NOTE: End date changed to 01/31/2023 per NSSC information (Ed., 2/1/22).

POWER SUPPLY: The Power supply () connects the wall outlet to two motors.

CONTROLLED MOTION: The DigiGait has 2 degrees of freedom, pitch and roll. In both directions (clockwise and counterclockwise, defined as forward and backwards), the motors can be moved by being given power. The power given directly corresponds to the force applied by the motor; this power is variable. More power can be provided to make the motor move more quickly. It is advantageous to gradually increase and decrease the power of the motors as it increases movement accuracy. Currently, the motors are given a gradual acceleration and deceleration period, and do not hold a constant velocity for any substantial time. This is not a limitation; a constant velocity can be held if desired. The rate of acceleration, as well as the duration of the periods, can be changed. Note that upon reaching any destination, the DigiGait takes some time to settle. This time is at a max of 5 seconds by default, but can be reduced substantially with slower speeds, or potentially increased with higher speeds.

The DigiGait is capable of simple and complex movement commands. The most basic command often utilized is "There and Back". An example use of this command is to roll the DigiGait 5 degrees and then bring it back 5 degrees. It is possible to vary the speed of this movement, as well as the distance traveled. It is also possible to add a pause at the target destination. Note that increasing the speed of the movement can result in decreased precision, while reducing the speed can increase precision. Tolerance is set at 1 degree by default, but can be reduced with slower movements. The accelerometer measures the movement and returns a value within 0.3 degrees of the actual positions.

A complex command, "Return to Zero", allows the DigiGait to return from any position to zero, within 0.5 degrees by default. This involves a series of gradual movements stepping towards zero. The speed of the command can be changed to adjust the tolerance; increasing the duration of command will lower the tolerance to as low as 0.3 degrees, which is the tolerance of the accelerometer.

Another complex command, "Shake", allows the DigiGait to rapidly move back and forth on either axis. It is possible to change how quickly this occurs, for how long it occurs, the direction this occurs in, and more. An example of this command is to shake in the pitch direction, varying position between 0 and 3.0 degrees, and taking 1 second to move between both ends of the range. Note that the shake command does possess an amount of uncontrollability associated with the settling of the DigiGait; uncontrollable does not mean unpredictable or unmeasurable, this simply means that we are relying on the settling of the DigiGait to contribute to the command.

RODENT STUDIES: The pilot study of twelve male mice included include both hind limb unloaded (HU) and full weight-bearing GROUND controls. Gait assessment of each mouse was be performed prior to HU or before the start of the study as a GROUND control, and then again after 14 days. From these mice we determined if: i] the system is functional and we can modify the design if necessary, and ii] the system can reliably measure gait data across a range of speeds and displacements

METRICS ASSESSMENT

1. General: We had originally planned to use 20º displacements as the baseline metric. However, when moving on the treadmill, displacements of this magnitude, even at relatively low speeds, can result in mice being pushed laterally and forward into the enclosure. Thus, displacements of 5º appear optimal for this sex and strain.

2. Speed: When the speed of the treadmill gets below ~12 cm/s, the rodents can essentially “ride” the displacement in the lateral direction. It remains useful for a forward pitch in terms of identifying if braking gait responses are altered. The utility in the lateral direction (roll) would be to identify if stability is altered by some stimulus (e.g., Radiation).

17.5 cm/s appears to be an optimal speed to promote forward locomotion and recovery with perturbations. Faster speeds and this strain of mouse runs to the back of the encasement.

3. Pitch: Forward pitch appears to provide a good metric of the ability to brake and remain in motion – both if the motion is continuous (without pause after forward pitch) and when a pause at a downward angle is provided. Time to return to linear locomotion is also a valued metric and is affected by treatment.

4. Roll: Rolling metrics (with and without pause) likewise appear to reflect stability, with important metrics including stance width and paw angle variability, with time to recovery of linear locomotion serving as a putative important metric regarding effects of treatments.

5. Pauses for both pitch and role: Pausing in the midst of motion (e.g., at 5 º) of roll or pitch permits the assessment of 2 “recoveries” – first from the initial perturbation (with associated gait pattern changes reflective of neuromotor and sensorimotor deficits) and then a timing of return to normal locomotion, if possible, and then with a return to baseline position, another set of recoveries.

6. Shaking: The shaking feature appears very useful in measuring gait pattern metrics associated with stability, and also a return to normal locomotor behavior upon cessation. We feel that for shaking especially, measuring gait parmaters prior to, during, and after the shaking challenge (and then comparison with baseline (“pre”) measures are of particular importance).

NOTE: End date changed to 01/31/2023 per NSSC information (Ed., 2/1/22).

To effectively move the DigiGait within the frame, as it weighs several hundred pounds, we sourced larger capacity 12V DC motors manufactured by Leeson. These motors are designed for large mechanical loads and hard-to-start applications as they generate 1 horsepower and 35 in-lb of full-load torque. At their max, these motors will draw 80 amps, which required us to source a marine battery as a power supply.

We tested the functionality of the Leeson motors using a Dart speed control system. This control uses a mechanical relay switch to control motor direction and a potentiometer to control motor speed. The benefit of this system is that it allows us to test motor response and determine the appropriate time needed to generate the desired displacements of the DigiGait™ treadmill surface.

We tested mice on the frame, moving at 17cm/s. The frame had the capacity to move the DigiGait as expected, and while forward linear movements were observed to be perturbed, the mice did recover linear walking as expected. We will then analyze these movements and animal postural patterns from images captured by the DigiGait in order to assess which parameters are important in determining performance deficits that can be measured by perturbing linear motion using the adjunct frame.

The harness of the exoskeleton attaches under and to the DigiGait on each side by way of bolts through an aluminum plate at the base, then again with a threaded rod through the center with nuts on each side to prevent slipping. Mounted bearings with 1” diameter are bolted to each end of this harness for the first rotation or roll movement. There is padding between this harness and the DigiGait. The inner frame is a rectangle connected to both the harness and the load-supporting frame.

The rotational accelerations about each axis are currently controlled by Two DC motors (24V, 41 Nm torque) to control rotation about each axis. Moreover, we created 3 different codes in MATLAB for frame movement, with the add-on for the Arduino Integrated Development Environment to control an Arduino Uno board with two BTS7960 motor drivers. Pulse width modulation (PWM) signals are sent to the DC motors to create displacements of the treadmill surface.