Explain how acetylcholine (ACh) initiates skeletal muscle contraction.

Analyse how capillary and mitochondrial densities affect slow twitch (type I) muscle fibres.

Diagram A, shows a gymnast balanced on a beam. In Diagram B, she is about to fall. Explain how a fall can be avoided by moving the body and therefore the centre of mass.

AB

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The diagram shows a cyclist.

[Source: elmm/Vector Images/Shutterstock]

Label the three forces: A, B, and C.

A.

B.

C.

Outline how wave drag can be reduced for a swimmer.

changes the electrical/neural impulse into a chemical stimulus at the motor end plate
OR
ACh is released when an action potential/nerve arrives at the motor end plate ✔

ACh binds to post synaptic receptors✔

increases membrane permeability to sodium ions/Na⁺ ✔

which causes Ca⁺⁺ to be released <into the muscle cell> ✔

ACh is broken down < by cholinesterase> to prevent continual muscle stimulation✔

Type I has a high density of capillaries and mitochondria ✔

high capillary density allows for increased oxygenation ✔

high mitochondrial density allows for increased use of oxygen
OR
high mitochondrial density allows for use of aerobic respiration «producing high amounts of ATP» ✔

both contribute to activities that require prolonged energy supply
OR
increases fatigue resistance ✔

in diagram A the centre of mass/COM is perpendicular to the contact point on the beam/ ✔

in diagram B the centre of mass/COM shifts to the left «of the beam» ✔

in order for the COM to remain above «the beam» the gymnast needs to move her body in line above the beam
OR
e.g the gymnast needs to move her legs further to the right in order to compensate for the torso moving to the left ✔

in diagram B the gymnast could lower their COM
OR
e.g by moving into a tucked position above the beam ✔

A: air resistance ✔

B: ground reaction force ✔

C: friction ✔

2 correct for [1] mark

3 correct for [2] marks

wave drag can be reduced by avoiding motion at the interface between air and water ✔

swimming underwater for as long as is allowed at the start of a race ✔

by using wave limiting lane ropes✔

Most candidates were able to identify that Ach stimulates the release of calcium ions. However, many candidates struggled to explain the change from neural to chemical message. Many candidates discussed sliding filament theory despite the question focusing on the initiation of a skeletal contraction.

Many candidates were able to identify that type 1 fibres will have high capillary and high mitochondrial density. However, few candidates were able to analyse how these factors would affect type 1 fibres.

All candidates attempted this question. Many candidates started their answer by defining COM which was not required for the question. This commonly used up most of their response lines. Candidates were able to identify that diagram B had COM outside of their body. However, there were unable to specify why gymnast A was stable. Many candidates were able to suggest a change in body position that would make gymnast B stable. However, further understanding of line of gravity and base of support is required.

All candidates attempted this question. Most candidates were able to identify air resistance and ground reaction force. However, very few candidates were able to identify friction. Some candidates confused ground reaction force and friction force.

All candidates attempted this question. Most started by describing wave drag, followed by outlining one method to reduce wave drag. Some candidates described streamlining as a method to reduce drag and this was often too vague. Only reference to reducing the disruption between the interface of the two fluids was accepted.