Muscular function practice questions.

A volleyball player lands from a block and the knees flex under control while the quadriceps group resists the downward motion. Which muscle action is the quadriceps performing?

• Eccentric, because the muscle lengthens while producing force
• Concentric, because the muscle produces force to control the load
• Isometric, because the joint angle at the knee is held constant
• Eccentric, because the muscle relaxes as the joint angle changes

Answer: Eccentric, because the muscle lengthens while producing force

In an eccentric action the muscle develops force while an external load lengthens it, which is how landing forces are absorbed. It is not relaxation: the muscle is fully active, which is why eccentric work is the main trigger of delayed onset muscle soreness.

A biopsy from a marathon runner's gastrocnemius shows fibres with many mitochondria, a dense capillary network and a slow contraction speed. Which fibre type is described?

• Type IIx, fast glycolytic
• Type IIa, fast oxidative glycolytic
• Type I, slow oxidative
• An even mixture of Type IIa and Type IIx with high glycolytic capacity

Answer: Type I, slow oxidative

Type I fibres are slow oxidative: mitochondrial density, myoglobin content and capillary supply are all high, so they resist fatigue at submaximal pace. Type IIx sit at the other extreme, with few mitochondria, fast contraction and rapid fatigue.

A weightlifter raises the force produced by the quadriceps from a light warm-up set to a near-maximal lift. Which change in the nervous system best accounts for the rise in force?

• Each active muscle fibre now contracts more strongly than before
• The motor neurons conduct their impulses over a longer distance
• The fibres change from Type I to Type IIx during the set itself
• More motor units, including larger ones, are recruited

Answer: More motor units, including larger ones, are recruited

A motor unit obeys the all-or-none law, so whole-muscle force is graded mainly by recruiting more and larger motor units (the size principle) rather than by each fibre contracting harder. Fibre type shifts over months of training, never within a single session.

During the upward phase of a dumbbell curl the elbow flexes. What is happening to the triceps brachii?

• It contracts concentrically in order to oppose the movement
• It lengthens under control as the antagonist
• It acts as the fixator, holding the scapula still
• It shows no electrical activity at all during the movement

Answer: It lengthens under control as the antagonist

The antagonist does not contract against the agonist; it lengthens in a controlled way so that the movement can occur, a pattern known as reciprocal inhibition. Holding the scapula steady is the task of a fixator such as the trapezius.

During the drive phase of a sprint the myosin heads attach to actin, pull and detach over and over. What does the binding of a fresh ATP molecule to the myosin head do in this cycle?

• It supplies the energy for the power stroke at the moment the head swivels
• It breaks the link between the myosin head and actin so that the head releases
• It moves tropomyosin aside so that the binding site on actin is uncovered
• It pumps calcium ions back into the sarcoplasmic reticulum to end the contraction

Answer: It breaks the link between the myosin head and actin so that the head releases

ATP binding breaks the cross-bridge, and it is the hydrolysis that follows which re-cocks the head ready for the next stroke. Uncovering the binding site on actin is the work of calcium acting on troponin, not of ATP.

The fibres of a weightlifter's gastrocnemius run at an angle to the tendon rather than along the line of pull. What does this pennate arrangement favour?

• A greater range of movement at the joint
• A higher shortening velocity for the muscle taken as a whole
• A greater force, because more fibres fit into the volume
• A lower energy cost for the same external work

Answer: A greater force, because more fibres fit into the volume

Angling the fibres packs many more of them across the line of pull, so the physiological cross-sectional area and therefore the force rise. The cost is paid in range and speed, because each fibre is shorter, which is why the long parallel-fibred muscles are the ones built for excursion.