DigiGait – chillax, and see how they run!

European researchers lead the way in advocating for the health and well-being of laboratory animals. These efforts are improving laboratory animal welfare worldwide, while driving greater awareness of refinement, replacement, and reduction of in vivo studies. Laboratory animals are used to model many human conditions that are characterized by movement disorders. Parkinson’s disease, multiple sclerosis, and arthritis, for example, disturb gait in humans; research scientists study the gait of animal models of these disorders to better understand and treat the human problems. What is the best way to comprehensively study their gait, without stress, and yet honor their great contribution to advances in improving health and well-being?

The DigiGait Imaging System was designed to provide high quality gait data from laboratory animals under the wide range of gait conditions normally encountered by rodents living in their natural environs. It is well known that laboratory mice can run ~5km in the course of 24 hours, and mice in the wild can exceed running speeds of 150 cm/s. The DigiGait Imaging System empowers researchers to study the posture and kinematics under many conditions that humans, and their lab animal surrogates, might encounter. For example, humans and rodents both may find themselves coerced into conditions under which they should run fast, up hills, and down declines. Humans, moreover, may enjoy walking or running on a treadmill, or may be asked to do so by their health care professionals, as either a diagnostic, or therapy. Laboratory mice most certainly select to run inside running wheels, often at fast speeds [exceeding 40 cm/s]. The DigiGait Imaging System, therefore, becomes a useful instrument to empower researchers to enable laboratory animals to naturally walk and run a wide range of speeds, up and down hills [were they entitled to live where they prefer, rather than in a laboratory cage].

Bear in mind, for pre-clinical gait data to be truly useful to research, metrics from at least 10 strides are needed, with all animals from a study walking the same speed. This helps to keep the standard errors low, rule out speed as a confounding factor, and improve the repeatability of studies. Mice usually take ~4 strides per second at a comfortable walking speed; the researcher then only needs a mouse to walk on the DigiGait treadmill for ~4 seconds, equipping the scientific community with data for ~16 strides . Challenging the animal to walk faster usually results in a higher step frequency, so a shorter test is possible. In contrast, a researcher attempting gait analysis with a voluntary overground paradigm, perhaps with a catwalk type device, and a goal box and other ruses to encourage the animals to cooperate with the testing, demand a lot more time from their animals in order to obtain gait data from a sufficient number of steps. The confines of the catwalk itself, and imaging limitations, only allow for about 4 or 5 strides. Repeat testing is necessary – at least 3 times – in order to obtain a robust set of signals. This adds time and additional demands of the animals on a catwalk device.

In contrast, the data coming from the brief ~ 4 second DigiGait treadmill study is far more relevant, quantitative, and useful than the data coming from an animal that might have voluntarily and reluctantly ambled or scurried across a walkway. For a researcher who has invested time, money, and possibly forced the animal into a murine research study, the quantitative data coming from numerous strides resulting from the animal’s DigiGait treadmill walking at known speeds far exceed the spurious data coming from some animals that might voluntarily walk a handful of strides at highly variable and unknown speeds. Mice can recruit their limbs ~10 times each second when fast running. To be able to quantify their kinematics at these speeds via DigiGait provides enormous opportunities for musculoskeletal research.

While recognizing the power of DigiGait to generate copious amounts of data about the animal’s posture and kinematics on the moving treadmill belt, we too asked whether aspects of DigiGait testing were stressful, or at least in comparison to other gait analysis methods. To determine whether the DigiGait treadmill walking might pose a stress to animals, we studied their heart rate inside the DigiGait walking compartment just before and just after treadmill walking. Increases in heart rate are known to be a metric of stress, at least in humans. If DigiGait were to be construed as a forced or forceful test, however, we would expect to see profound increases in heart rate and reductions in heart rate variability

 

To study the ECG, we placed the ECGenie recording platform inside of the DigiGait walking compartment atop of the treadmill belt, and placed the animal inside the walking compartment, with the speed of the belt at zero. After letting the animal acclimate to the chamber, we acquired the ECG signals for later ECG signal analyses. We removed the ECGenie platform and turned the DigiGait treadmill motor on, and allowed the mouse to run 24 cm/s for ten seconds, and for 36 cm/s for ten seconds, and acquired movies of the ventral view for later DigiGait analyses. Upon terminating the motor, we quickly re-inserted the ECGenie recording platform, and again recorded the ECG. The time between termination of the treadmill belt and capture of the signal was < 10 seconds. The animal was then returned to its cage, within two minutes of being removed from its cage initially.

Neither the heart rate nor the heart rate variability were appreciably affected by the brief treadmill walking and running for the typical study protocols, indicating that DigiGait does not cause stress to the animals. We do not yet know how much the heart rate and heart rate variability change with increases in treadmill speed, and longer bouts of walking and running. It might be interesting to quantify, for example, how the ECG and gait are affected in mice prone to arrhythmia, either due to a gene mutation or drug administration, after a 3 minute DigiGait run at 70 cm/s. This type of study, not possible via a catwalk type apparatus, would translate to the very real occurrence of sudden death in athletes.

tachogram_chillaxBlog_figureFigure 1 is an ECG from an animal inside the DigiGait with the treadmill speed at 0. Heart rate from this B6 female mouse was 744 bpm. Figure 2 is a tachogram, which reflects the RR interval for ~20 complexes. Shown are two plots, one for when the animal was first introduced to the walking chamber, and the other after 2 bouts of walking, once at 24 cm/s, and once at 36 cm/s. Each bout of walking was ~10 seconds. The tachogram for post-walking was obtained within about ten seconds after termination of running. The data indicate that the heart rate and heart rate variability were not affected by the brief bouts of treadmill walking.