Antagonistic pairs at five joints
Every joint movement uses a pair. The agonist (prime mover) contracts to cause the movement; the antagonist relaxes and lengthens to allow it. 0413 needs a named pair at five joints: shoulder (deltoid / latissimus dorsi), elbow (biceps brachii / triceps brachii), hip (hip flexors / gluteus maximus), knee (quadriceps / hamstrings), ankle (gastrocnemius / tibialis anterior). Roles swap when the movement reverses.
Isotonic vs isometric contraction
Muscles work in three ways. An isotonic contraction changes length and moves a joint, in two forms: concentric = the muscle shortens, creating movement (biceps lifting a dumbbell); eccentric = the muscle lengthens while still under tension, controlling movement (biceps lowering a dumbbell slowly). An isometric contraction produces tension but stays the same length, with no joint movement (a scrum, a plank).
Slow-twitch vs fast-twitch fibres
Muscle contains a mix of two fibre types. Slow-twitch (Type I) fibres produce a low force but are highly fatigue-resistant, using the aerobic system; they suit endurance -- marathon running, distance cycling. Fast-twitch (Type II) fibres produce a high force quickly but fatigue quickly, using the anaerobic system; they suit short, explosive efforts -- sprinting, jumping, weight training. Most performers have a mix.
Drawn from real examiner reports.
Concentric vs eccentric contraction
The key definition trap. Concentric contraction is the muscle shortening to create movement; eccentric is the muscle lengthening while still under tension to control it. Candidates mix them up. Lock a word to each: concentric = SHORTENS, eccentric = LENGTHENS but still WORKING. A relaxed muscle produces no force, so eccentric is never "relaxing".
Digest 1.4: concentric and eccentric contraction are frequently confused with each other (s22 P12 Q6a; s23 P11 Q9).
Fast-twitch vs slow-twitch swapped
Candidates swap the two fibre types -- e.g. saying fast-twitch fibres resist fatigue, or slow-twitch fibres use anaerobic energy. Keep the three linked features straight: slow-twitch = low force, fatigue-resistant, aerobic; fast-twitch = high force, fatigues quickly, anaerobic. One feature backwards drags the others with it, so learn each type as one profile.
Digest 1.4: fast-twitch and slow-twitch fibres are muddled with each other (s22 P11 Q3b(iii); s22 P12 Q8c; w23 P11 Q4a).
A "difference" needs both fibre types
Asked for a DIFFERENCE between the fibre types, candidates give a fact about only ONE type (e.g. "fast-twitch fibres fatigue quickly") -- which scores nothing. A difference must name BOTH sides: "fast-twitch fatigue quickly, WHEREAS slow-twitch are fatigue-resistant". Build it on the three features: force, fatigue tolerance, and aerobic vs anaerobic energy.
Digest 1.4: asked for a DIFFERENCE between the two fibre types, candidates often give a fact about only one fibre type instead of a genuine comparison (s22 P11 Q3b(iii); s22 P12 Q8c; w23 P11 Q4a).
Agonist vs antagonist role
It is easy to transpose which is which. The agonist (prime mover) contracts to cause the movement; the antagonist relaxes and lengthens to allow it. Lifting a bicep curl, the biceps brachii is the agonist and the triceps brachii the antagonist. Reversing their roles loses the mark just as surely as naming the wrong muscles.
Isotonic vs isometric
Both involve a contracting muscle, so they get confused. An isotonic contraction changes the muscle's length and moves the joint (lifting or lowering a weight). An isometric contraction produces tension but stays the same length with no joint movement (a plank or a scrum). Quick test: if the body part moves it is isotonic; if held still under load it is isometric.
The same agonist works both phases
When lowering a weight under control, candidates wrongly switch the agonist to the opposite muscle. In a bicep curl the biceps brachii is the agonist for BOTH phases: it contracts concentrically to lift, then eccentrically (lengthening under tension) to lower. The antagonist does not take over -- the same muscle keeps working, just eccentrically.
Eccentric contraction is not relaxing
Students assume that because a muscle gets LONGER in an eccentric contraction, it must be relaxing -- the opposite is true. It is still a contraction: the muscle keeps producing tension while stretched under load, which lets it control the movement (lowering a weight slowly, landing softly). A slow "braking" movement is the agonist working eccentrically, not relaxing.
Digest 1.4: eccentric contraction is misdescribed as the muscle lengthening and relaxing rather than lengthening while still under tension (s22 P12 Q6a; s23 P11 Q9).
Cover both halves of the performance
When applying fibre types to a performer, cover BOTH halves of the performance. For a long-distance cyclist, do not only explain the fast-twitch sprint finish -- also explain the slow-twitch, aerobic work across the rest of the ride. Missing one half loses marks.
Name the specific muscle pair
When describing antagonistic action, name the specific agonist and antagonist, e.g. "biceps brachii as the agonist while triceps brachii relaxes as the antagonist". Writing generically about "the muscles" rarely earns full credit -- the named pair is what scores.
Apply it to the named performer
Generic, textbook answers are the most penalised habit in 0413. When a question names a performer, apply every point to it explicitly -- link the fibre type or contraction to the real movement in that sport, not a "typical athlete". Choose a named activity you can develop.
At every joint, muscles work in antagonistic pairs.
| Joint | Movement example | Agonist | Antagonist |
|---|---|---|---|
| Shoulder | Raising the arm forward, e.g. a bowling action | Deltoid | Latissimus dorsi |
| Elbow | Bending the arm, e.g. a bicep curl | Biceps brachii | Triceps brachii |
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