Original Article J. Phys. Ther. Sci. 13: 139–143, 2001 Individual Cervical Muscle Function in Biomechanical Studies: A Review of Literature

ASGHAR REZASOLTANI, PhD, PT1)

1)Faculty of Rehabilitation, University of Sh. Beheshti: Damavand St., Faculty of Rehabilitation, Dept. of Physiotherapy, Tehran-Iran. E-mail: [email protected]

Abstract. The human cervical structure is a complex arrangement in which an important array of bones, soft tissues and vital organs are collected in a closely-packed area. There are numerous small and large muscles which act together to induce head and motion in a certain direction. The cervical muscles are also involved in many audiovisual reflexes, which are a complicating factors in clinical evaluations. Because of this anatomical compaction and the complexity of the upper motor neuron reflexes involving the cervical muscles, there is as yet no general understanding of the anatomy and function of the neck muscles. This gap in our knowledge may in part be due to a lack of proper examination tools or to a failure to examine the applicability of the present methods for evaluating cervical muscle function. Today the field of biomechanical evaluation of the cervical spine needs an easy and practical method which would also be replicable in follow-up studies such as rehabilitation assessments. Within the last decade, parallel to methods like electromyography and muscle strength tests, a few imaging techniques, particularly computerized tomography, magnetic resonance imaging and ultrasonography have been used to evaluate the function of the cervical muscles. In the present article, the application of the current biomechanical methods in the assessment of the individual cervical muscle function is discussed. Key words: Individual, Cervical, Muscle, Function.

(This article was submitted Feb. 21, 2001, and was accepted May 20, 2001)

