Teacher sheet · Ages 15–16 · about 45–50 minutes
Muscle contraction, energy and forces
Lesson page: echoesof.science/school/muscles/ages-15-16/ · Pupil worksheet: echoesof.science/school/muscles/ages-15-16/print/
Learning aims
- Describe the structure of skeletal muscle down to the sarcomere.
- Explain the sliding filament model and the roles of Ca²⁺, troponin, tropomyosin and ATP.
- Compare the ATP sources and link them to fatigue.
- Apply moments to the biceps and evaluate a fatigue investigation with uncertainty.
Lesson plan
| Time | Activity |
|---|---|
| 5 min | Starter: hold a book with your arm bent; who is pulling harder, your biceps or the book? |
| 10 min | Read the text; draw a relaxed and a contracted sarcomere. |
| 20 min | Fatigue and recovery investigation. |
| 5 min | Worked example: 470 N. |
| 5 min | Quiz or contraction/relaxation game. |
Materials for the experiment
- 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.
Common misconceptions
| Pupils may think… | Actually… |
|---|---|
| The filaments themselves get shorter. | The filaments slide past each other; the A band stays the same width and the sarcomere shortens. |
| ATP is only needed to make the muscle pull. | ATP is also needed for myosin to detach from actin and to pump Ca²⁺ back for relaxation. |
| Lactic acid is the proven single cause of fatigue. | The textbook says lactic acid may contribute; fatigue has several possible causes. |
Worksheet answers
- Myosin heads pull actin past myosin; Z lines move closer, the I band narrows, the A band stays the same width; the sarcomere shortens.
- Acetylcholine from the motor neuron starts an action potential; this releases Ca²⁺ from the sarcoplasmic reticulum; Ca²⁺ binds troponin, moving tropomyosin off actin; myosin heads attach, pull and detach using ATP (cross-bridge cycling).
- Weights: 2.50 × 9.80 = 24.5 N and 4.00 × 9.80 = 39.2 N. F × 0.0400 = 24.5 × 0.160 + 39.2 × 0.380 = 3.92 + 14.9 = 18.8 N m, so F ≈ 470 N.
- Answers depend on data; uncertainties include effort and motivation, judging a full squeeze, counting errors, timing, order effects and different pupils.
Quiz answers
- When a sarcomere contracts, which band stays the same width? The A band. The A band (myosin length) stays the same; the I band narrows.[4]
- What does Ca²⁺ bind to at the start of contraction? Troponin. Ca²⁺ binds troponin, moving tropomyosin off actin's binding sites.[4]
- Why is ATP needed for a myosin head to let go of actin? Without ATP the myosin head cannot detach. ATP binding lets the head detach and re-cock.[4]
- About how long can creatine phosphate supply energy? About 15 seconds. Creatine phosphate gives roughly 15 s of contraction.[4]
- A 30 N load is held 30 cm from the elbow; the muscle attaches 5 cm from the elbow (forearm weight ignored). Muscle force? 180 N. F = 30 × 30 / 5 = 180 N.[6]
- Skeletal muscle uses 15 W of an 85 W basal metabolic rate. What percentage is that? About 18%. 15 / 85 ≈ 0.18 = 18%.[7]
Going further
The joint force in the textbook example is 407 N. Use the sum of vertical forces to show where this number comes from.
Sources used in this lesson
- [2] Anatomy and Physiology 2e, 10.1 Overview of Muscle Tissues (OpenStax, Rice University): reliability High
- [3] Anatomy and Physiology 2e, 10.2 Skeletal Muscle (OpenStax, Rice University): reliability High
- [4] Anatomy and Physiology 2e, 10.3 Muscle Fiber Contraction and Relaxation (OpenStax, Rice University): reliability High
- [6] College Physics 2e, 9.6 Forces and Torques in Muscles and Joints (OpenStax, Rice University): reliability High
- [7] College Physics 2e, 7.8 Work, Energy, and Power in Humans (OpenStax, Rice University): reliability High
Full list with links and a note on each source's reliability: echoesof.science/school/muscles/sources/
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