Updated: Sep 29, 2021
by Dr. Chris Knudsen
Muscle IQ Physical Therapy
I recently read an interesting study in the Journal 'Brain'. The study looked at how muscle contractions can be controlled by the spinal cord, even during complex activities like walking, in people with spinal cord injuries. I like this study because it helps to show how important reflexes are in normal everyday muscle functioning. I want to use this study as a backdrop for teaching the principle that muscles work by reflexes. This is what we teach our patients at Muscle IQ Physical Therapy. We teach our patients that when you apply a force to a muscle the muscle will automatically, or reflexively, contract in order to equal that force. I call this a "Normal Muscle Response". For this example, the decision for the muscle to contract is a "No Brainer". It does not require a decision to be made by the brain.
Weak Muscles and Muscle Inhibition
How a muscle responds to an applied force is very important to understand when treating painful injuries. Every patient who walks into our clinic at Muscle IQ Physical Therapy with low back pain, neck pain, or any other pain has a muscle weakness somewhere. We look for weak muscles because we believe that restoring muscle strength is key to eliminating pain and getting back to normal activities. Most people do not know why their muscles are weak. They might think that their muscles are weak because of de-conditioning or muscle atrophy. We are now learning that when someone is injured the muscle can be weak because the spinal cord is telling the muscle to be weak. We test for muscle strength and instead of getting a Normal Muscle Response (see Image 1 below) the muscle response is weak, or 'inhibited' (see Image 2). Muscle Inhibition can happen when the spinal cord has decided there is a problem or danger, and in order to protect you the spinal cord will make certain muscles or muscle groups weak. By understanding how this muscle reflex works we can identify where the weakness is coming from, treat the involved tissues, restore a "Normal Muscle Response", and turn off the muscle inhibition reflex.
Image 1: Normal Muscle Response
(Electrical impulses travel between muscles and the spinal cord 500 times a second in order for a muscle to contract fully. Graphic created by Howard Knudsen, DPT.)
Image 2: Muscle Inhibition
(During muscle inhibition the spinal cord can decrease the speed of electrical impulses to the muscles from 500 per second to 250 per second because of a "danger" message. This is a protective reflex which weakens the normal muscle contraction. Graphic created by Howard Knudsen, DPT.)
What is Afferent Input?
Afferent Input is any message being sent from the tissues back to the spinal cord. Efferent input are the messages going from the spinal cord to the muscles. The importance of afferent input is clarified in this study. The final sentence in the Brain study summary says, "We propose that the human spinal cord can interpret complex step-related, velocity-dependent afferent information to contribute to the neural control of stepping." What the researchers in this study are saying, in layman's terms, is that muscles can act purely from spinal reflex even when performing complex functional movements. The muscles during walking sense the ground forces and send this information to the spinal cord. We call these messages Afferent Input. The tissues sense the force and send a signal to the spinal cord and the spinal cord responds by sending signals back to the muscles. The muscle response does not have to be controlled by the brain, the muscle contraction can be initiated by the spinal cord. The spinal cord takes in messages from lots of tissues all over the body. It interprets this information and sends signals back out to the muscles. Some muscles are told to relax and some muscles are told to contract.
Danger Messages and Muscle Inhibition
Afferent Input can be a message from injured tissues in the body that are sending a danger message to the spinal cord. The spinal cord wants to protect this injured tissue so it responds by telling certain muscles or a muscle group to relax, or not contract fully. This is Muscle Inhibition. We see this in every patient who presents with pain. No matter how hard the patient tries to make their weak muscle respond in a strong way during testing, they cannot, because the muscle is being told to be weak by the spinal cord. The brain is not able to overpower the messages from the spinal cord. Then, after just a few minutes of treatment directed at the appropriate tissues, the weak muscle is retested and found to have a Normal Muscle Response.
This Study was a 'No Brainer'
What is unique about the study from the Brain journal is that the test subjects all had spinal cord injuries. The test subjects could not walk on their own because of their spinal cord injuries. They needed to walk on a treadmill with a harness system to decrease their body weight and some of them needed a person to lift their legs forward. The researchers measured the electrical responses from several different leg muscles during the walking tests. The researchers were able to more easily show that the muscle activity that was recorded during the walking movements was being created by reflexes from the spinal cord and not messages from the brain. The test subjects legs were experiencing forces being applied to the muscles and the muscles were responding to these forces. The faster the treadmill speed the higher the muscle response recorded. This study proves what we are seeing in our clinic, which is that when we are testing muscle strength we are really testing reflexes. A normal muscle response is for the muscle to respond to applied force equal to that force. A weak muscle response to force is happening because of Muscle Inhibition, a protective reflex from the spinal cord, not due to a 'weak' muscle, and not due to the brain.
Next time you have an injury you should consider that some of your muscles have become weak. This muscle weakness can be found during muscle testing. In order to fully recover you must treat the right tissues so that those muscles can become strong again.
To learn more about Muscle Inhibition and the best ways to recover from injury please check out our website at MuscleIQ.com or give us a call at 801-310-0851.