Optimal Intensity of Reactive Balance Training Post-stroke

Remaining active after stroke is essential to recovery, maintaining quality of life, and reducing secondary stroke risk. However, impaired balance control post-stroke increases fall risk, contributes to fear of falling, and reduces overall mobility, activity, and community integration. Therefore, effective interventions to improve balance control and reduce fall risk are essential to ongoing recovery and quality of life post-stroke. However, while there is unequivocal evidence that exercise, specifically balance training, is the most effective intervention for preventing falls in older adults without stroke, it is unclear if exercise prevents falls after stroke. Effective balance reactions (e.g., reactive stepping) are essential to avoid falling following a loss of balance, or balance perturbation. Reactive balance training (RBT), where clients experience repeated balance perturbations, is a novel type of exercise that aims to improve control of balance reactions. The investigators found that RBT improves reactive stepping ability in people with stroke. The investigators' recent meta-analysis found that RBT reduces rate of falls in daily life by ~40% among older adults and people with neurologic conditions, including stroke.

Clinicians are unsure how to optimally prescribe RBT. Exercise is often prescribed considering frequency, intensity, time, and type; these parameters are interdependent. For example, as intensity increases, frequency or time can decrease to achieve similar benefits. Clinicians report that limited time in rehabilitation services and competing rehabilitation goals and priorities mean that there is little time available to include RBT in a client's treatment plan. Therefore, it would be valuable to know if a short duration of high-intensity RBT can improve reactive balance control post-stroke. However, it is possible that people with stroke would not tolerate high intensity perturbations and, therefore, require lower-intensity but long-duration RBT. No study has directly compared different RBT training intensities among people with stroke.

The purpose of this study is to determine the optimal intensity of RBT post-stroke. The 'optimal' intensity is the intensity that improves reactive balance control in fewer sessions, without any apparent negative consequences (i.e., no increase in adverse outcomes). Responses to backward-fall perturbations will be assessed at the end of each session, and at the one-week retention time-point. An untrained perturbation (i.e., backward-fall perturbation) will be used to test transfer of learning to a novel context. Participants will experience 3 forward-directed platform translations, evoking a backward loss of balance, at the multi-step threshold. Because backward falls are more challenging than forward or lateral falls, a backward-fall perturbation at the forward-fall multi-step threshold should evoke a multi-step reaction, at least prior to training. Testing at each participant's multi-step threshold ensures that participants will initially experience challenges responding to the perturbation and there is room for improvement with training. The primary outcome will be number of steps taken to recover balance.

The investigators will calculate the learning rate for each participant by fitting an exponential function, a+b(e^(- t/x) ), to their data (average number of steps taken at each assessment time point), where a and b are constants, t is the assessment time point, and x is the learning rate.

The investigators expect that high-intensity training (50% above the multi-step threshold) will improve reactive stepping ability faster than moderate-intensity training (at the multi-step threshold). If the first reactive step is not effective in avoiding a fall after a loss of balance, additional steps must be taken; therefore, the number of steps needed to recover from a loss of balance is a global indicator of the effectiveness of balance reactions. The investigators will assess reactions to novel, untrained, balance perturbations throughout training, and calculate the rate of decline in average number of steps taken to recover balance (i.e., learning rate). Retention of learning will be assessed one week post-training. The investigators' primary hypotheses are:

1. Adaptation rate will be faster for high-intensity RBT than moderate-intensity RBT;
2. Adaptation rate will be faster for both RBT groups than a walking control group; and
3. Both RBT groups will have better retention of learning than the walking control group; i.e., the RBT groups will take fewer steps to respond to the novel perturbation than the walking control group one week post-training.

Secondary objectives are to compare the rate of adverse outcomes between groups, and to determine the effect of different intensities of RBT on the mechanisms underlying improved reactive stepping ability, functional balance, falls efficacy, and participation in daily activities.