Editorial/Note on Pearson et al "Evaluating the Structural Properties of Suprahyoid Muscles and their Potential for Moving the Hyoid": The concept of Hyoid Posture

In their paper "Evaluating the Structural Properties of Suprahyoid Muscles and their Potential for Moving the Hyoid", Pearson et al. present the results of calculations based upon measurements from anatomical dissections to provide a background for understanding the forces that, potentially, can be generated by selected suprahyoid muscles. As they clearly point out, such results are useful only in the context of specific additional physiological information.

What that physiological context is, and how one should interpret their statement that "muscle function is limited by structural data" is not made clear. The information necessary to understand normal, and hence abnormal hyoid movement, is scattered across many papers and a myriad of subdisciplines. The goal of this note is to point to some of the disparate sources of information that are relevant to hyoid movement, including Pearson et al.'s paper, in order generate a better understanding of the wide variety of factors that may affect hyoid movement.

Hyoid movement is critical to swallowing, mastication, intraoral transport, vocalization, and respiration. Equally important for some of those behaviors, is non-movement, or stabilization of the hyoid. "Hyoid posture" is the concept that the position of the hyoid and the dynamic maintenance of that position is critical to function. Airway patency or anchoring the base of the tongue for speech or oral function are examples of when constancy of position is critical. Because the hyoid is a "floating bone", and its position is maintained by a sling of muscles, some of which do not articulate with another fixed bone (e.g., middle pharyngeal constrictor, hyoglossus), the limits and capabilities of the hyoid musculature are essential for hyoid posture.

The paucity of information on several aspects of hyoid muscle contraction contribute to the difficulty of predicting hyoid behavior from the anatomy of its muscles; one is the lack of basic information on the length-tension and force-velocity characteristics of individual hyoid muscles. For several functions, the anterior and posterior suprahyoid and infrahyoid muscles are active simultaneously (van Lunteren 1987a; van Lunteren and Manubay 1992; Perlman et al. 1999; Thexton et al. 2007; German et al. 2009). Such simultaneous activities of agonist and antagonist muscles is important for stabilization and precise trajectories of movement (Özkaya and Nordin 1999). Excitation patterns also vary tremendously within the hyoid muscles (Thexton et al. 2007; German et al. 2009; Thexton et al. 2009); multiple fine-wire or micropatch electrodes in geniohyoid and mylohyoid have demonstrated that the patterns of EMG activity not only varied with the specific function (suck, swallow) but also varied with the intramuscular site within the individual muscles.

The relationship between EMG activity and muscle strain, or length change, is also complex. The differences in kinematics that result from concentric (muscle shortening), isometric (static length) and eccentric (muscle length increasing) contractions suggest that knowledge of the anatomical properties of a muscle, including its origin, insertion and effective fiber population, are still not sufficient to permit prediction of the movements that result from its activation. Furthermore, it is entirely possible that different portions of a muscle can generate different contraction patterns (Lapatki et al. 2006; Konow et al. 2010). Recent work (Wenzel et al - need cite) also shows that significant regionalization of function exists in geniohyoid and sternohyoid during a simple reflex behavior.

While factors, such as fatigue resistance, muscle fiber twitch times and tetanic fusion frequency etc., have been investigated with respect to specific, even isolated, hyoid related muscles in a few species (van Lunteren et al 1987; van Lunteren and Manubay 1992) there is a general lack of such information for the hyoid musculature as a whole and a serious deficiency of information in the case of man. There are also a number of other considerations that have yet to be adequately investigated, such as elasticity and hysteresis in the hyoid musculature and the extent to which successive positions of the hyoid are determined by passive equilibria or actively by positional sensory feedback to motoneurons; the investigation of the latter is complicated by the apparent paucity and uncertain nature of the sensory receptors in those muscles. At the very least, in order to gain a very basic understanding of hyoid movement, the EMG and kinetic activity of the sets of agonist and antagonist muscles, such as geniohyoid, mylohyoid, sternohyoid and stylohyoid should be considered simultaneously with cranio-cervical posture.

Such studies may be problematic in man. There is also a potential problem in interpreting EMG activity, recorded from hyoid muscles in humans, using conventional surface electrodes. Conventional electrodes detect signals from a volume that is likely to contain not only different hyoid muscles but different motor unit populations within those muscles. The functional heterogeneity, that has already been identified among the motor units of hyoid muscles by other methods ((Tsuiki 2000; van Lunteren and Dick 2001; Thexton et al. 2007; Konow et al. 2010) consequently be unlikely to be detected, If the goal of a study was to detect, within an EMG recording, signs of a particular neurological deficit, such conventional surface electrode recording methods could mask the specifics of that pathophysiology.

Pearson et al conclude that the geniohyoid "has the most potential to displace the hyoid in an anterior direction" but do acknowledge that additional information on “fiber types, passive forces …‥ and motor units recruited for a specific task …… are essential to understanding function”. The lack of information on the level of recruitment of geniohyoid motor units to move the hyoid forward does, however, tend to undermine the use of physiological cross sectional area (PCSA) as a predictor of forward pull on the hyoid although the PCSA may well indicate the upper limit of that pull. Before firm conclusions can be drawn, it is necessary to obtain detailed in vivo data both on the hyoid kinematics and on the distribution of the intramuscular electromyographic activity simultaneously within the individual muscles that make up the three-way lines of action on the hyoid.