Lactate metabolism during vigorous exercise (the conversion of pyruvate to lactate, oxygen debt and the reconversion of lactate), and the structure, properties and uses of slow-twitch (type 1) and fast-twitch (type 2) skeletal muscle fibres.

What this dot point is asking

The SQA wants you to explain why muscle cells produce lactate during vigorous exercise, describe oxygen debt and the fate of lactate, and compare the structure, properties and uses of slow-twitch and fast-twitch muscle fibres.

Lactate metabolism

When you exercise hard, your muscle cells use ATP faster than the blood can deliver oxygen. Without enough oxygen, the electron transport chain cannot run, so NADH cannot be unloaded and NAD runs out. Because glycolysis needs NAD, it would stop, and so would ATP production. To prevent this:

Oxygen debt and recovery

The build-up of lactate creates an oxygen debt. After exercise, the body keeps breathing hard and the heart keeps beating fast to take in extra oxygen. This oxygen is used to convert the lactate back to pyruvate, which is then either respired aerobically or used to make glucose in the liver. The continued heavy breathing after you stop exercising is the body repaying its oxygen debt.

Muscle fibre types

Skeletal muscle contains a mixture of two fibre types, and the proportion varies between people and between muscles.

Slow-twitch (type 1) fibres:

• Contract relatively slowly but can keep going for a long time without fatigue.
• Rely on aerobic respiration, so they have many mitochondria, a good blood supply and a large store of the oxygen-binding pigment myoglobin, which makes them look red.
• Suit endurance activities such as long-distance running, cycling and swimming.

Fast-twitch (type 2) fibres:

• Contract quickly and powerfully but fatigue rapidly.
• Rely on glycolysis and produce lactate, so they have few mitochondria, a poorer blood supply and little myoglobin, which makes them look white.
• Suit short bursts of intense activity such as sprinting, jumping and weightlifting.

Endurance athletes tend to have a higher proportion of slow-twitch fibres, while sprinters and power athletes have more fast-twitch fibres.

Examples in context

Example 1. The sprinter versus the marathon runner. A 100-metre sprinter relies heavily on fast-twitch fibres for explosive power and accepts rapid fatigue and lactate build-up. A marathon runner relies on slow-twitch fibres that respire aerobically and resist fatigue over hours, showing how fibre type matches the event.

Example 2. The burning feeling in hard exercise. As lactate accumulates during an all-out effort, the muscles feel they are burning and tire quickly. When the effort stops and the oxygen debt is repaid, the lactate is cleared and the feeling fades, illustrating lactate metabolism and recovery.

Try this

Q1. State what pyruvate is converted into in muscle cells when oxygen is in short supply. [1 mark]

• Cue. Lactate.

Q2. Explain why slow-twitch fibres can keep contracting for a long time without fatigue. [1 mark]

• Cue. They respire aerobically, with many mitochondria, a good blood supply and myoglobin to store oxygen, so they do not rely on lactate-producing glycolysis.