A review of the H-reflex and M-wave in the human triceps surae

Abstract

The soleus is the most commonly used muscle for H-reflex studies in humans, while limited comparable data have been produced from the gastrocnemii muscles. This article reviews the fundamental differences between the structure and function of the human soleus and gastrocnemii muscles, including recent data published about their complex innervation zones. Protocols for eliciting, recording, and assessing the H-reflex and M-wave magnitude in the human triceps surae are also discussed.

Introduction

Many studies have now been completed on the Hoffmann reflex (H-reflex) and direct motor response (M-wave) in the triceps surae, with greatest focus on the soleus because of the large separation between the stimulus thresholds of its H-reflex and M-wave (Capaday, 1997). There are however limitations to extrapolating complete and comparative results from many of these investigations due to the variability that exists between the experimental protocols of different authors.

Firstly, variability exists in the preferred placement of stimulating electrodes between some investigators. The bipolar stimulating electrodes are placed longitudinally on the skin above the tibial nerve in the popliteal fossa by some investigators (Burke, Adams, & Skuse, 1989), while others place the electrodes on either side on the leg at the level of the popliteal fossa (Tucker & Türker, 2004) (see Section 6.1). The stimulation duration and frequency (see Section 6.2) are also variable, however authors generally use either a 0.5 ms or 1 ms stimulus duration (Burke, Gandevia, & McKeon, 1984), and a 2–10 s interval between stimuli (Ferris et al., 2001, Goldberg et al., 1992).

Secondly, the position and type of recording electrodes are quite variable, with some authors suggesting monopolar recording is superior to bipolar (Gerilovsky, Tsvetinov, & Trenkova, 1989), in direct contrast to others who prefer bipolar recording (Hugon, 1973) (see Section 6.5). The position of recording electrodes also becomes particularly important when the test muscles’ innervation zones are considered (see Sections 2.3 Motor points, 6.5 Recording electrodes and Fig. 1). It is well understood that if bipolar electrodes are used, both electrodes need to be placed either superiorly or inferiorly to the test muscles’ innervation zones to reduce the chance that a great proportion of the signal will be lost due to common mode rejection (see Section 6.7). In the case of the soleus, where most of the innervation zone lies under the gastrocnemius muscle, this will not be a problem given that the electrodes will be placed well inferior to the muscle’s motor points. However in the case of the gastrocnemius, the innervation zones of both the medial and lateral heads are quite varied and therefore the chance of signal cancellation must be considered in this muscle (Childers, 2004, Parratte et al., 2002, Saitou et al., 2000) (see Section 6.7 and Fig. 1).

Once optimal stimulating and recording protocols are met, variability in the maximal M-wave magnitude between conditions, and over time must be considered (see Section 3.1). During reflex studies, it is common to use either the maximal M-wave magnitude as a normalisation factor, or as an indicator of stimulus efficiency throughout an investigation (reviewed in Zehr, 2002).

To begin, this article reviews the structure and function of the muscles of the human triceps surae. Secondly, the methods for eliciting, recording and assessing the H-reflex and M-wave magnitude in the human triceps surae are discussed, including methodological considerations such as cross-talk, filtering and signal cancellation. Finally, the modulatory effects of central and peripheral stimuli on the H-reflex and M-wave magnitude are also considered and new research in this field is summarised.