One limitation with how balance assessments are often done is that they heavily emphasize reflexive actions without a need to revise automatic postural reactions. Such an exclusive focus on highly stereotypical reactions would fail to address how we can modify such reactions should the need arise. For example, avoiding an obstacle with a recovery step.
The present protocol offers a way to require suppression of automatic yet inappropriate corrective balance reactions and force a selection among alternative action choices to successfully recover balance following postural perturbation. What is unique in the current approach is that access to vision is precisely controlled while the subject remains fixed and their response environment is altered around them to create different action opportunities and/or constraints. By manipulating the presence of obstacles and affordances for action, this method emphasizes cognitive processes such as decision making and response inhibition in relation to balance recovery.
Prior to testing, have the participant provide written informed consent to procedures. Acquire data and bandpass filter using a data acquisition interface and appropriate software. After fixing the surface EMG electrodes onto two intrinsic hand muscles on the right hand and ankle dorsiflexors on both legs, collect EMG data from the target muscles.
Use a custom made lean and release cable system to impose forward perturbations. Have the participant stand in a forward lean position with their feet approximately hip width apart. To control vision, put on liquid crystal goggles for the participant and limit the vision to the timeframe just before postural perturbation.
When closed, the goggles prevent access to the visual scene so the participant is unaware of the forthcoming response condition. To maintain this forward lean, fasten a support cable to the back of a body harness. Fix the support cable to the wall by a magnet.
Verify that the participant can actually see the handle and leg block when wearing the goggles. Begin each trial by instructing the participant to look directly at a fixed point on the floor about three meters in front of them while holding their head in a comfortable position. Position the body to ensure that the handle is within graspable range.
Have the participant lean forward while keeping both feet in contact with the floor. During trials where the handle is uncovered, a leg block is presented in front of the participant's legs. The leg block impedes a step but is not rigidly set in place.
On certain trials, use a black tarp to cover the handle to prevent direct visual access and to prevent any supportive grasp. Then remove the leg block to allow a step reaction if necessary. Change the specific configuration of the leg block and handle availability for each trial while the goggles are closed so that the participant needs to quickly perceive the environment once the goggles open.
Move the handle cover and the leg block into position via computer-triggered servo motors at the start of each trial. Prior to testing, briefly familiarize the participant with how to reach the handle and step forward from a leaning position. Throughout testing and practice, instruct the participant to remain relaxed unless prompted to move by a sudden cable release.
Randomly change the response setting between trials. If released from the support cable, the participant must regain stability by either reaching for the wall-mounted safety handle or stepping forward if the step path is clear. Always close the occlusion goggles at the beginning of each trial at which time the response setting will be altered.
Close the goggles for a randomized period usually three to four seconds to allow the setting to change. When the goggles open, provide one of two possible response settings. First, the leg block is present and the support handle is present, or second, no leg block is present and no support handle is present.
On trials where a perturbation does occur, release the cable shortly after the goggles open. Have each trial last 10 seconds with a short pause between trials to allow participants a chance to reset as needed. Give participants a brief rest period in between each test block and allow them to sit.
In one version of our balance task, a stepping response is more frequent and thus more automatic. For example, 30%of trials use a leg block. The average waveforms of individuals that were on opposite ends of the continuum for suppressing step-related leg activity are shown here.
The scatterplot depicts a small but significant correlation between the ability to suppress a blocked step and response inhibition as measured by the stop signal reaction time which is a measure derived from a seated cognitive task. In a separate study using transcranial magnetic stimulation, the results indicate that simply viewing a support handle resulted in facilitation of an intrinsic hand muscle. This figure depicts the size of the motor evoked potential in different conditions.
Another separate study found evidence of a widespread shutdown across the motor system when an automatic step is abruptly stopped. In this particular study, the handle was not available. Properly positioning the participant is critical to ensure they can see the handle and leg block and that these items are within grasping and kicking range.
Another technique that we have used is transcranial magnetic stimulation to probe activation of the motor system. This provides insight into preparatory brain mechanisms in a reactive balance context. Our approach emphasizes the need for higher brain processes to avoid a fall.
Consequently, this could be used to expose brain deficits in clinical groups with cognitive decline.
