Characterisation of oxygen uptake response to linearly increasing work rate during robotics-assisted treadmill exercise in incomplete spinal cord injury

Abstract

Background Robotics-assisted treadmill exercise (RATE) is a new mode of exercise available to people with an incomplete spinal cord injury (SCI) that allows them to utilise their lower limb muscles during stepping. Pilot data suggest that RATE elicits a non-linear oxygen uptake ( V ˙ O 2 ) response corresponding to a linear increase in work rate. However, a linear V ˙ O 2 response during an incremental exercise test (IET) may be important to enable accurate estimation of key cardiopulmonary performance parameters. Aim This study aims to characterise the linearity of the V ˙ O 2 response elicited by a linearly increasing work rate during robotics-assisted treadmill exercise in subjects with incomplete SCI. Methods Utilising the Lokomat system, 10 subjects each performed two IETs on a robotics-assisted treadmill to the limit of their tolerance. By employing work rate estimation algorithms, subjects were asked to use cognitive feedback and volitional control of their contribution to the exercise to follow a linearly increasing target work rate that was displayed on screen. Pulmonary gas exchange and ventilatory measurements (including V ˙ O 2 ) were continuously measured throughout the exercise using a breath-by-breath respiratory monitoring system. Linear and 3rd-order non-linear approximations with comparable R 2 values were computed for each subject’s V ˙ O 2 response to the linear increasing work rate. Results R 2 values for the non-linear approximations were 9% higher on average ( p = 0.015 ) than the corresponding R 2 values for the linear approximations. Conclusion The V ˙ O 2 response elicited by a linearly increasing work rate during robotics-assisted treadmill exercise in those with incomplete SCI is non-linear. To ensure the intensity of exercise increases linearly, a more appropriate IET may be implemented by employing feedback control of V ˙ O 2 to track a linear target.