The Navy SEAL Physical Fitness Guide 111
perform a martial arts kick. Static or passive flexibility refers to the maximal range of
motion of a joint during passive movement induced by an external source (e.g., a partner,
equipment, gravity). The range of static flexibility is always greater than that of dynamic
flexibility.
The Stretch Reflex and the Lengthening Reaction
The stretch reflex and the lengthening reaction are joint-protective mechanisms in
which sensory organs, located in the muscles and tendons surrounding a joint, are activated
when muscles are stretched. As seen in Figure 7-1, the two sensory organs involved in
monitoring muscle tightness are the muscle spindle cells and golgi tendon organs (GTOs).
Figure 7-1. Graphical Representations of Muscle
Spindles and Golgi Tendon Organs
The stretch reflex involves muscle spindles which lie parallel to the muscle fiber.
These spindles are very sensitive to changes in muscle length. When the muscle stretches,
muscle spindles send signals to the spinal cord, which in turn, sends signals to the muscle
telling it to contract in order to protect the muscle from potential tissue damage. The classic
example of the stretch reflex occurs when a physician taps a patient just below the kneecap.
Muscle Spindle CellGolgi Tendon OrganSensory NeuronsInhibitory Neuron