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Assessment of the kinetics of replenishment of the phosphocreatine pool after exercise using <jats:sup>31<\/jats:sup>P magnetic resonance spectroscopy can provide an <jats:italic>in vivo<\/jats:italic> measure of mitochondrial function; however, the wider application of this technique appears limited by complex or expensive MR-compatible exercise equipment and protocols not easily tolerated by frail participants or those with reduced mental capacity. Here we describe a novel in-scanner exercise method which is patient-focused, inexpensive, remarkably simple and highly portable. The device exploits an MR-compatible high-density material (BaSO<jats:sub>4<\/jats:sub>) to form a weight which is attached directly to the ankle and a one-minute dynamic knee extension protocol produced highly reproducible measurements of post-exercise PCr recovery kinetics in both healthy subjects and patients. As sophisticated exercise equipment is unnecessary for this measurement, our extremely simple design provides an effective and easy-to-implement apparatus that is readily translatable across sites. Its design, being tailored to the needs of the patient, makes it particularly well suited to clinical applications and we argue the potential of this method for investigating <jats:italic>in vivo<\/jats:italic> mitochondrial function in new cohorts of growing clinical interest.<\/jats:p>","DOI":"10.1038\/srep19057","type":"journal-article","created":{"date-parts":[[2016,1,11]],"date-time":"2016-01-11T10:05:06Z","timestamp":1452506706000},"update-policy":"https:\/\/doi.org\/10.1007\/springer_crossmark_policy","source":"Crossref","is-referenced-by-count":20,"title":["Simple and effective exercise design for assessing in vivo mitochondrial function in clinical applications using 31P magnetic resonance spectroscopy"],"prefix":"10.1038","volume":"6","author":[{"given":"Alison","family":"Sleigh","sequence":"first","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Victoria","family":"Lupson","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Ajay","family":"Thankamony","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"David B.","family":"Dunger","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"David B.","family":"Savage","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"T. 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