VARIATIONS IN NECK MUSCLE FASCICLE LENGTHS WITH HEAD POSITION

A. N. Vasavada (1,2), S. Li (1), S. L. Delp (1,2)
(1) Sensory Motor Performance Program, Rehabilitation Institute of Chicago
(2) Biomedical Engineering Department, Northwestern University
345 East Superior Street, Chicago, IL 60611

Presented at the 20th Annual Meeting of the American Society of Biomechanics
Atlanta, Georgia. October 17-19, 1996

INTRODUCTION

The muscles of the neck generate head movements and maintain the stability of the cervical spine. The roles of individual neck muscles in this complex musculoskeletal system are not well understood. However, the unique features of cervical spine anatomy, kinematics and muscle architecture may influence neck muscle function at different postures. In this study, we found that neck muscle fascicles operate on different parts of the active muscle force-length curve during various movements and that muscles with similar functions may have very different fascicle operating ranges.

REVIEW AND THEORY

The head-neck system includes over 20 pairs of muscles. Many of these muscles can generate moments about more than one axis of rotation. The complex anatomy makes it difficult to use many common experimental techniques of musculoskeletal biomechanics, such as electromyography or measurement of moment arms. Thus, detailed, anatomically-based musculoskeletal models are important to understand neck muscle function.

Documenting the physiological operating ranges of muscle fascicles provides insight into the part of the force-length curve on which muscles operate, and therefore how force-generating capacity varies with head position. The objective of this study was to quantify neck muscle fascicle lengths over a range of head positions in three directions of motion.

PROCEDURES

A graphics-based musculoskeletal model of the head and neck was developed using Software for Interactive Musculoskeletal Modeling (Delp and Loan, 1995). Description of the head-neck model and preliminary results have been presented by Li and colleagues (1995).

The isometric force-generating properties of muscles were obtained by scaling a generic Hill-type model of muscle (Zajac, 1989). The four scaling parameters per musculotendon actuator are optimal fascicle length, tendon slack length, pennation angle and peak isometric force. Muscle architecture parameters were collected through a collaboration with Queen's University. The experimentally obtained parameters were muscle fascicle and sarcomere length at the neutral head position, physiological cross-sectional area and pennation angle. The muscles included in this analysis were sternocleidomastoid, longus capitis, scalenus (anterior, medius, and posterior), trapezius, splenius (capitis/cervicis), semispinalis (capitis/cervicis), rectus capitis posterior major, rectus capitis posterior minor, obliquus capitis superior and obliquus capitis inferior.

Cervical spine kinematics were defined based on data in the literature (White and Panjabi, 1990). The total angular range of motion of the head relative to the trunk was 136s in flexion/extension (pitch), 154s in axial rotation (yaw) and 122s in lateral bending (roll), and motions at the intervertebral levels were distributed as a percentage of the total head rotation angle.

Fascicle lengths at neutral were measured in anatomical studies. Changes in fascicle length were calculated in the model from muscle attachment sites and coordinate transforms defined by joint kinematics. Fascicle lengths were normalized to optimal length and superimposed on an active muscle force-length curve for analysis.

RESULTS

This analysis showed that the operating ranges of neck muscle fascicles vary considerably among muscles and movement directions. For example, fascicle lengths of sternocleidomastoid remain in the plateau region during pitch, enter the descending limb of the force-length curve during yaw, and encompass almost the entire active force-length curve during roll (Figure 1). Certain muscles which are commonly considered to have similar functions were found to operate on very different portions of the force-length curve. Splenius capitis and semispinalis capitis both generate an extension moment. However, splenius capitis fascicles act on the descending limb and plateau region during pitch, while semispinalis capitis fascicle lengths operate on the ascending limb and the plateau region (Figure 2). For all muscles studied, fascicle length variations were greatest for those directions with the largest moment arms.

Figure 1: Fascicle operating ranges of sternocleidomastoid during head movements in pitch, yaw and roll. The vertical line indicates fascicle length at neutral head position.

Figure 2: Fascicle operating ranges of splenius capitis (dotted) and semispinalis capitis (dashed) during pitch movements (head flexion and extension). Vertical lines indicate fascicle lengths at neutral head position.

DISCUSSION

The range of joint angles over which a muscle can generate active force is proportional to the ratio between optimal fascicle length and moment arm. A larger ratio indicates less variation in force-generating capacity with changes in head position. The ratio of optimal fascicle length to moment arm varies among neck muscles, more often because of differences in moment arm rather than fascicle length. For example, sternocleidomastoid has optimal fascicle length of 10.8 cm and flexion moment arm less than 1 cm. The optimal fascicle length of splenius capitis is 9.5 cm, but the extension moment arm is close to 5 cm. This explains why sternocleidomastoid remains on the plateau region during flexion-extension movements while splenius capitis undergoes large changes in fascicle length.

Differences in fascicle operating ranges in muscles with apparently similar functions may influence load sharing strategies. For example, splenius and semispinalis both have large extension moment arms. Splenius has greatest force-generating capacity at relatively shorter lengths (corresponding to head extension), while semispinalis can generate greater force at longer lengths (corresponding to head flexion).

The magnitude of the force decreases at the ends of the range of motion indicates that force-length effects can potentially play a large role in modulating muscle moment generating capacity. However, the extreme postures analyzed in this study may not occur during natural movements. In cats, fascicle lengths of most neck muscles remained on the plateau region during head and neck movements measured while tracking a drinker (Statler et al., 1995). Furthermore, the interaction of fascicle length and moment arm changes will influence the overall moment-generating capacity. The decreases in muscle force may be either offset or augmented by the changes in moment arm. Thus, characterizing the interaction of neck muscle architecture and musculoskeletal geometry is important to understanding muscle moment-generating and stabilizing capability at different head postures.

REFERENCES

Delp, S. and Loan, J. Comput Biol Med, 25, 21-34, 1995.

Hoy, M. et al. J Biomech, 23, 157-169, 1990.

Li, S. et al. Third Int Symp on the Head/Neck System, 1995.

Statler, K. et al. Third Int Symp on the Head/Neck System, 1995.

White, A. and Panjabi, M. Clinical Biomechanics of the Spine (p. 92-98), J. B. Lippincott, 1990.

Zajac, F. CRC Crit Rev in Biomed Eng, CRC Press, 1989.

ACKNOWLEDGMENTS

The authors gratefully acknowledge the work of Drs. Lynne Kamibayashi and Frances Richmond in collecting neck muscle architecture parameters. This study was supported by funding from NIDCD-NASA center grant for vestibular research and an NSF predoctoral fellowship.