MUSCLE FLEXIBILITY
Passive Resistance to Stretch
When a muscle is stretched it resists stretch
similar to when a
rubber band is pulled. At the beginning of the stretch the muscle or
rubber band moves a large distance with little force input. As you near
the end of the stretch you reach a point where the muscle or rubber band is less
elastic. At this point, you need to exert a much greater force on the
muscle to get it to move. The relationship between a
muscle's resistance to force and muscle length is exponential and is exhibited
below. In our laboratory, the length of the muscle is given by the hip
angle (in radians) relative to the floor and the resistance to stretch is measured
in newtons.
In this figure, the top red line represents the total forc
e
measured by the force transducer, the middle line represents hip angle, and the
bottom line represents the resistive force exerted by the muscle during
stretching. Most studies that have examined flexibility
training have used a joint's range of motion as their measure. It appears
that improvements in range of motion may be due to tolerance to the stretch and
not actually due to change in the elastic properties of the muscle.
Results from our laboratory confirm this and indicate that moderate duration
stretching programs (4-8 weeks) have little effect on the stiffness of skeletal
muscle.
Peak Torque/Peak Torque Angle
There is a relationship between a muscle's length and its
maximum force production. If a muscle is completely contracted it doesn't
have room to contract any more and its ability to produce force is
decreased. Similarly, if a muscle is stretched to a great extent its
ability to produce force will be lower th
an at its optimal length. In the
figure above this is demonstrated. The line intersects the net force curve
at its highest point. The hip angle at this point can also be determined
and represents the optimal length of the muscle. We are currently
interested if flexibility training can alter the length/force relationship of
muscle and change the muscle length where maximum force production occurs.
Our results indicate that there is no evidence of
a change in optimal muscle length following four weeks of stretching. This
suggests that our stretching intervention caused no change in the resting length
of the muscle. Additionally, stretching had no effect on measures of
active force production as has been proposed by others.
Supported
by the Vermont Genetics Network
through
NIH Grant Number P20 RR16462
from
the BRIN Program of the National Center for Research Resources
