Background: Renal blood flow is important for maintaining kidney functions and is controlled by an autonomic nervous system (ANS). An ankle exercise is widely used for improving blood flow in lower limb, but its effects on renal blood flow have not been examined.
Objective: To investigate the effects of the active ankle movements (AAM) on renal arterial hemodynamics and autonomic nervous system (ANS) responses in community-dwelling elderly participants.
Design: The present study is a prospective randomized controlled trial. Twenty-six community-dwelling elderly participants were randomized to control (n = 13) and AAM (n = 13) groups.
Materials and Methods: The AAM group was instructed by physiotherapists to perform the combined active movements of plantar flexion, dorsi flexion, inversion, and eversion at the frequency of 60 times/min for 15 min; while the control group was instructed to rest for another 15 minute. Primary outcomes [volumetric arterial blood flow (VF), peak systolic velocity (PSV), end diastolic velocity (EDV), and resistive index (RI)] were determined by radiologists using a Doppler ultrasound on the left renal artery at baseline and post-intervention. Secondary outcomes [heart rate variability (HRV) parameters low-frequency spectral power (LF), high-frequency spectral power (HF), LF/HF ratio, standard deviation of normal-to-normal intervals (SDNN), and root mean square
of successive differences (RMSSD)] were determined as ANS proxies.
Results: After the AAM, VF in the left renal artery significantly increased from 327.7+47.4 to 402.7+56.5 ml/min (p<0.05) whereas PSV, EDV, and RI were not changed. HRV measurements showed that LF (a proxy of sympathetic cardiac control) decreased from 5.21+0.22 to 4.85+0.15 ms2 (p<0.05).
Conclusion: The AAM increased renal arterial blood flow putatively by reducing sympathetic input to the renal artery.

Keywords: Active ankle movements, Renal blood flow, Doppler ultrasound, Elderly, sympatho-vagal balance