How mice gain locomotive speed

Abstract

Tests for evaluating, topical anesthetics, pain relief medications, and counterirritants in rodents are often based on the latency of the test subjects’ responses to painful stimuli. However, for many clinical applications, efficacy of agents may be reflected by restoration of normal activity. In orthopedic applications, restoration of normal gait or mobility improvement may be the objectives. Yet, gait dynamics in mice are not well characterized. Accordingly, we evaluated how faster locomotive speed is achieved in mouse, with the aim of establishing markers for healthy vs. pathological gait in mouse models.

Methods   Figure 1.   Gait dynamics in an adult male C57BL/6 mouse (24 weeks, obtained from The Jackson Laboratory) were monitored using The DigiGait Imaging system. Neither restraint nor anesthetic was required. Baseline recordings were performed after ambulation of 22 cm/s for 2 min. The treadmill speed was increased 50% to 33 cm/s for 2 min. Imaging of the ambulating mouse was immediately repeated. e-MOUSETM signal detection algorithms were used to interpret the gait signals.1

Results

Discussion

We show that gait dynamics were significantly altered by a 50% increase in locomotive velocity. The increase in speed was accomplished predominantly by nearly a 40% decrease in propulsion time and secondarily by a ~15% decrease in braking time. Swing time was not changed by the increase in speed from 22 cm/s to 33 cm/s, median and high locomotive velocities for mice.2 In rat, swing time does not vary as the animal increases its speed of walking, despite making larger strides.3 As shown in Figure 1, stride frequency increased only modestly, from 2.9 Hz to 3.4 Hz, necessitating a 30% increase in stride length, from 7.5 cm to 9.7 cm. Mice can increase their ambulatory speed by increasing their stride length or stride frequency. As emphasized previously in a study of rats, the two options are deployed in combination and not necessarily in equal proportion.4 Increases in stride length and decreases in swing time may become important at higher speeds. As in dog5, braking duration was significantly greater in the forelimbs, whereas propulsion duration was generally longer in the hind limbs (data not shown). Dynamic analyses of gait in mice could accelerate understanding and treatment of pain, arthritis, neuromuscular disease, and other entities that affect coordinated locomotion.

References

1. Chu, V. et. al. 2001. BMC Physiology 1:6.
2. Clarke, K.A. et al. 1999. Physiol. and Behav. 66:723-729.
3. Hruska, R.E. et al. 1979. Life Sciences 25:171-180.
4. Clarke, K.A. and A.J. Parker. 1986. Physiol. and Behav. 38:345-351.
5. Budsberg, S.C. et al. 1987. Am. J. Vet. Res. 48:915-918.