All skeletal muscles have a resting length. When our muscles are stretched to the ideal length, it can maximize muscular contraction. This lesson explains the length-tension relationship in skeletal muscle and explores how the arrangement of myofilaments in sarcomere can impact tension and contraction.
Muscles Are Stretched At RestTendons are organs that attach our skeletal muscles to our bones. When our muscles are resting, that is, not contracting, they are actually stretched to what we call a resting length by these attachments. How do we know this? Let's take a look at the frog's gastrocnemius, or calf muscle.
Tendons attach our skeletal muscles to our bones
If you were to remove the muscle from the body of the frog, its length would shorten. Therefore, the muscle is stretched to its resting length within the body. As the muscle is stretched, so are the muscle fibers that make up the muscle organ.
As it turns out, the natural resting length of our skeletal muscles maximizes the ability of the muscle to contract when stimulated. If the resting length is shorter or longer, contraction is compromised. The effect of resting fiber length on muscular contraction is referred to as the length-tension relationship. This lesson will describe the anatomical arrangement of the muscle at rest and explain how this helps with muscular contraction.
Length-Tension RelationsLet's do an experiment using the gastrocnemius muscle of a frog to examine the relationship between resting muscle fiber length and contraction.
First, remove the gastrocnemius from the frog. Then, clamp the muscle between a fixed position and aforce transducer, which is an instrument that will record how much contraction occurs when the muscle contracts. We can move the clamp to change the resting length of the muscle - in other words, how long the muscle is before it contracts. We will then record contraction after stretching the muscle 1mm each time.
Let's start with a short length at which the muscle is pretty loose. When the muscle contracts at this short resting length, we see a small amount of force development, as illustrated by the small blip on the screen.
Now let's stretch the muscle a little bit, so we increased its resting length by just 1mm. As you can see, the muscle contracts with more force at this longer resting length. If we stretch the muscle once again to now 2mm beyond what it was originally, it develops even more force. However, if we stretch the muscle 3mm beyond the original length, now the force developed is less. When we stretch the muscle 4mm, the muscle force development is even less.
Our results can be graphed to illustrate the resting length on the x-axis versus the tension or force development on the y-axis. As you can see, tension development increases as we increase the resting length to a point, and then tension or force development decreases with further stretch. As it turns out, the resting length that produces maximum tension just so happens to be the resting length of the frog's muscle in the body.
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