INTRODUCTION cervical injuries and to gain an understanding of the instability caused by cervical trauma (Yoganadan Biomechanics is the science that examines the and Pintar 1997). Grauer et al. (1997) examined the action and interaction of various forces upon and mechanism of whiplash injury by monitoring the within a biological structure (Nigg and Herzog motion of individual spinal vertebrae during 1996). According to Miller and Nelson (1973), the whiplash trauma. The authors found that during a main aim of developing biomechanical components whiplash injury the cervical spine was bent into a s- (models) is to provide a simple and accurate shaped curve and subjected to hyperextension estimation of the force exerted on different deformities. biological structures. Nigg and Herzog (1996) stated that biomechanics involves studies of BIOMECHANICAL PROPERTIES OF disabled, non-disabled, athletic and non-athletic HUMAN CERVICAL MUSCLES subjects. Clinical biomechanical methods have already In vitro laboratory studies of cervical been used to analyse the mechanisms behind biomechanics have been undertaken with both 140 J. Phys. Ther. Sci. Vol. 13, No. 2, 2001 human and animal subjects (Nolan and Sherk 1988, Staudte and Dühr (1994) measured cervical Dickman et al. 1994, Chen et al. 1994). Dickman et extension force and neck length and calculated al. (1994) compared and analysed the function of torque by multiplying cervical extension force by the upper cervical spine in baboons and humans. neck length in men and women aged between 14 The authors concluded that as regards the anatomy and 74. They found that torque declined with and biomechanics of their upper cervical spine, increasing age because the neck became shorter and these two species resembled each other. the neck muscles weaker. In the other studies, Kamibayashi and Richmond (1998) provided Ylinen and Ruuska (1994), Ylinen et al. (1998), descriptive morphometric data on individual Julin et al. (1998), and Ylinen et al. (1999) cervical muscles in humans. Basing their model on measured absolute neck maximal extension force a microdissecting approach, the authors evaluated and reported higher values for athletes than for non- the muscle mass, pennation angle, fascicle length athletes or cervical patients. and sarcomere length of the fourteen cervical Load sharing among the cervical muscles during muscles. Drawing on Kamibayashi and isometric contraction has been studied by EMG, Richmond’s (1998) results from cadaver studies, MRI and computer motion analysis (Steen 1966, Vasavada et al. (1998) developed a computer- Mayoux-Benhamou et al. 1989, Conley et al. 1995, graphic model of the cervical muscles and Vasavada et al. 1998). While a few authors have calculated the movement-generation properties of granted that measurements based on the external individual neck muscles for a range of head and torque of the cervical muscles have some neck positions. According to the authors, neck applicability in EMG studies, others have more muscles with similar action may have some faith in absolute neck force measurements. Harms- essential differences in their neural activation Ringdahl et al. (1986) calculated the cervical load patterns, moment arm and their force generation moment induced by the weight of the head and the capacity. neck about the bilateral axis of the C7-T1 spinal In vivo indirect biomechanical methods such as motion segment. The authors located the centre of surface electromyography and muscle force cervical motion (the fulcrum) at a site halfway measurements have been employed in studies of the between the spinous process of C7 and the upper biomechanical properties of the cervical muscles margin of the manubrium sterni. They reported that (Harms-Ringdahl et al. 1986, Schüldt et al. 1988, with the head and neck in flexion, the load moment Mayoux-Benhamou and Revel 1993, Queisser et al. of the cervical extension force for the C7-T1 joint 1994). Using surface electrodes, Queisser et al. was 3.6 times as high as the value for the neutral (1994) measured the myoelectrical signals position of the head. Queisser et al. (1994) generated by four cervical muscles, the semispinalis calculated the external torque of the post-cervical capitis muscle, the splenius capitis muscle, the muscles by multiplying the absolute force and the levator scapula and the muscles during external lever arm for cervical extension. These MVC in cervical extension at four head and neck authors estimated the external lever arm (moment positions. The authors reported a linear relationship arm) by determining the distance between the centre between the electromyographic signals of the of pressure under the head and the C7-T1 spinous semispinalis capitis muscle and external torque, process. whereas the splenius capitis muscle results suggested that the relationship between the signals CERVICAL MUSCLE DIMENSIONS AND generated by the splenius capitis muscle and MUSCLE FORCE external extension torque tends to be non-linear. The authors suggested the effect on EMG signals of Imaging methods such as computerized the shifting of the end plates in different head and tomography scanning and MRI have been used to neck positions as a possible source of error. Schüldt estimate the relationship between cervical muscle et al. (1988) applied a similar vector line to force and size in humans (Mayoux-Benhamou et al. estimating cervical muscular moment. The authors 1989, Conley et al. 1995). Mayoux-Benhamou et used surface EMG and reported a non-linear EMG/ al. (1989) measured the total cross-sectional area of moment relationship for the erector spine/trapezius, the posterior cervical muscles and absolute neck splenius, and levator scapula muscles. extension force. The highest correlation found by 141 these authors was between absolute neck extension Regarding the literature, such a single-vector model force and the cross-sectional area of the may not work satisfactorily or produce inaccurate semispinalis capitis muscle. They suggested that results since there are many other factors not because of individual anatomical and physiological considered in it (Mayoux-Benhamou et al. 1989, differences, using any vectorial construction might McGill and Sharratt 1990). McGill and Sharratt lead to an oversimplified picture of the actual (1990) assumed that a single vector may not muscle strength. adequately represent the strength of the spinal In studies of muscle size and related muscle force, extensor muscles since the force generated by the muscle size has been measured separately from muscles is parallel to the axis of compression, with maximum voluntary contraction (Mayoux- the result that it may cancel the shearing force. In Benhamou et al. 1989, Freilich et al. 1995, Ikai and addition, a single equivalent vector cannot enable a Fukunaga (1968). The first simultaneous comprehensive perspective on the mechanics measurement of muscle response and muscle force involved because of the intervertebral disc load, was reported by Hicks et al. (1984). The authors facet joint reaction forces, the force of gravity and found a significant decrease in the echo amplitude the multitude of ligaments and muscles. of the quadriceps muscle during MVC as compared Using an external lever arm, Schüldt et al. (1988) to measurements taken with the muscle at rest. and Queisser et al. (1994) gained contradictory Furthermore, Henriksson-Larsen et al. (1992) results on the relationship between EMG activity scanned the vastus lateralis muscle at rest and and cervical extensor torque. Regardless of the during the MVC force of knee extension. The limitations of EMG recording of the cervical authors also revealed a significant increase in the muscles, the discrepancies might be partly due to angle of the muscle during MVC as the complexity of modelling the moment arm for compared to a state of rest. Likewise, in another muscle force prediction. The contradictory findings ultrasonographic study, the mean fiber angle regarding EMG and cervical extension torque may pennation of the vastus lateralis and vastus also be a result of different biomechanical models intermedius muscles decreased as the muscles first being applied by different authors. In a spinal contracted or shortened and then stretched. muscle strength study, Rantanen et al. (1994) came (Rutherford and Jones 1992). The same authors to paradoxical results when they compared maximal also reported a significant positive correlation trunk flexion and extension force and torque using a between the fiber angle and the CSA of the vastus different level of the fulcrum. The authors lateralis and vastus intermedius muscles. concluded that trunk flexion and extension torque In this regard, muscle outline and dimensions increased in direct proportion to the height of the may be altered during muscle contraction. The fulcrum. The authors further stated that lowering result of our recent study revealed a significant the fulcrum (increasing the external lever arm) may variation in the lateral and anterio-posterior allow the hip flexor and extensor muscles to also dimensions of the neck semispinalis capitis muscle contribute to the generation of torque in back when subjects supported the weight of their heads in flexion and extension. a prone position and muscle size was measured Another factor which must be taken into account simultaneously (Rezasoltani et al. 1998a). Our in the force production of a single muscle are the other results also showed a significant correlation special biomechanical characteristics of the muscle between the dimension of the muscle at rest and studied. Vasavada et al. (1998) found that in certain cervical extension force during MVC (r=0.82, movements some cervical muscles, such as the p<0.001) in a group of males and females sternocleidomastoid, may provide two moment (Rezasoltani et al. 1998b). arms. The semispinalis capitis muscle consists of two medial and lateral parts named as a digastric of DISCUSSION the neck (Kapandji 1974), and each part may have different physiological and mechanical In biomechanical studies, there are a few authors characteristics in maximal and submaximal who have modelled the musculature of the human contraction. Both parts of the semispinalis capitis cervical spine as a single equivalent force vector muscles have a common insertional attachment to (Harms-Ringdahl et al. 1986, Queisser et al. 1994). the . The lateral part originates from 142 J. Phys. Ther. Sci. Vol. 13, No. 2, 2001

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