[4]
Muscles can only contract, so they act in antagonistic pairs (biceps flexes the elbow, triceps extends it). Because muscles attach close to joints, their mechanical advantage is much less than one: with the biceps 4.00 cm from the elbow, a 2.50 kg forearm (centre of gravity at 16.0 cm) and a 4.00 kg book at 38.0 cm, the biceps force is 470 N, 7.38 times the 63.7 N weight supported.[6]
In return, a small shortening of the muscle gives a large, fast movement of the hand and a wide range of motion. At rest, skeletal muscle uses about 15 W, 18% of a basal metabolic rate of 85 W.[6][7]
Try it: Fatigue and recovery: does arm position matter?
You need
- A spring clothes peg (or a hand-grip trainer) per pair
- Stopwatch
- Results table and graph paper
Safety
Stop at once if there is pain, tingling or dizziness. Holding an arm up is tiring: lower it if it becomes uncomfortable. Pupils with a hand, arm or shoulder condition act as timer and recorder. Use ordinary pegs or a light grip trainer.
What to do
- 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.
- Condition A: arm relaxed by your side. Count peg squeezes in six consecutive 20-second intervals with no rest. Record each count.
- Rest for at least 5 minutes, then do condition B with the same hand, holding the arm straight up above your head.
- 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.
- 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.
What happens, and why?
The count usually drops as the muscles fatigue: they can no longer contract as fully in response to nerve signals. Stored ATP and creatine phosphate last only seconds, so sustained work depends on glycolysis and on aerobic respiration, which needs a steady oxygen supply; a lack of oxygen leads to lactic acid, which may contribute to fatigue. Whether the arm-up condition really changes this is what your data must show; motivation, technique and order effects can be as large as the effect you are looking for.[4]
Play: Contraction or relaxation?
Each card is one step in a muscle fibre. Does it belong to contraction or to relaxation?
Your best score is kept only on this device. Nothing is sent anywhere.
Quiz
Other ages: Ages 6–7 · Ages 10–11