Researchers Adrian Lai (left) and James Wakeling (right) used an ergometer to collect data for their recent study.

Researchers in the department of Biomedical Physiology and Kinesiology have discovered that muscles appear to know which fibres to recruit depending on demands of the task.

The universal theory governing muscle recruitment is known as the “size principle”. It describes how motor units; the building blocks of muscles, are recruited in a simple and elegant way to make a muscle contract and move the body. 

Researcher Adrian Lai from the Neuromuscular Mechanics Lab at SFU explains, “Size principle found that smaller motor units consisting of slower, less energy costly muscle fibres are always recruited before larger motor units, which consist of faster, more high powered but more costly muscle fibres”. This strategy would ensure that slow and postural contractions are achieved in an efficient and fatigue-resistant manner.

But there is a paradox.

 Lai says, “At fast, low-intensity contractions, it makes more sense to recruit faster muscle fibres over slower ones because they can generate greater power and are more efficient during these conditions.”

Along with his advisor, James Wakeling, Lai set out to understand why, for speed and economy, tasks requiring higher velocity contractions would not prefer to recruit faster muscle. 

Using computer simulations of the human body, the two were able to confirm previous suggestions, shown using electromyography, that there may be more strategies at work depending on the conditions.

The team first simulated isometric (i.e. when the muscle does not change length) and slow contractions where the strategy should match size principle. Then, they simulated faster and more powerful contractions using data collected on cyclists. They found that once the cycling speed exceeded 100 RPM, the “size principle” was replaced by a more mechanically and metabolically sensible “preferential” recruitment strategy resulting in the activation of faster motor units over the slower ones.

The team also found that the body was at least 10% more efficient when faster motor units were preferred during these conditions.

Lai says that the computer model confirms that if the body wants to work in the most efficient manner, then it should vary the strategies it uses to recruit motor units.  In this way, different motor units are recruited in the most mechanically and metabolically sensible way to match the most economical strategy for a given movement.

“This simulation provides a novel framework to further investigate coordination between muscles with different proportions of fibre types and relate recruitment strategies to other movements such as walking and running,” he says.

Wider applications to this work could eventually include the treatment of motor impairments such as stroke and motorneurons diseases as well as informing how cyclists and athletes should train their muscles to maximize their performance.

The full study can be found here.