For countless individuals who enjoy walking and running, nothing beats a walk through the woods, a run along the beach, or laps around a track. The conveniences of a stationary treadmill to satisfy the desire for exercise or maintain health cannot be disputed. Treadmills are used routinely by millions of individuals as an alternative to walking and running overground. Treadmills, moreover, are used regularly as diagnostic tools for cardiovascular diseases and movement disorders. Millions of laboratory animals are used to research cardiovascular diseases and movement disorders.  Forced to live in small enclosures, they still manage to traverse their cages and activity wheels several kilometers a day. Given the opportunity to walk or run on a motorized treadmill for even a few moments, most laboratory mice do so enthusiastically.

Gait analysis is increasingly being recognized as an important assessment tool for understanding and developing therapies for numerous movement disorders and neurodegenerative diseases.  The differences between treadmill and overground walking are fairly obvious, but experts agree that the fundamental mechanisms of muscle strength, balance, and coordination for treadmill and overground walking are basically the same, for humans (1, 2) and rodents (3,4). Published studies underscore the remarkable similarities between overground and treadmill walking, and point to subtle differences in some aspects such as proprioception and somatosensory feedback (3). Comparisons of mice walking overground and on treadmills indicates that the relationship of the stance time and the swing time against the stride frequency are the same in both conditions (3,4). Having established that the mechanics of treadmill and overground gait are essentially equivalent, clinical movement analysis routinely employs treadmill-based protocols (2). Key benefits of treadmill walking, for clinical and preclinical studies, include a) equal walking speeds for all study groups; b) the flexibility to study a range of walking and running conditions; and c) data collection and analyses based on numerous strides, resulting in tighter data with lower standard errors.

To analyze gait in rodents, expecting them to walk “on command” and having all animals walk at the same speeds are not likely to occur. The video below, provided by a purveyor of a catwalk apparatus, over which an animal is expected to walk on command, illustrates the typical scenario of the hesitant and irregular stepping of the subject. Although the behavior of the animal might be of interest, it is not possible to robustly quantify the gait of this subject.

The “gait” in these instances is obfuscated, rather, by the exploratory behavior of the animal.  Dark boxes and rewards are sometimes provided as incentives to incite the animals to actually walk. This, in and of itself, may introduce some variability because of differences in the visual or olfactory acuity of animals to comparably respond to the incentives. A short cat walk can limit the number of strides at a constant speed that can actually be recorded. Physics mandate that the animal start and end the traverse at 0.0 cm/s. Therefore, if an animal traverses the catwalk somewhat smoothly, that means it has accelerated and decelerated through most of it traverse. It also means that the faster speeds that lab animals are capable of cannot be evaluated at all by this paradigm. Laboratory mice can exceed speeds of 100 cm/s in their activity wheels and on treadmills. Even a walkway length of 130 cm will likely not capture an animal walking much faster than ~40 cm/s. Many subtle motor disturbances are highlighted at faster walking speeds, rendering the overground paradigm not useful for studying faster gaits (5).

Too, even with wider angle camera lenses placed sufficiently far from the animals [with concomitant loss in spatial resolution], the range of motion of the animals that can be captured by camera is limited to a few steps; multiple trials are needed to generate a sufficient complement of strides for analyses, and it is highly unlikely that the repeated trials are performed at the same walking speed as the initial trial. Indeed, few strides with compromised spatial resolution at indeterminate/variable speeds are the results of these efforts.

The treadmill paradigm afforded by DigiGait [and copied by an admirer] provides greater camera resolution than is possible with the catwalk – almost double – and enables the animals to be studied over numerous strides at known and comparable speeds.

The DigiGait Imaging System enables the analysis of gait in mice, rats, guinea pigs, and hamsters for high throughput and high content analysis of gait disturbances. Arthritis, ataxia, CNS disorders, neurodegenerative diseases…anything that affects how we walk, there is an animal model that can be characterized with DigiGait.

