Endurance athletes may suffer from muscle fatigue when contraction of the muscle fibres is disrupted. It is thought that the disruption of the contraction of muscles is caused by a lack of calcium ions within the muscle fibre. In an investigation, a group of cyclists completed a 30 km time trial in a simulator. The blood lactate concentration was measured at rest, during exercise and after exercise. The results are shown on the graph. State one variable that would need to be controlled in this investigation and how this would be achieved. (1) ................................................................................................................................................................................................................................................................................ ................................................................................................................................................................................................................................................................................

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