TO THE EDITOR: In their Viewpoint, Adami and Rossiter (1) eloquently explore the strengths and weaknesses of combining near-infrared spectroscopy (NIRS) with repetitive postexercise arterial cuff occlusions to measure skeletal muscle oxidative capacity in vivo. Before the advent of this technique, the assessment of mitochondrial function was limited to invasive muscle biopsies and/or expensive and time-consuming magnetic resonance spectroscopy (MRS) techniques. Now, clinicians and researchers

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... ike have a robust, high-throughput clinical platform to noninvasively assess muscle oxidative capacity across a wide range of muscles and disease states. That this approach is easily transportable and relatively low cost opens new possibilities for bedside medicine, clinical decision making, and incorporation into large multicenter clinical trials. However, this increased emphasis on clinical populations also emphasizes the need for future technology development to overcome the well-established limitations of NIRS with regards to limb adiposity (2). While Adami and Rossiter appropriately highlight the relatively large testretest variability with this technique compared with the differences often observed between health and disease, it is important to emphasize that this variability is similar to that achieved with MRS (5) or muscle biopsies (4). This does highlight the importance of careful experimental/clinical design, however. Indeed, over 40% of the investigations referenced in this Viewpoint studied locomotor muscle groups, which are highly dependent on physical activity level (3). Targeting nonlocomotor muscle groups should help limit within-group variability. With these considerations in mind, we believe NIRS-derived muscle oxidative capacity assessments hold great promise in clinical translational medicine. 2. Ferrari M, Mottola L, Quaresima V. Principles, techniques, and limitations of near infrared spectroscopy. . 3. Larsen RG, Callahan DM, Foulis SA, Kent-Braun JA. Age-related changes in oxidative capacity differ between locomotory muscles and are associated with physical activity behavior. Appl Physiol Nutr Metab 37: 88 -99, 2012. doi:10.1139/h11-135. 4. Ryan TE, Brophy P, Lin CT, Hickner RC, Neufer PD. Assessment of in vivo skeletal muscle mitochondrial respiratory capacity in humans by near-infrared spectroscopy: a comparison with in situ measurements. . A crossvalidation of near-infrared spectroscopy measurements of skeletal muscle oxidative capacity with phosphorus magnetic resonance spectroscopy.