Skeletal muscle phosphorus-31 31 P MRS is the oldest MRS methodology to be applied to in vivo metabolic research. The technical requirements of 31 P MRS in skeletal muscle depend on the research question, and to assess those questions requires understanding both the relevant muscle physiology, and how 31 P MRS methods can probe it. Here we consider basic signal-acquisition parameters related to radio frequency excitation, TR, TE, spectral resolution, shim and localisation. We make specific recommendations for studies of resting and exercising muscle, including magnetisation transfer, and for data processing. We summarise the metabolic information that can be quantitatively assessed with 31 P MRS, either measured directly or derived by calculations that depend on particular metabolic models, and we give advice on potential problems of interpretation. We give expected values and tolerable ranges for some measured quantities, and minimum requirements for reporting acquisition parameters and experimental results in publications. Reliable examination depends on a reproducible setup, standardised preconditioning of the subject, and careful control of potential difficulties, and we summarise some important considerations and potential confounders. Our recommendations include the quantification and standardisation of contraction intensity, and how best to account for heterogeneous muscle recruitment. We highlight some pitfalls in the assessment of mitochondrial function by analysis of phosphocreatine (PCr) recovery kinetics. Finally, we outline how complementary techniques (near-infrared spectroscopy, arterial spin labelling, BOLD and various other MRI and 1 H MRS measurements) can help in the physiological/metabolic interpretation of 31 P MRS studies by providing information about blood flow and oxygen delivery/utilisation. Our recommendations will assist in achieving the fullest possible reliable picture of muscle physiology and pathophysiology.

C J Gorter Center for High Field MRI Department of Radiology Leiden University Medical Centre Leiden the Netherlands

Center for Child Development and Exercise Wilhelmina Children's Hospital University Medical Center Utrecht Utrecht the Netherlands

Center for Medical Physics and Biomedical Engineering Medical University of Vienna Vienna Austria

Cognitive Neuroscience Center University Medical Center Groningen Groningen the Netherlands

DBMR and DIPR University and Inselspital Bern Switzerland

Department of Imaging Methods Institute of Measurement Science Slovak Academy of Sciences Bratislava Slovakia

Department of Kinesiology University of Massachusetts Amherst MA USA

Department of Musculoskeletal Biology and Liverpool Magnetic Resonance Imaging Centre University of Liverpool Liverpool UK

Department of Physical Therapy University of Florida Gainesville Florida USA

Department of Radiology Amsterdam University Medical Center|site AMC Amsterdam the Netherlands

Department of Radiology and Nuclear Medicine Radboud University Medical Center Nijmegen The Netherlands

Department of Radiology University Medical Center Utrecht the Netherlands

Duchenne Center The Netherlands

High Field MR Center Medical University of Vienna Vienna Austria

Institute for Applied Life Sciences University of Massachusetts Amherst MA USA

MR Unit Department of Diagnostic and Interventional Radiology Institute for Clinical and Experimental Medicine Prague Czech Republic

NIHR Wellcome Trust Clinical Research Facility Cambridge University Hospitals NHS Foundation Trust Cambridge UK

NMR Laboratory Neuromuscular Investigation Center Institute of Myology AIM CEA Paris France

Norwich Medical School University of East Anglia Norwich UK

Oxford Centre for Clinical Magnetic Resonance Research RDM Cardiovascular Medicine BHF Centre of Research Excellence University of Oxford Oxford UK

Wellcome Trust MRC Institute of Metabolic Science University of Cambridge Cambridge UK

Wolfson Brain Imaging Centre University of Cambridge Cambridge UK

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