Fundamentals of Biomechanics

Proprioception of Muscle Action
and Movement
Considerable information about the body
and its environment are used in the regulation
of many movements. While persons
use all their senses to gather information
about the status or effectiveness of their
movements, there are musculoskeletal receptors
that provide information to the
brain to help produce movement. These receptors
of information about the motion
and force in muscles and joints are called
proprioceptors. While we usually do not
consciously attend to this information, this
information and the various reflexes they
initiate are important in the organization of
movement. A reflex is an involuntary response
initiated by some sensory stimulus.
Reflexes are only initiated if the sensory
stimulus is above some threshold.
There are many proprioceptive receptors
that monitor aspects of movement.
Information about joint position is provided
by four kinds of receptors. The vestibular
system of the inner ear provides information
about the head's orientation with
respect to gravity. This section will summarize
the important MTU proprioceptors
that provide information on muscle length
(muscle spindles) and force (Golgi tendon
organs). Human movement performance
relies on an integration of all sensory organs,
and training can be quite effective in
utilizing or overriding various sensory or
reflex responses. A dancer spinning in the
transverse plane prevents dizziness (from
motion in inner ear fluid) by spotting—rotating
the neck opposite to the spin to keep
the eyes fixed on a point followed by a
quick rotation with the spin so as to find
that point again. Athletes in “muscular
strength” sports not only train their muscle
tissue to shift the Force–Velocity Relationship
upward, they train their central
nervous system to activate more motor
CHAPTER 4: MECHANICS OF THE MUSCULOSKELETAL SYSTEM 99
units and override the inhibitory effect of
Golgi tendon organs.
When muscle is activated, the tension
that is developed is sensed by Golgi tendon
organs. Golgi tendon organs are located at
the musculotendinous junction and have
an inhibitory effect on the creation of tension
in the muscle. Golgi tendon organs
connect to the motor neurons of that muscle
and can relax a muscle to protect it from
excessive loading. The intensity of this autogenic
inhibition varies, and its functional
significance in movement is controversial
(Chalmers, 2002). If an active muscle were
forcibly stretched by an external force, the
Golgi tendon organs would likely relax that
muscle to decrease the tension and protect
the muscle. Much of high speed and high
muscular strength performance is training
the central nervous system to override this
safety feature of the neuromuscular system.
The rare occurrence of a parent lifting part
of an automobile off a child is an extreme
example of overriding Golgi tendon organ
inhibition from the emotion and adrenaline.
The action of Golgi tendon organs is
also obvious when muscles suddenly stop
creating tension. Good examples are the
collapse of a person's arm in a close wrist
wrestling match (fatigue causes the person
to lose the ability to override inhibition) or
the buckling of a leg during the great loading
of the take-off leg in running jumps.
Muscle spindles are sensory receptors
located between muscle fibers that sense
the length and speed of lengthening or
shortening. Muscle spindles are sensitive to
stretch and send excitatory messages to activate
the muscle and protect it from
stretch-related injury. Muscle spindles are
sensitive to slow stretching of muscle, but
provide the largest response to rapid
stretches. The rapid activation of a quickly
stretched muscle (100–200 ms) from muscle
spindles is due to a short reflex arc. Muscle
spindle activity is responsible for this myo-