MUSCLE-TENDON FLEXIBILITY CELLULAR CHANGES WITH AGE
1. Viscoelastic Deformation (VED)
With the application of stretching typical of that practiced in rehabilitation and sports, the biomechanical effect of viscoelastic deformation can be quite minimal and of such a short duration that it may not have influence on subsequent stretching. In a study of hamstrings, it was discovered that a static stretch of 45 seconds duration did not have a significant effect on the next stretch performed 30 seconds later. With 3 consecutive static stretches of 45 seconds (30-second rest intervals between stretches), each stretch showed a 20% VED during the static retention phase. However, the muscles had already recovered from relaxation in the next stretch. Flexibility changes with aging: what can we do?
2. Plastic deformation of connective tissue
(Plastic deformation means that the tissue does not return to its resting length once the tension has been removed)
The classic plastic deformation model would require a stretching intensity sufficient to pull the connective tissue into the muscle beyond the elastic limit and into the plastic region of the torque / angle curve so that once the stretching force is removed, the muscle does not return to its original length but would remain permanently in an elongated state.
Muscle studies show a markedly different curve. A plastic deformation phase would be reflected in the passive length / tension curve due to a decrease in its slope. (Indicating that there is not as much passive resistance to stretching as expected) And as such, a classic phase of plastic deformation does not seem to occur in the muscle, which means that the stretched muscle almost always returns to its resting state after stretching.
3. Increase in serial sarcomeres
Animal studies have shown that the amount of serial sarcomeres of a muscle can be modified by prolonged immobilization in extreme positions. That is, when the muscles are immobilized in fully extended positions, there is an increase in the number of sarcomeres in series.
This theory has been successfully applied to short-term intermittent stretching (which I would do as part of a stretching routine) with the view that this type of stretching regime will cause similar increases in serial sarcomeres and a concurrent increase in the length of the stretched muscle.
4. Neuromuscular relaxation
There was a time when it was believed that through the activation of the Muscle Spindle / Golgi-Tendon complex within all muscle / tendon units, greater flexibility could be achieved.
This form of stretching has been shown to be short-term only without any current research that indicates that neuromuscular stretching increases long-term flexibility.
5. Reduction of tendon stiffness
According to my last blog article (The tendon changes with aging and how to reduce the possibility of injury), I indicated that the structure of the tendon changed with aging, but that regular exercise (important during maturation but also of value after the maturation phase) was important to maintain the functioning of the tendons. and keeping the collagen content aligned and well lubricated.
The striking feature of collagen is its ability to withstand tensile loads. In general, it shows a minimum elongation (less than 10%) under tension; a proportion of this elongation is not the result of the true elongation of the individual fibers, but of the straightening of the fibers that are packaged in several three-dimensional assemblies. In contrast, elastic fibers can increase their length by 150%, but still return to their previous configuration .
This indicates that the tendon when healthy will be stretched substantially (but will always return to resting length). As the tendon ages, there is a general and progressive loss in collagen and an increase in the concentration of mature cross-links (which increases the “adhesion” of the tissue, the fibers cannot slide so smoothly by themselves). Such alterations in the characteristics of collagen are responsible for muscle hardening (less flexible) with age.
6. Sensory theory to increase muscle extensibility
There have been several recent studies that suggest that increases in muscle extensibility (range of motion through a joint) observed immediately after stretching and after short-term stretching programs (3 to 8 weeks) are due to an alteration of sensation only and not an increase in muscle length.
In the previous graph, when muscle lengthening occurs, the curve shifts to the right, reflecting a greater muscle length with a given passive tension force (the tensor muscle is applied while stretching).
After any stretching protocol, without a simultaneous right shift in the previous torque / angle curve, there is no evidence of an increase in muscle length.Read more