The effects of hypoxia on muscle deoxygenation and recruitment in the flexor digitorum superficialis during submaximal intermittent handgrip exercise

Nell et al. BMC Sports Science, Medicine and Rehabilitation https://doi.org/10.1186/s13102-020-00163-2 (2020) 12:16 RESEARCH ARTICLE Open Access The effects of hypoxia on muscle deoxygenation and recruitment in the flexor digitorum superficialis during submaximal intermittent handgrip exercise Hayley J. Nell1, Laura M. Castelli1, Dino Bertani1, Aaron A. Jipson1, Sean F. Meagher1, Luana T. Melo1 , Karl Zabjek1,2 and W. Darlene Reid1,2,3* Abstract Background: Decreased oxygenation of muscle may be accentuated during exercise at high altitude. Monitoring the oxygen saturation of muscle (SmO2) during hand grip exercise using near infrared spectroscopy during acute exposure to hypoxia could provide a model for a test of muscle performance without the competing cardiovascular stresses that occur during a cycle ergometer or treadmill test. The purpose of this study was to examine and compare acute exposure to normobaric hypoxia versus normoxia on deoxygenation and recruitment of the flexor digitorum superficialis (FDS) during submaximal intermittent handgrip exercise (HGE) in healthy adults. Methods: Twenty subjects (11 M/9 F) performed HGE at 50% of maximum voluntary contraction, with a duty cycle of 2 s:1 s until task failure on two occasions one week apart, randomly assigned to normobaric hypoxia (FiO2 = 12%) or normoxia (FiO2 = 21%). Near-infrared spectroscopy monitored SmO2, oxygenated (O2Hb), deoxygenated (HHb), and total hemoglobin (tHb) over the FDS. Surface electromyography derived root mean square and mean power frequency of the FDS. Results: Hypoxic compared to normoxic HGE induced a lower FDS SmO2 (63.8 ± 2.2 vs. 69.0 ± 1.5, p = 0.001) and both protocols decreased FDS SmO2 from baseline to task failure. FDS mean power frequency was lower during hypoxic compared to normoxic HGE (64.0 ± 1.4 vs. 68.2 ± 2.0 Hz, p = 0.04) and both decreased mean power frequency from the first contractions to task failure (p = 0.000). Under both hypoxia and normoxia, HHb, tHb and root mean square increased from baseline to task failure whereas O2Hb decreased and then increased during HGE. Arterial oxygen saturation via pulse oximetry (SpO2) was lower during hypoxia compared to normoxia conditions (p = 0.000) and heart rate and diastolic blood pressure only demonstrated small increases. Task durations and the tension-time index of HGE did not differ between normoxic and hypoxic trials. (Continued on next page) * Correspondence: 1 Department of Physical Therapy, University of Toronto, 160-500 University Avenue, Toronto, ON M5G 1V7, Canada 2 KITE, Toronto Rehab-University Health Network, 550 University Ave, Toronto, ON M5G 2A2, Canada Full list of author information is available at the end of the article © The Author(s). 2020 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 licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence 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 licence, visit http://creativecommons.org/licenses/by/4.0/. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated in a credit line to the data. Nell et al. BMC Sports Science, Medicine and Rehabilitation (2020) 12:16 Page 2 of 10 (Continued from previous page) Conclusion: Hypoxic compared to normoxic HGE decreased SmO2 and induced lower mean power frequency in the FDS, during repetitive hand grip exercise however did not result in differences in task durations or tension-time indices. The fiber type composition of FDS, and high duty cycle and intensity may have contributed greater dependence on anaerobiosis. Keywords: Near-infrared spectroscopy, Hypoxia, Exercise, Skeletal muscle Introduction Optimal oxygen delivery and utilization by skeletal muscle is essential to maximize muscle performance and exercise capacity. It is well established that poor delivery of oxygen to muscles greatly compromises performance and exercise capacity [1, 2]. Previous studies have demonstrated that acute exposure to hypoxia impairs muscle endurance and performance in large muscle groups [3, 4]. Similarly, the effects of hypoxia on smaller muscle groups has also been associated with reduced muscular endurance and performance, albeit to a lesser degree [5]. High-intensity intermittent static contractions of adductor pollicis muscle resulted in reduced endurance times during hypobaric hypoxia compared with normoxia [5–7]. A review by Perrey & Rupp outlined that acute hypoxic exposure when compared to normoxic conditions leads to a decline in muscular endurance time when protocols employed submaximal intermittent isometric contractions [8]. On the other hand, acute hypoxia exposure on maximal voluntary force generating capacity of small muscle groups appears to have minimal to no reduction in force production and the rate of decline of force compared to normoxic conditions [6, 9]. Near-infrared spectroscopy (NIRS) can estimate muscle oxygenation during exercise via continuous-wave emissions of near-infrared at approximately 760 nm and 850 nm, which is absorbed by deoxygenated (HHb) and oxygenated hemoglobin (O2Hb), respectively. The change in concentration of these chromophores can be estimated from the modified Beer-Lambert law, which compensates for light scattering during emission through tissues [10]. Using spatially resolved NIRS, the saturation of muscle oxygenation (SmO2) can be quantified [11]. NIRS has been shown to be reliable [12], and valid to measure muscle oxygenation at rest [13] and during exercise [14, 15]. Its wireless, non-invasive application provides an unobtrusive tool for measuring muscle oxygenation to further explore the range of deoxygenation to improve muscle function while minimizing adverse effects. Muscle oxygenation and deoxygenation profiles in small muscle groups such as the forearm and finger flexors is of particular relevance to the climbing population, where finger and grip strength is of upmost importance. Previous studies in climbers examining muscle oxygenation in small muscle groups during continuous and intermittent isometric testing protocols have shown a greater level of muscle deoxygenation and faster rates of reoxygenation in elite climbers compared to controls [16–18]. When examining muscle oxygenation profiles in small muscle grou (...truncated)