Proprioception, Performance and Injury
Terms proprioception and
proprioceptive reflexes are often used when discussing muscle performance and
injury prevention. The term proprioception is broadly defined as the awareness
of posture and movement. Movements of joint systems are constantly monitored by
various sensors called proprioceptors. The reflexes that arise from these
proprioceptors can initiate, inhibit or fine tune joint movements by actions on
the muscle motor neurons. By influencing muscle activation and contraction,
these reflexes play an important role in stabilizing joints and preventing
injury.
What is Proprioception?
Information about joint movement, muscle length and force
are all provided by proprioceptors, small structures that are found in muscle,
ligaments and tendons that are connected to the spinal cord by neurons. Well
known proprioceptors include the muscle spindle which monitors muscle length,
tendon organs that monitor that amount of force applied to the tendon, and
joint receptors that monitor joint position, movement and acceleration.
Proprioception operates under the concept of feedback. When a proprioceptor
identifies an unwanted movement, it sends a signal from the joint, muscle or
ligament to the spinal cord. Within the spinal cord, the signal can either
initiate or inhibit the motor neuron responsible for muscle contraction. That
is, proprioceptors can cause a muscle to contract or relax. These reflexes are
very fast, occurring within 20-50 thousandths of a second (millisecond) and are
designed to protect the joint from unwanted movements.
The best example of a proprioceptive reflex is the knee
jerk. The knee jerk begins when the patellar ligament is tapped (this is the
thick band that attaches the patella or knee cap to the tibia or shin bone).
This causes rapid knee extension or a “jerk”. The reflex happens as a result of
the tap causing a small but rapid stretch of the quadriceps muscles. The rapid
stretch activates the muscle spindle that lies within the muscle. This stretch
causes the muscle spindle to send a signal to the spinal cord. There, the motor
neurons controlling the quadriceps are activated, causing the muscle to contract
and shorten, counteracting the stretch. An important concept is that this
reflex does not involve information processing by the brain. It is far too
rapid. The muscle spindle is stretched, a signal is sent to the spinal cord,
the muscle contracts and shortens. The brain is aware of what has happened but
the movement is initiated within the spinal cord.
Many feel that the stretch reflex is very important in
stabilizing joints and preventing ligament injury. For example, a player cuts
by planting her left foot and accelerating to the right. As force is applied to
the foot, the ankle begins to turn inward (inversion). This, in turn causes a
small, but rapid stretch of the muscles that turn the ankle outward (eversion).
The stretch activates the muscle spindles of the stretched muscles (the muscles
on the lateral side of the lower leg) and causes them to contract. The force
exerted by these muscles counteracts to inward movement. In this case, the
stretch reflex aids in preventing the athlete from “rolling” her ankle and
injuring the ankle ligaments.
Ligaments also have proprioceptors that exert protective
reflexes. A number of researchers feel that proprioceptors within the anterior
cruciate ligament (ACL) are sensitive to tension placed on the ligament. One
function of the ACL is preventing the tibia from sliding forward with respect
to the femur (the bone of the upper leg). When the tibia moves forward and the
ACL is stretched, these proprioceptors trigger the hamstring muscles to
contract. The hamstring force pulls the tibia backwards. This stabilizes the
knee, reduces ACL tension and reduces the risk of being damaged. This reflex is
thought to play a role in protecting against ACL injuries. Its importance is
seen in athletes who have undergone ACL reconstruction. In these athletes, the
reflex is greatly diminished and may even be absent and may contribute to the
high rate re-injury.
Not all proprioceptive reflexes activate muscle. Some are
inhibitory. The Golgi tendon organ (GTO) is located in the tendons of most
major muscles. This proprioceptor is sensitive to the amount of force exerted
by the muscle. If, during muscle contraction, excessive force is placed on the
tendon, the GTO sends a signal to the spinal cord. This signal inhibits the
motor neuron and causes the muscle to relax. This inhibitory reflex is designed
to protect the tendon from being damaged by excessive muscle force. Better to
relax the muscle than to have it ruptured or torn away from the bone.
Force production by muscles during dynamic activities such
as landing, cutting and running is a complex interaction of activating signals
originating from the brain (voluntary control) and modulating signals arising
from proprioception (reflex control). The brain activates specific muscles for
a specific task and the proprioceptive reflexes modify contractions to
accommodate unexpected changes in movement.
Can We Train the Proprioceptive System?
When an untrained individual lands a jump, there is a
brief period of muscle relaxation (around 50 msec) that is quickly followed by
contraction. As the person lands, the knees and ankles flex stretching the
quadriceps and calf muscles. This should trigger the stretch reflex and cause a
rapid contraction. However, the excessive force of lengthening (or eccentric)
contractions seems to trigger the GTO and cause a brief period of relaxation,
about 50 msec. Shortly after the relaxation period, the brain initiates
contraction of the hip, knee and ankle extensor muscles so that the athlete can
land the jump without collapsing.
Several research studies also show that trained athletes
have enhanced proprioceptive reflexes. In the example above, that brief period
of relaxation when landing a jump is replaced by a period of enhanced muscle
activation. Training seems to either improve the stretch reflex or diminish the
GTO reflex. Either way, the proprioceptive reflex is enhanced following
training. This results in greater and more rapid force production at landing as
well as improved height of a subsequent rebound jump.
Proprioceptive training involves exercises such as jumps,
cutting maneuvers and balancing activities. They are designed to evoke rapid
changes in movement of the knee and ankle. The idea is to place stress on the
joint by simulating “unwanted” joint movements very controlled conditions.
These movements are thought to “Train” the proprioceptive reflexes as well as
build strength of the musculature. Research has shown that programs targeting
proprioception, balance and strength training do indeed result in reduced
injury risk.
Summary
Proprioception or the awareness of body position and joint
movement is an important aspect of normal neuromuscular function. The reflexes
that arise as a part of the proprioceptive system are critically important for
peak performance and reducing the risk of joint injury. Coaches and athletes
should remember that a part of any comprehensive training program should
include exercises designed to enhance proprioceptive reflexes.
Further Reading
Ergen E, Ulkar B (2008) Proprioception and ankle injuries
in soccer. Clinics in Sports Medicine,
27:195-217.
Hewett TE, Paterno MV, Meyer GD (20020) Strategies for
enhancing proprioception and neuromuscular control of the knee. Clinical
Orthopaedics and Related Research, 402:76-94.
Silvers HJ, Mandelbaum BR (2007) Prevention of anterior
cruciate ligament injury in the female athlete. British Journal of
Sports Medicine, Supplement 1:i52-i59..