Exercise physiology and nutrition of the human body practice questions.

A weightlifter increases the force produced by the quadriceps as the load on the bar is raised. Which change in the nervous input to the muscle produces most of this extra force?

• Only the largest motor units are recruited, and the smaller ones stay silent
• The firing frequency of the motor units already active falls
• More motor units are recruited and each fires more often
• Recruitment of motor units stops once half of them are active

Answer: More motor units are recruited and each fires more often

Force is graded by recruiting additional motor units and by raising the firing frequency of those already active. Recruitment follows the size principle, so the small fatigue-resistant units are brought in first and the largest are added only for the heaviest lifts.

During the steady state of a 1500 m race, which stimulus provides the main chemical drive to ventilation detected by the central chemoreceptors?

• A fall in the partial pressure of oxygen in arterial blood
• A rise in the partial pressure of carbon dioxide
• A rise in the concentration of glucose in arterial blood
• A fall in the concentration of lactate in the blood

Answer: A rise in the partial pressure of carbon dioxide

Central chemoreceptors in the medulla respond to hydrogen ions generated from carbon dioxide, so carbon dioxide is the dominant chemical drive. Oxygen partial pressure is monitored by the peripheral chemoreceptors and only becomes an important drive at altitude or in severe hypoxia.

Which pair of changes is produced by increased sympathetic activity in a footballer waiting for kick-off?

• Heart rate falls and the bronchioles constrict
• Heart rate falls and blood flow to the gut rises
• Heart rate rises and the bronchioles constrict
• Heart rate rises and the bronchioles dilate

Answer: Heart rate rises and the bronchioles dilate

Sympathetic outflow prepares the body for work by accelerating the heart, widening the airways and diverting blood away from the gut. The parasympathetic division produces the opposite pattern and dominates during recovery.

A gymnast holds a balance on the beam with her eyes closed. Which receptors supply the information about joint and muscle position that makes this possible?

• Baroreceptors in the walls of the aortic arch and carotid sinus
• Chemoreceptors in the medulla oblongata
• Proprioceptors in muscles, tendons and joint capsules
• Thermoreceptors in the skin of the hands and feet

Answer: Proprioceptors in muscles, tendons and joint capsules

Muscle spindles, Golgi tendon organs and joint receptors feed limb position to the central nervous system continuously, which is why a skilled gymnast can hold a balance without vision. Baroreceptors monitor blood pressure, not position.

A rugby player stands in cold rain during a stoppage and begins to shiver. In the negative feedback loop controlling core temperature, the skeletal muscles are acting as which component?

• The effector
• The receptor
• The control centre
• The stimulus

Answer: The effector

Effectors carry out the corrective response, here generating heat by involuntary contraction. The receptors are thermoreceptors in the skin and hypothalamus, and the control centre is the hypothalamus itself.

A distance runner competes in dry conditions at 28 degrees Celsius. Which route removes most heat from her body during the race?

• Radiation from the skin to the surroundings
• Conduction from the feet through the shoes to the road
• Convection from the skin to the surrounding air
• Evaporation of sweat from the skin

Answer: Evaporation of sweat from the skin

As skin temperature approaches air temperature the radiation and convection gradients shrink, so evaporation carries almost the whole heat load. This is why the same runner overheats in humid air that is no warmer.