Ventral view of mouse walking on DigiGait Imaging System motorized transparent treadmill belt. Inherent in the system are better camera resolution, faster imaging capture rate, and more data from animals talking multiple strides at known and equal walking speeds.

People who walk on treadmills know that the speed of the treadmill can be fixed and held stably by the individual. Likewise, the speed of an animal walking on a treadmill with a fixed speed is equal to that speed, unless the animal fails to perform the test. Most mice are natural walkers and runners. Clarke and Still showed that Swiss Webster mice walk speeds of ~14 to 40 cm/s (6).  Numerous studies by Theodore Garland’s group have shown that mice in activity wheels can approach speeds of 100 cm/s (e.g., 7). Since speed is the most important confounder in the interpretation of gait data, it is highly important to have all animals in a research study walk the same speed. It is well established that injured or moribund animals will walk more slowly, and therefore the gait metrics are affected by the slower speed. Even Professor Frank Hamers, largely responsible for the development and promotion of CatWalk, a commercially available overground catwalk type gait analysis tool, has reported that locomotion speed affects several key metrics of gait (8). Others have demonstrated the profound affect that overground walking speed has on stride length, stride frequency, stance time, swing time, and duty factor (3), to name a few.

A B6 laboratory mouse takes about 3-10 strides per second depending on its walking speed (higher stride frequency at higher speeds). For example, in a study of mice with Huntington’s disease, a brief ~5 second bout on the DigiGait treadmill set to a speed of 30 cm/s provides about 28 strides for gait analysis for all of the mice in the study.  ~28 strides for gait analysis at a known speed of 30 cm/s, for all animals, healthy and sick—wow—simply not possible with a catwalk paradigm.

Emphasized here are  a) numerous strides; b) known speed; c) same speed for all animals via treadmill gait analysis. The DigiGait Imaging System captures the paws, limbs, and body of the mouse with twice the camera resolution than the catwalk paradigm allows, and with significantly faster temporal resolution [+150 frames per second]. This results in more data with higher spatial and temporal resolution with lower stand error and greater accuracy and sensitivity.

Herbin_speed

A recent publication describing gait in rodents emphasizes the effect of different walking speeds on numerous gait metrics (3). Differences in walking speed is the most important confounder in the interpretation of gait data. The DigiGait imaging system enables all animals to be studied at known and equal speeds.

References:

1) Lemke K, Cornwall MW, McPoil TG, Schuit D. Comparison of rearfoot motion in overground versus treadmill walking.  J Am Podiatr Med Assoc. 1995 May;85(5):243-8.

2) Riley PO, Paolini G, Della Croce U, Paylo KW, Kerrigan DC.  A kinematic and kinetic comparison of overground and treadmill walking in healthy subjects.  Gait Posture. 2007 Jun;26(1):17-24.

3) Herbin M, Hackert R, Gasc JP, Renous S. Gait parameters of treadmill versus overground locomotion in mouse. Behavioural Brain Research 2007;181:173–9.

4) Pereira JE, Cabrita AM, Filipe VM, Bulas-Cruz J, Couto PA, Melo-Pinto P, et al. A comparison analysis of hindlimb kinematics during overground and treadmill locomotion in rats. Behavioural Brain Research 2006;172:212–8.

5) Milligan C et al. Gait in SOD1 G93A mouse model of ALS. 2013, in press.

6) Clarke KA, Still J. Gait analysis in the mouse.  Physiol Behav. 1999 Jul;66(5):723-9.

7) Garland T Jr, Kelly SA, Malisch JL, Kolb EM, Hannon RM, Keeney BK, Van Cleave SL, Middleton KM. How to run far: multiple solutions and sex-specific responses to selective breeding for high voluntary activity levels.  Proc Biol Sci. 2011 Feb 22;278(1705):574-81.

8) Koopmans GC, Deumens R, Brook G, Gerver J, Honig WM, Hamers FP, Joosten EA. Strain and locomotor speed affect over-ground locomotion in intact rats.  Physiol Behav. 2007 Dec 5;92(5):993-1001.