The tibialis posterior reflex is an intrinsic reflex of the tibialis posterior muscle (posterior tibial muscle) that is responsible for supination, the elevation of the inner edge of the foot. The reflex is triggered by a small blow with the reflex hammer just above or below the medial malleolus. The reflex arc is controlled by the tibial nerve, a major branch of the sciatic nerve. Failure of the reflex may indicate problems in the L5 vertebra.
What is the tibialis posterior reflex?
The tibialis posterior reflex is an intrinsic reflex of the tibialis posterior muscle (posterior tibial muscle) that is responsible for supination, the elevation of the inner edge of the foot. The tibialis posterior reflex is an intrinsic reflex that, when triggered, causes the sole of the foot to tilt laterally inward. The inner edge of the foot is pulled upwards and at the same time the outer edge of the foot moves downwards, a typical tilting movement inwards (supination), which also occurs in an overstretched form during the so-called twisting of the foot. Since this is an intrinsic reflex that is triggered by the same organ where the stimulus arrives, reaction times are extremely short. The nervous circuitry and control of the reflex runs through only one or a few spinal ganglia. The tibialis posterior reflex is triggered by a small blow with the reflex hammer on the tendon of the tibialis posterior muscle, the posterior tibial muscle, just above or below the medial malleolus. The muscle reports the abrupt stretch to the spinal cord via afferent fibers of the tibial nerve. The impulse is processed directly and the reflex contraction of the muscle is initiated via efferent motor fibers of the nerve. Artificial triggering of the reflex is mainly used to demonstrate the function of the nerve crossing sites in the L5 region of the spine.
Function and task
The tibialis posterior reflex is a stretch reflex that belongs to the group of intrinsic reflexes because the affected organ-in this case, the tibialis posterior muscle-sends the stretch message itself to the spinal ganglion in the spinal cord via afferent fibers, and postliminarily triggers the involuntary brief contraction of the muscle via efferent motor fibers. Stretch reflexes are involuntary muscle reflexes that cannot be controlled voluntarily and, unlike extrinsic reflexes, cannot be trained away or modified. Because the processing of the sensory perception of muscle stretch by the muscle spindles occurs directly in the spinal ganglion in the spinal cord and is responded to with the appropriate contraction command, the reflex is extremely fast. The time from the triggering stimulus of the reflex to the incoming motor signal for muscle contraction is only about 30 to 50 milliseconds (ms). Prior processing in the motor centers of the central nervous system (CNS) would take significantly more time. The short reflex arc from the triggering stimulus to the action serves to protect the muscle from overstretching. However, the reflex-triggering sensory message is sent not only to the spinal ganglion, but also in parallel (collaterally) to the antagonistic muscle. This ensures that the latter is inhibited and cannot tense at the same time. The triggering blow with the reflex hammer on the tendon of the posterior tibial muscle, immediately above or below the ankle, results in only a slight but all the more rapid stretching of the entire muscle, so that practically all muscle spindles report the rapid stretching of the muscle as a sensory message to the spinal ganglion. The spinal ganglion responds immediately and sends the contraction command to protect the muscle, without prior consultation with the CNS. The advantage of the rapid response time is offset by the disadvantage that the reflex cannot be influenced at will. The tibialis posterior reflex functions even after many rapid repetitions. Like all other muscle stretch reflexes, it does not wear out. It is therefore an innate, nonmodifiable, unconditional reflex.
Diseases and disorders
The most common problems and complaints associated with muscular intrinsic reflexes and the tibialis posterior reflex arise from impairments in nerve conduction. Not only does this cause the reflex to weaken or fail altogether, but sensory and motor problems are usually present in specific areas of the body.Artificial triggering of the tibialis posterior reflex with the reflex hammer can provide an indication of problems in the area of the lumbar vertebra L5 if the reflex is weakened or cannot be activated. Often, problems with nerve conduction are caused by mechanical forces acting on the nerves. Predestined for such mechanical nerve damage are the crossing points of the nerves on the spine in the area of the intervertebral discs. A diminished muscle reflex can then provide an initial suspicion of the presence of disc damage in a particular vertebra. In the case of the tibialis posterior reflex, however, caution is required, since the reflex is sometimes only weakly expressed even in healthy people and is absent in some individuals. Therefore, it is important to perform the reflex on both feet to make a comparison. Basically, muscle reflexes with unilateral weakening or strengthening usually indicate nerve damage. After a stroke, a strengthened muscle reflex can often be detected on a paralyzed muscle. In rare cases, clonus occurs after a stroke. This is a rhythmic twitching of the muscle after only a single stimulus. The contractions are apparently triggered by motor nerves originating in the CNS.
