Length-Tension: one relationship we all need to understand.

When it comes down to exercise selecation, one of the most important elements is understanding the length-tension relationship of a muscle.

Within a muscle fibre we have two components – actin and myosin. Their role is to attach to one another and pull together to contract and shorten the muscle.

These two components work best when they are in their optimal position for attachment. This happens to be when they have the right amount of overlap – in mid-range.

When a muscle is lengthened the distance is too great to create a strong attachment and pull together. When the distance is too short there’s too much overlap and an inability to create a strong link for contraction.

A muscle working in a lengthened or shortened state has less ability to attach strongly, and therefore less ability to apply a strong force. A muscle in mid range position is in an optimal position for attachment, contraction and therefore force production.

If a muscle is short or long it needs to recruit more muscular fibres (higher muscle activation) to have as strong a contraction as the muscle working in mid-range. Ie. It’s in a far less efficient state.

How does this look functionally?

Imagine you’re on a treadmill and are at a comfortable walk. The speed is slowly increased gradually by an unnamed practitioner.

You’re cruising along, until all of a sudden you’re not. The speed has increased and walking all of a sudden feels very effortful. Your calf is getting tired, your heart rate is up and breathing is laboured.

The treadmill goes up two more clicks and you find yourself all of a sudden shifting into a run rather than a walk. ‘Ahh that feels much better.’ The jog feel effortless and you begin to cruise again.

What happened? Why did the shift from a walk to a jog happen and what caused that shift in state?

It all comes down to length-tension.

The main propulsion generator for walking is our calf muscle. It’s a force production king and it’s ability to apply force determines the length of our step.

As the treadmill speed increases, the hip is forced back quickly into extension. This leaves the calf in a longer and longer position. Eventually the calf is trying to apply force in such a lengthened state that it reaches a point of diminishing returns.

At a microscopic level the actin and myosin are so far apart that the calf is having to recruit excessive muscle fibres to apply only small amounts of force. Ie. It’s extremely inefficient.

The nervous system makes a change; shifting state (phase shift) into a jog. All of a sudden the calf is back in mid-range and can apply that all important propulsion without having to recruit a whole lot of unneeded muscle fibres.

The calf is now producing more force into the ground with less muscle activation. Efficiency at its best.

If length-tension of the calf muscle is the rate limiting factor in shifting from walking to running, then it’s got to be a very important principle to understand in the scheme movement and exercise science.