Muscles: what pulls, what it is made of, and what the other side is doing

The body contains three muscle tissues, and it is worth two lines to separate them before spending the lesson on one.

• Cardiac muscle — found only in the heart. Involuntary, striated, and self-exciting: it generates its own contraction rhythm. Lesson 4 returns to it.
• Smooth muscle — in the walls of blood vessels, the airways and the digestive tract. Involuntary and non-striated. Lessons 5 and 6 both depend on it.
• Skeletal muscle — attached to bone by tendons. Voluntary and striated, and roughly 40 per cent of body mass. This is the muscle of movement and the subject of the rest of this lesson.

Now the constraint that shapes everything else. A muscle can only pull. It cannot push. When it is stimulated it shortens, drawing its insertion towards its origin; when it stops, nothing returns it to length except something else pulling the bone back. This single fact has two large consequences.

• Muscles must work in pairs. If the biceps pulls the forearm up, only the triceps can pull it back down. Every joint has opposing muscle groups, and neither can do the other's job.
• Force is always transmitted to a lever. The muscle's pull reaches the bone through a tendon, and the bone rotates about the joint. That is exactly the lever system Lesson 1 described — and the reason the dot point asks about the two systems together rather than separately. Neither system produces movement alone. Bones cannot generate force; muscles cannot generate movement without something rigid to pull on.

Learn each muscle with the joint action it produces, never as a name on a diagram. A name alone answers no question in this course; a name plus an action answers most of them.

Two group names are worth expanding, because questions use them. The quadriceps are four muscles on the front of the thigh — rectus femoris, vastus lateralis, vastus medialis and vastus intermedius. The hamstrings are three on the back — biceps femoris, semitendinosus and semimembranosus. Notice that the hamstrings appear twice in the action column: they cross both the hip and the knee, which is precisely why they are the muscle group most often strained in sprinting.

Muscles are not uniform. Every muscle contains a mixture of fibre types, and the proportion differs between people and between muscles in the same person. The syllabus names the comparison directly: slow versus fast twitch muscle fibres.

Four things about this table are examinable beyond the table itself:

• The characteristics are not a random list — they follow from the energy system. Slow twitch fibres work aerobically, which requires oxygen, which requires capillaries to deliver it, mitochondria to use it and myoglobin to store it. That is why those three rows move together, and saying so is worth more than reciting them.
• Postural muscles are predominantly slow twitch. The soleus and the erector spinae hold you upright all day at low force. That is exactly the slow twitch job description.
• Fibre proportion is largely genetically determined, which is a real and uncomfortable constraint on who can excel at which event. Elite sprinters and elite marathon runners have measurably different fibre profiles.
• Training does not convert slow to fast. What training does is shift fibres within the fast twitch category — IIx behaving more like IIa with endurance work — and improve the qualities of whatever fibres you have: more mitochondria and capillaries with aerobic training, greater size and force with resistance training. Lesson 12 develops this.

The fibres are recruited in order of size: light effort calls up slow twitch fibres first, and fast twitch fibres are added only as the force required increases. This is why a jog and a sprint feel like different activities using the same legs — they are, in the sense that they are using different fibres.

The syllabus names three: isotonic concentric, isotonic eccentric and isometric. Isotonic means the muscle changes length and movement happens; isometric means it does not.

• Isotonic concentric. The muscle shortens under tension, because the force it generates exceeds the resistance. This is the phase people think of as "the work": the upward phase of a biceps curl, the push-up on the way up, the drive phase of a sprint.
• Isotonic eccentric. The muscle lengthens while still under tension, because the resistance exceeds the force generated. The muscle is not switching off — it is braking. Lowering the curl, descending into a squat, and the landing after a jump are all eccentric.
• Isometric. The muscle develops tension but does not change length, so the joint angle does not change and no movement occurs. A plank, a wall sit, holding a scrum, or gripping a bar at the top of a hold.

Three further facts about eccentric contractions, each of which regularly appears in questions:

• Eccentric contractions produce the greatest force of the three types. A muscle can lower more weight than it can lift.
• They use the least energy for a given load, which is why walking downhill feels easier than walking up.
• They cause the most muscle damage, and are the principal cause of delayed onset muscle soreness — the stiffness that peaks a day or two after unfamiliar exercise, and which is markedly worse after downhill running or a heavy lowering session.

Because a muscle can only pull, movement requires muscles to work in coordinated relationships. The syllabus names three roles, and the crucial point is that these are roles, not muscles — the same muscle is the agonist in one movement and the antagonist in another.

• Agonist (prime mover). The muscle chiefly responsible for producing the movement. It contracts.
• Antagonist. The muscle on the opposite side of the joint, producing the opposite action. It relaxes and lengthens in a controlled way to let the movement happen. If it did not, the two would fight each other and the limb would not move. This coordinated relaxation is called reciprocal inhibition.
• Stabiliser (fixator). A muscle that contracts, usually isometrically, to hold a bone or joint still so the agonist has a firm base to pull from. Without stabilisers a muscle's contraction would move the wrong end of itself.

One refinement that repeatedly separates good answers from adequate ones. Lowering a weight does not swap the agonist. Curling a weight up makes the biceps the agonist working concentrically; lowering it slowly leaves the biceps as the working muscle, now contracting eccentrically, with the triceps still the antagonist. The roles are assigned by which muscle is controlling the movement, not by which direction the joint happens to be travelling.

Take the skill you analysed in Lesson 1 — you already have its joints and joint actions. Now add the muscular half, and you will have a complete movement analysis, which is the standard tool this dot point is asking you to be able to build.

A coach tells a 15-year-old that she should stop training for sprints because she "does not have the fibre type for it". Your task is to evaluate that claim properly, which means neither accepting it nor dismissing it.

• What is true in it? Write down what the evidence actually supports about fibre proportion, heritability, and elite performance at the extremes of an event range.
• What does it overstate? Consider what training genuinely changes: fibre qualities, shifts within Type II, recruitment, technique, and everything that is not fibre type at all.
• What would you need to know to make the claim at all — and is that measurement realistically available to a school athlete? Consider what determining fibre proportion actually requires.
• The Module 1 question. What is the effect on participation of telling a young person their biology has decided their ceiling? Module 1 established that participation is itself a determinant of health. Weigh that against whatever accuracy the claim has.

Write a response to the coach in one paragraph. It should concede what is correct, correct what is overstated, and close on what you would recommend instead.