To design and construct a low-cost MR compatible ergometer that enables human subjects to perform a plantar flexion exercise protocol while inside a whole–body MR scanner.Phosphorus (³¹P) Magnetic Resonance Spectroscopy (MRS) is a well-established method for measuring the rate of phosphocreatine (PCr) resynthesis after exercise, which is an index of mitochondrial oxidative metabolism in the muscle, ^1-3 and has been used extensively to differentiate between normal and pathological states.^4-7 Acquisition of ³¹P-MR data during a rest-exercise-recovery protocol requires an ergometer that records the force exerted during exercise and is compatible with the high magnetic field of modern MR scanners. In this work, we present an in-house developed MR compatible ergometer, which enables human subjects to perform a plantar flexion exercise protocol while ³¹P-MR data are acquired.The mechanical body of the ergometer, including the foot pedal and mounting board (Fig.1) was made entirely of custom machined plastic to enable use in the MR environment. Forces applied to the pedal were continuously monitored with non-magnetic force transducer and angle sensors (Tekscan, MA). The analog force and angle signals were digitized outside the magnet on an Arduino Uno Rev 3 board and inputted to Matlab for real-time processing and display. The angle sensor (Fig.1b,d) triggered a LED on top of the pedal that provided real-time feedback to the subject, indicating when the predetermined flexion angle threshold had been reached (Fig.1c). Rubber bands were used to generate the defined forces that counteract the plantar flexion, ranging from 0 to approximately 250 N. The total cost of the plastic materials and electronics to construct the ergometer was under 500 USD. We tested the ergometer on a 7 T whole-body magnet (Siemens Medical Solutions, Erlangen, Germany) with a dual tuned (³¹P/¹H) quadrature transmit-receive kneel coil (Rapid MRI Ohio). A 32 year old male subject performed a 2.5-min plantar exercise at 0.33 Hz to an acoustic cue. The ergometer was used to measure plantar force exerted during the experiment, while PCr signal in the gastrocnemius was acquired with a spectrally selective three-dimensional turbo spin echo (TSE) sequence with a temporal resolution of 6-s.¹² A PCr image in a cross-section of the lower leg is shown in Figure 2a. During the plantar flexion, PCr signal decreases and recovers after the plantar flexion is stopped. The signal in a segmentation of the gastrocnemius is shown (Fig.2b). The forces generated during the plantar flexion exercise are continuously monitored as shown in Figure 2c.In this work, we designed and constructed a robust low-cost ergometer that can be used to quantify force generated inside the magnet while acquiring ³¹P-MR data. The main advantage of this pedal is that it is easy to construct at a very low cost (under 500 USD) and can be easiliy connected and placed on the magnet bed. Despite its low cost, it allows continuous measurement of the effort exerted by the exercising subject. That is mainly because of the use of inexpensive, yet efficient componensts, such as the Arduino microcontroller. One of the limitations of this ergometer is that resistance forces cannot be changed during the experiment. Such changes can be more easily performed using pneumatic resistances.¹¹ Also, visual feedback is only given by an LED light when subjects reach the desired displacement angle. More sophisticared approaches have incorporated MR compatible mounted displays, which would increase the total cost significantly.¹³We constructed a low cost MR compatible ergometer. This ergometer allows subjects to perform a plantar flexion exercise protocol while ³¹P-MR data are acquired. It is easy to build and can be used on whole-body MR scanners.