What is the H-Reflex, and how does it affect the stretch-shorten cycle of the lower extremity?
The H-reflex, short for the Hoffmann reflex, was first described by Dr. Paul Hoffmann in 1910. In order to best describe the H-reflex, we need to first describe a monosynaptic reflex. The stretch-reflex is one of the most common forms of a monosynaptic reflex. Figure 1 below depicts the classic "elbow tap" often used by physicians to test the quality of reflex in an individual. To elicit a stretch reflex, there needs to be sufficient stimulus; in other words, the tendon must be struck with enough force or there is no reflex. When the tendon is struck with an appropriate force, there will be a pull or stretch on the musculature. When this stretch occurs, it will activate the muscle spindles. The muscle spindles are located in parallel inside muscle (see Figure 1). When the muscle is stretched rapidly, so too are the muscle spindles. When the spindles stretch at a rate beyond its threshold speed, they send a signal to the spinal cord via the Ia afferents. In the spinal cord, there is activation of the alpha-motorneurons (α-motorneuron), which if sufficiently stimulated will send a signal to the musculature, causing a contraction of the stretched muscle and a relaxation in the antagonist or opposite muscle. If one were to place electromyography (EMG) sensors on the stretched muscle, a signal would appear just before contraction. Another signal would appear as the muscle began to contract.
Figure 1. Schematic depicting the stretch reflex in the upper arm.
The H-reflex is exactly the same as the stretch reflex; however, there is no stretch to activate the Ia afferents. Instead, an external electric stimulus is applied (see Figure 2 below, number 1). The stimulus is usually applied from the surface to a mixed nerve (e.g., the posterior tibial nerve). The electrical stimulus travels toward the spinal column and, if potent enough, will stimulate the α-motorneuron, thus sending a signal toward the musculature (see Figure 2 below, number 3). Again, an EMG recording device is used to pick up the signal prior to contraction (H-reflex) and then when contraction occurs (M-wave).
Figure 2. Schmatic depicting the method of eliciting and recording the Hoffmann reflex.
Now that we have an understanding of the H-reflex, what does it all mean? The H-reflex can be used to examine the response of the nervous system in various neurological disorders, musculoskeletal injuries, therapeutic modalities and exercise training. One example how the H-reflex is used is to test the response prior to an exercise bout training then to reexamine the response. If there is a reduction in the response, this could give insight as to the fatigued state of the musculature. This is a common practice when examining the effects of stretch-shortening cycle (SSC) exercise on reflex responses. Paavo Komi has extensively studied the mechanisms underlying the SSC.
Recently, he reviewed and discussed some possible effects of fatigue on SSC. One of the conclusions derived from the review was that when the musculature is fatigued, there is deterioration in of the α-motornneuron pool. There is also an increased inhibition within the spinal cord. Finally, there may be actual damage of the muscle spindle itself. This is very interesting in that these results indicate that as a muscle is fatigued, a greater stimulus (stretch) is required to elicit a SSC response. The H-reflex technique is helpful in uncovering these mechanisms.
The Hoffman reflex is a unique tool for assessing the neurological system. It can be used to examine the effect SSC exercise has on the reflex response. However, it does not actually affect the SSC in the lower extremity or any muscle.
- Komi, P.V. (2000). Stretch-shortening cycle: a powerful model to study normal and fatigued muscle. Journal of Biomechanics. 33: 1197-1206.
- Misiaszek, J.E. (2003). The H-reflex as a tool in neurophysiology: its limitations and uses in understanding nervous system function. Muscle and Nerve. 28: 144-160.
- Palmieri, R.M., Ingersoll, C.D., and Hoffman M.A. (2004). The Hoffmann reflex: methodologic considerations and applications for use in sports medicine and athletic training research. Journal of Athletic Training. 39:268–277.