Every fact in the three muscle lessons is listed here with where it comes from. The numbers in [brackets] on the lesson pages link to this list.
How we rate sources
High: space agencies (NASA), peer-reviewed open textbooks (OpenStax, Rice University) and professional scientific societies (American Physical Society).
Medium-high: reputable science news reporting directly on a primary document.
Medium: single-author essays and popular history sites. We only use these for history details, and never on their own: each one is paired with a second source.
Wikipedia was not used as a source. Numbers are rounded (for example 9.8 m/s² for Earth's gravity). The text was drafted with help from AI (Claude) and then checked against these sources. Corrections: hello@echoesof.science.
Simplifications and care points: this is general science education, not medical or training advice. Muscle fatigue has several possible causes and is still studied; the textbook names falling ATP and lactic acid lowering the pH as possible contributors, not as the whole story. The times for the ATP sources (a few seconds, about 15 seconds, about 1 minute) are textbook approximations. The forearm calculation uses the textbook's simplified lever model. The clothes-peg tests measure how many squeezes a pupil manages, which also depends on effort and motivation, so results vary a lot between people and between tries. Pupils with an injury or a condition that affects their muscles or joints can time, count or record instead; questions about a pupil's own muscles go to a parent, the school nurse or a doctor.
Type: Children's health education site of a paediatric health system, medically reviewed
There are three kinds of muscle: skeletal muscles (attached to bone by cord-like tendons, in the legs, arms and face), smooth muscles (in the walls of the stomach, intestines and blood vessels) and cardiac muscle (in the walls of the heart). Skeletal muscles are voluntary, because you control them; smooth and cardiac muscles are involuntary and are controlled automatically by the nervous system. Muscles can pull bones but cannot push them back, so they work in pairs: flexors bend a limb at a joint and extensors straighten it; the biceps at the front of the upper arm is a flexor and the triceps at the back an extensor. Muscles let us smile and talk; exercise helps us stay fit and healthy.
[2] Anatomy and Physiology 2e, 10.1 Overview of Muscle Tissues
OpenStax, Rice University · Reliability: High
Type: Peer-reviewed open textbook
The body has three types of muscle tissue: skeletal and cardiac muscle are striated, smooth muscle is not. Skeletal muscle depends on signals from the nervous system; cardiac muscle in the heart is connected so the heart contracts as one unit; smooth muscle is found in artery walls, skin, organs and internal passageways. Muscle is excitable (it carries an action potential along its membrane), contractile (it pulls on its attachment points and shortens with force), elastic (it recoils to its original length) and extensible (it can stretch).
[3] Anatomy and Physiology 2e, 10.2 Skeletal Muscle
OpenStax, Rice University · Reliability: High
Type: Peer-reviewed open textbook
Skeletal muscles produce movement, keep posture against gravity, stabilise joints, control openings of internal tracts, protect abdominal and pelvic organs, and produce heat (for example during exercise or shivering). Each muscle fibre is a long cylindrical cell up to 100 μm across with many nuclei, formed when many embryonic myoblasts fuse. Connective tissue layers: epimysium around the whole muscle, perimysium around bundles of fibres (fascicles), endomysium around each fibre. The sarcomere, from one Z-disc to the next, is the functional unit; the orderly actin and myosin give the striped (striated) look.
[4] Anatomy and Physiology 2e, 10.3 Muscle Fiber Contraction and Relaxation
OpenStax, Rice University · Reliability: High
Type: Peer-reviewed open textbook
A motor neuron releases acetylcholine, which starts an action potential in the muscle fibre; this releases Ca²⁺ from the sarcoplasmic reticulum; Ca²⁺ binds troponin, which moves tropomyosin off the binding sites on actin. Myosin heads attach, pull the thin actin filaments past the thick myosin filaments, detach and re-cock, powered by ATP; the Z lines move closer, the I band gets smaller and the A band stays the same width. To relax, acetylcholine release stops and ATP-driven pumps return Ca²⁺ to the sarcoplasmic reticulum. Stored ATP lasts only a few seconds of contraction; creatine phosphate gives about 15 seconds; glycolysis about 1 minute; aerobic respiration is the most efficient but needs a steady supply of oxygen. Without enough oxygen lactic acid forms, which may contribute to fatigue. Fatigue is when a muscle can no longer contract in response to nerve signals.
[5] Anatomy and Physiology 2e, 11.1 Interactions of Skeletal Muscles, Their Fascicle Arrangement, and Their Lever Systems
OpenStax, Rice University · Reliability: High
Type: Peer-reviewed open textbook
Muscles pull on bones to change the angle at a joint. Tendons are strong bands of dense connective tissue that connect muscles to bones. The insertion is the movable end attached to the bone being pulled; the origin is attached to a fixed bone. The prime mover (agonist) is the main muscle for a movement, for example the biceps when lifting a cup; the triceps, which extends the forearm, is its antagonist. Bones act as levers and joints as fulcrums.
[6] College Physics 2e, 9.6 Forces and Torques in Muscles and Joints
OpenStax, Rice University · Reliability: High
Type: Peer-reviewed open textbook
Muscles can only contract, so they occur in pairs: the biceps flexes the elbow and the triceps extends it. Example 9.4: biceps attached 4.00 cm from the elbow, a 2.50 kg forearm at 16.0 cm and a 4.00 kg book at 38.0 cm give a biceps force of 470 N, 7.38 times the 63.7 N weight supported; the elbow joint force is 407 N. Most muscles attach close to joints, so their mechanical advantage is much less than one; in return a small contraction gives a large, fast movement and a wide range of motion.
[7] College Physics 2e, 7.8 Work, Energy, and Power in Humans
OpenStax, Rice University · Reliability: High
Type: Peer-reviewed open textbook
The basal metabolic rate (BMR) is the total energy conversion rate of a person at rest; in the textbook table it is 85 W, of which skeletal muscle uses 15 W (18%) and the brain 16 W (19%). Climbing stairs at 116 steps per minute uses about 685 W (76 kg male).