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. 2017 Oct 16;7:13300. doi: 10.1038/s41598-017-13369-1

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© The Author(s) 2017

Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/.

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Pick-up volume and inter-electrode distance. A schematic representation of the tissue volume sampled by pairs of electrodes with different inter-electrode distances is shown. Dashed lines indicate the pick-up volume of individual pairs of electrodes; i.e., the region within which action potentials contribute substantially to the surface EMGs. By substantially we mean the amplitude of action potentials within the pick-up volume is at least 10% of that of action potentials located immediately below the recording electrodes^14,31. Note the number of gastrocnemius fibres included in the pick-up volume increases with the inter-electrode distance. Surface EMGs detected by electrodes A and C sample predominantly from the most distal gastrocnemius fibres; unrepresentative muscle sampling. By increasing the inter-electrode distance (electrodes B-D), surface EMGs sample from more proximal gastrocnemius fibres. After a certain distance (cf. electrodes C-E in Fig. 1), fibres from the deep, soleus muscle are expected to contribute substantially to the surface EMG; EMGs are not specific for gastrocnemius.