It is easy to assume that harder training always produces better results. In resistance training, which includes lifting weights or using resistance machines, taking every set to the point where you cannot complete another repetition may create more fatigue without producing better results.

A repetition, or rep, is one complete movement of an exercise. A set is a group of repetitions performed without a long rest.

Training to failure means continuing a set until you cannot complete another full repetition with acceptable technique, despite making your best effort. This is sometimes called momentary muscular failure.

An alternative is to stop while you could still perform one or more repetitions. This is described as repetitions in reserve, or RIR. If you could complete ten repetitions but stop after eight, you have two repetitions in reserve, usually written as 2 RIR.

Why might training close to failure help?

Muscles are made up of individual cells called muscle fibres. These fibres are broadly classified as type 1 or type 2 fibres and are controlled by the nervous system. The type 2 fibres produce more force and can grow more than type 1 fibres.

A light load is generally one that you could lift for more than 15 repetitions, whereas a heavy load limits you to completing fewer than 10 repetitions. When lifting a light load, the type 1 fibres are active. As the set approaches failure, the nervous system activates type 2 fibres to keep the weight moving.

Lifting a heavy weight activates type 2 muscle fibres from the........

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