Creep behavior of passive bovine extraocular muscle

Abstract

This paper characterized bovine extraocular muscles (EOMs) using creep, which represents long-term stretching induced by a constant force. After preliminary optimization of testing conditions, 20 fresh EOM samples were subjected to four different loading rates of 1.67, 3.33, 8.33, and 16.67%/s, after which creep was observed for 1,500 s. A published quasilinear viscoelastic (QLV) relaxation function was transformed to a creep function that was compared with data. Repeatable creep was observed for each loading rate and was similar among all six anatomical EOMs. The mean creep coefficient after 1,500 seconds for a wide range of initial loading rates was at 1.37 ± 0.03 (standard deviation, SD). The creep function derived from the relaxation-based QLV model agreed with observed creep to within 2.7% following 16.67%/s ramp loading. Measured creep agrees closely with a derived QLV model of EOM relaxation, validating a previous QLV model for characterization of EOM biomechanics.

Figure 1

Mean normalized displacement J(t) plotted for two specimens each of the six bovine EOMs in both linear and semi-log scales. All EOMs exhibited similar creep to 1.34 ± 0.02 (SD) maximum.

Figure 2

Mean creep for 1.67%/s loading rate for 5 EOM specimens. (a) Force versus displacement. After initial ramp loading achieved at around 6 mm deformation, the force was held constant at 0.2 N. (b) Reduced creep coefficients over 1,500 seconds. Maximum creep coefficient SD was 0.023.

Figure 3

Comparison of creep at 4 different initial loading rates. The left arrow indicates that a lower reduced creep coefficient is reached at the end of initial ramp loading as the ramp-loading rate increases. The right arrow indicates that a higher creep coefficient is reached after 1500 seconds as ramp-loading rate increases.

Figure 4

Observed reduced EOM creep function, and theoretical function derived from the reduced relaxation function.