Exploring the Links Between Human Movement, Biomechanics & Gait
Why is that muscle so tight?
We often think of neurological reasons (increased facilitation of the agonist, decreased reciprocal inhibition of the antagonist, increase gamma drive, etc), but how about the series elastic element (ie the connective tissue)? Or perhaps the sarcomere (individual contractile unit of the muscle)? How can we fix that? It is easier than you thought!
An oldie but a goodie. A great FREE FULL TEXT paper on sarcomere loss and how to prevent it. Yep, would you have guessed static stretching? Yes, this study was on mice and it seems plausible that it would be applicable to humans as well.
“When muscle is immobilised in a shortened position there is both a reduction in muscle fibre length due to a loss of serial sarcomeres and a remodelling of the intramuscular connective tissue, leading to increased muscle stiffness. Such changes are likely to produce many of the muscle contractures seen by clinicians, who find that such muscles cannot be passively extended to the full length, which normal joint motion should allow, without the production of muscle pain or injury.
…These experiments show that in addition to preventing the remodelling of the intramuscular connective tissue component daily periods of stretch of ½ h or more also prevent the loss ofserial sarcomeres which occurs in mouse soleus muscles immobilised in the shortened position.”
Foot instrinsic dialogue Motor Control. 2014 Jul 15. [Epub ahead of print]
Quantifying the Contributions of a Flexor Digitorum Brevis Muscle on Postural Stability. Okai LA1, Kohn AF.
There are many factors in adults that impair gait. It is not all biomechanical. This is part of our ongoing dialogue on the aging population and why gait impairments and falls are so prevalent. Acta Bioeng Biomech. 2014;16(1):3-9. Differences in gait pattern between the elderly and the young during level walking under low illumination. Choi JS, Kang DW, Shin YH, Tack GR.
We get a lot of interest in our posts on stretching. Seems like this is a pretty hot subject and there is a lot of debate as to whether it is injury preventative or not. Are you trying to physically lengthen the muscle or are you trying to merely bring it to its physiological limit? There’s a big difference in what you need to do to accomplish each of these goals. Lets take a look at each, but 1st we need to understand a little about muscles and muscle physiology.
Muscles are composed of small individual units called sarcomeres. Inside of these “sarcomeres” there are interdigitating fibers of actin and myosin (proteins) which interact with one another like a ratchet when a muscle contracts. Sarcomeres can be of various lengths, depending on the muscle, and are linked and together from one end of the muscle to the other. When a muscle contracts concentrically (the muscle shortening while contracting) the ends of the sarcomere (called Z lines or Z discs) are drawn together, shortening the muscle fiber over all (see the picture above).
Signals are sent from the brain (actually the precentral gyrus of the cerebral cortex areas 4, 4s and 6) down the corticospinal tract to the spinal cord to synapse on motor neurons there. These motor neurons (alpha motor neurons) then travel through peripheral nerves to the muscles to cause them to contract (see picture above).
The resting length of the muscle is dependent upon two factors: The physical length of the muscle 2. The “tone” of the muscle in question.
The physical length of the muscle is determined by the length of the sarcomeres and the number of them in the muscle. The “tone” of the muscle determined by an interplay of neurological factors and the feedback loops between the sensory (afferent) receptors in the muscle (Ia afferents, muscle spindles, Golgi tendon organs etc.), relays in the cerebellum and basal ganglia as well as input from the cerebral cortex.
If you’re trying to “physically lengthen” a muscle, then you will need to actually add sarcomeres to the muscle. Research shows that in order to do this with static stretching it must be done 20 to 30 minutes per day per muscle.
If you were trying to “bring a muscle to its physiological limit” there are many stretching methods to accomplish this. Pick your favorite whether it be a static stretch, contract/ relax, post isometric relaxation etc. and you’ll probably be able to find a paper to support your position.
Remember with both not to ignore neurological reflexes (see above). Muscle spindle loops are designed to provide feedback to the central nervous system about muscle length and tension. Generally speaking, slow stretch activates the Ia afferent loop which causes causes physiological contraction of the muscle (this is one of the reasons you do not want to do slow, steady stretch on a muscle in spasm). This “contraction” can be fatigued overtime, causing the muscle to be lengthened to it’s physiological limit. Do this for an extended period of time (20-30 mins per day) and you will physically add sarcomeres to the muscle.
Next time you are stretching, or you were having a client/patient stretch, think about what it is that you’re actually trying to accomplish because there is a difference.
We are and remain The Gait Guys. Bald, good-looking, and above-average intelligence. Spreading gait literacy with each post we publish.
thanks to scienceblogs.com for the corticospinal tract image