Muscle Length and Force: All Things Passive & Active

Now that you have a basic idea of How a Muscle Contracts and the Brain's Involvement, lets start diving into the concept of "Force and Length".

As you know, during a contraction a muscle shortens through a variety of mechanisms. When a muscle is relaxing after a contraction, it lengthens. So basically, after shortening, the muscle needs to return back to its original length and position, but how does that happen?

One of the ways that it returns back to its original length and position is through the assistance of antagonistic muscles, which move the limb in opposite directions. For example, the Triceps are antagonistic to the Biceps, contracting to help move the limb in the direction in which the Biceps can relax(lengthen).

The other ways a muscle returns back to its original length and position is by external forces like gravity or by the energy stored inside elastic tissues. The focus of this page is going to be on the stored elastic energy and how it adds to the total force a muscle can generate.

But first, there a few terms that need to be covered in order to understand what's coming later. These terms are as follows:

Positive & Negative Work

When a muscle lengthens by some type of energy, the work that causes it to lengthen is called Negative Work. When there's work done to shorten it, its called Positive Work.

Different Types of Contractions

Isometric: contraction in which the overall length of the muscles and tendons doesn't increase or decrease. The contraction is against a constant load...think of it as holding a weight in place without moving it.

Isotonic: contraction that causes a shortening of the muscle against a constant load. This would be your typical lifting movement such as curling a dumbbell towards your shoulder.

Isokinetic: constant rate of contration that occurs by varying the load. Here the load doesn't remain constant, in changes throughout the movement and this usually requires some type of special machinery or equipment.

How Stretching(Increasing the Length) Affects Force

Think of the way a rubber band works. When you stretch it, there's a tension(force) that you can feel up until a certain point, and if you keep pulling past that point the band breaks.

This tension that you feel when pulling a rubber band can be described as its elastic property, and muscle has an elastic property as well. When muscle is stretched, there is tension that can be measured.

At a relaxed length, a non-contracting muscle does not have any tension. But, if it is passively stretched beyond the resting length, the tension continues to increase, just as it does in a rubber band.

Where does the tension come from? This will be covered further down this page after the following section.

Contracting Muscle's Force at Different Lengths

As mentioned above, when a muscle is stretched passively, it generates tension(force) even without contracting.

Now when a muscle contracts, the tension varies at different lengths of the muscle. The highest amount of tension from contraction is generated near the resting length of the muscle.

Basically, the range at which a muscle is strongest, capable of producing the highest force, is between slightly shorter and slightly longer than resting length, at about a 5% variation. So if it shortens or lengthens greater than 5%, the contracting force will reduce sharply.

In addition, if a contracting muscle is stretched, there is additional force coming from the elastic property of the muscle as well, meaning that overall force is increased by the combination of contracting force and stretch force.

The simple explanation as to why there is reduced force the further you move away from resting length is because the maximum number of crossbridges cannot be formed in the sarcomere. In other words, the number of crossbridges is maximized right around the resting length (plus/minus 5%), allowing for the highest force potential.

When you hear about strongest & weakest "range of motion", the above explanation describes what's happening in the various ranges where you're stronger and weaker.

Where Does "Stretch Force" Come From?

During an active contraction, the force that a sarcomere generates is exerted on its tendon, which then transfers the force to bone. The tendon is capable of storing and transferring energy and it is also slightly stretchy, meaning that it has its own tension(passive force) when stretched.

During a passive stretch, when the muscle is not contracting, there is still tension but its not coming from the sarcomeres or the tendons. This passive force is coming from another elastic element, the Fascia, which is made up of elastic fibers that are capable of generating tension.

The Fascia, due to its ability to absorb energy, effectively protects a muscle from ripping or tearing while being stretched. Basically, it resists stretching beyond a certain point and attempts to bring everything back to its resting position.

The bottomline is that whether a muscle is shortening or lengthening, there are a variety of active and passive forces that interact together to generate force, protect the muscle and help us get a great workout in the gym!