Lean and Release Mechanism
The lean and release mechanism is a custom-built device used to provide destabilizing balance disturbances by leaning participants forward via a waist-supporting cable and breaking the cable’s connection to evoke a step response. This device features a trigger mechanism that initially grips the end of the cable when closed, and is opened by activating a 12 V DC solenoid. A load cell is incorporated in order to monitor cable tension, and lean angle may be adjusted with a rope cleat attached near the participant’s waist (coarse adjustment) or by turning a ball screw on which the trigger assembly is mounted (fine adjustment).
Waist Pull Mechanism
The waist-pull mechanism is a custom-built device used to provide destabilizing or non-destabilizing pulls at the waist in the posterior or lateral directions. This is accomplished by dropping weights with a computer-controlled mechanism while engaging a cleat that grips the cable used to transfer the postural disturbance to the subject. Disturbance magnitude may be varied by adjusting the amount of weight or the dropping height used, and cable force may be measured with an incorporated custom-built 2-channel load cell.
Three AMTI OR6-series force plates are used for balance experiments.These plates feature 500 lb shear force capacity and 1000 lb vertical force capacity; and 10,000 in-lb and 5000 in-lb moment capacities about the horizontal (x and y) and vertical (z) axes, respectively. The plates also feature horizontal force and moment sensitivities of 3 μV/(V-lb) and 0.18 μV/(V-lb), respectively; and vertical force and moment sensitivities of 0.75 μV/(V-lb) and 0.382 μV/(V-lb), respectively. The natural frequencies of the OR6-6 model (primarily used for pre-perturbation stance measurements) are 430 Hz and 900 Hz for the horizontal and vertical forces, respectively; and the natural frequencies for the OR6-7 model (primarily used for post-perturbation step landing measurements) are 300 Hz and 480 Hz for the horizontal and vertical forces, respectively.
The Optotrak Certus motion capture system (Northern Digital Inc.) is used for any experiment where kinematics are needed. The system offers an accuracy of up to 0.1 mm and resolution of 0.01 mm, which produces research-grade measurements for almost all biomechanic experiments. In addition, it offers a maximum marker frequency of 4600 Hz, and it can track up to 512 markers; making it a perfect device for full body experiments. Finally, the Optotrak Certus delivers true real-time data 100% of the time. The exceptionally low latency of this data makes it ideal for virtual reality and other feedback applications.
Four MX3 cameras are used for gait, upper and lower extremities motion experiments. These cameras provide a maximum frequency of 242 frames per second. The accuracy and resolution of these cameras depend on the test volume of the experiment, which can be adjusted. Finally, this device does not require wired markers, making the testing experience more comfortable for the subjects.
A treadmill (True model 500, O’Fallon, MO, USA) is used in the lab to facilitate lower-extremity fatigue. The treadmill features manual speed control ranging from 0 to 10 mph and grade control ranging from 0 to 15%. It is also equipped with a heart rate monitor that may be used to control the treadmill’s speed and grade in order to maintain a desired heart rate.
The EMG system is a Bagnoli 8-channel system manufactured by Delsys, Inc. Each output channel has selectable gains of 100, 1000, and 10,000 V/V with a frequency bandwidth of 20 – 450 Hz. Output voltage is ± 5V, which is accessible through BNC or composite connectors. This system is used with Delsys DE-3.1 double-differential surface electrodes, which feature 10 +2% V/V gain and supply voltage ranging from ± 4.5 V to ± 15 V at ± 2.5 mA.
Computer Hardware and Software
The Biodynamics Laboratory is equipped with three computers that are strictly dedicated to record data. One computer is used to record data either using a NI SCB-68 DAQ Board and LabVIEW, or a CED Power mkII and Spike2. Both of these data acquisition systems and programs are accepted in the literature, and can measure up to frequencies of 2000 Hz. The second computer is dedicated to record motion data using the Optotrak Certus; while the third computer is dedicated for motion data coming from the Vicon cameras. All systems can be used at the same time, since triggering electric circuits are available.