The relationship between bone mechanical properties and ground reaction forces in normal and hypermuscular mice

Abstract

Understanding the relationship between external load and bone morphology is critical for understanding adaptations to load in extant animals and inferring behavior in extinct forms. Yet, the relationship between bony anatomy and load is poorly understood, with empirical studies often producing conflicting results. It is widely assumed in many ecological and paleontological studies that bone size and strength reflect the forces experienced by the bone in vivo. This study examines that assumption by providing preliminary data on gait mechanics in a hypermuscular myostatin-deficient mouse model with highly mineralized and hypertrophied long bones. A small sample of hypermuscular and wild-type mice was video recorded while walking freely across a force platform. Temporal gait parameters, peak vertical and transverse (mediolateral) ground reaction forces (GRFs), vertical impulse, and loading rates were measured. The only gait parameters that differed between the two groups were the speeds at which the animals traveled and the transverse forces on the hind limb. The myostatin-deficient mice move relatively slowly and experienced the same magnitude of vertical forces on all limbs and transverse forces on the forelimb as the wild-type mice; though the myostatin-deficient mice did experience lower mediolateral forces on their hindlimbs compared with the wild-type mice. These preliminary results call into question the hypothesis that skeletal hypertrophy observed in hypermuscular mice is a result of larger GRFs experienced by the animals' limbs during locomotion. This calls for further analysis and a cautious approach to inferences about locomotor behavior derived from bony morphology in extant and fossil species.

Figure 1

A) Schematic diagram of the set-up for the mouse ground reaction force data collection. The runway covers but does not touch the force platform (black). Steps only register on the force platform when mouse paws touch the strip of wood covering the center of the plate (*). (B) Image of a mouse making a single forelimb contact with the wood strip (*).

Figure 2

(a) Faxitron radiographs of the humerus in wild type (top row) and myostatin-deficient mice (bottom row) showing the expanded deltoid crest (asterisk) and transverse dimension of the diaphysis (arrows) in the mice lacking myostatin. (b) Radiographs of the femur in normal (top row) and myostatin-deficient mice (bottom row) showing the expanded third trochanter (asterisk) and transverse dimension of the diaphysis (arrows) in mice lacking myostatin.

Figure 3

Demonstration of the various parameters measured on the vertical ground reaction force curve in this study. Shown is a force trace of the vertical ground reaction force of a single forelimb contact of a walking mouse, measured in Newtons. Parameters measured are (A) loading rate: the average slope of the initial loading period as defined in the text, (B) the peak vertical force at the end of the initial loading period, (C) the peak vertical force for the entire contact time, and (D) impulse: the area under the curve (shaded gray).

Figure 4

A representative mediolateral (ML) force (N) trace for the forelimb (FL) and hind limb (HL) from a single individual walking at 0.18 m/s.

Figure 5

Comparisons of some of the locomotor parameters for the wild-type (WT) and myostatin-knockout (KO) mice. The mean and one standard deviation for the walking speeds and peak vertical forces for forelimbs and hind limbs of the two groups of mice are compared. Significance values are indicated in the upper left corner. The designation “n.s.” indicates that the values are not significantly different from each other. Note that the myostatin-deficient (KO) mice walk significantly more slowly but the peak vertical forces are not significantly different from that of the wild-type (WT) mice.

Figure 6

Bivariate plot of peak vertical ground reaction force (divided by body weight) on the forelimb versus velocity (m/s) for the wild-type mice (dark gray triangles) and mysotatin-deficient knock-out mice (light gray circles).

Figure 7

Bivariate plot of peak transverse ground reaction force (in Newtons) on the forelimb versus velocity (m/s) for the wild-type mice (dark gray triangles) and mysotatin-deficient knock-out mice (light gray circles).