Christina D Bruce, Mathew I B Debenham, Brian H Dalton, Chris J McNeil
{"title":"Acute normobaric hypoxia causes a rightward shift in the torque-frequency relationship but has no effect on postactivation potentiation.","authors":"Christina D Bruce, Mathew I B Debenham, Brian H Dalton, Chris J McNeil","doi":"10.1152/japplphysiol.00378.2024","DOIUrl":null,"url":null,"abstract":"<p><p>Low fractions of inspired oxygen ([Formula: see text]; i.e., hypoxia) affect aspects of skeletal muscle contractility in humans, but it remains unclear if postactivation potentiation (PAP) and the torque-frequency (T-F) relationship are altered. We investigated the effects of 2 (H2) and 4 (H4) h of normobaric hypoxia ([Formula: see text] = 0.11 ± 0.47) on the magnitude of PAP of the knee extensors and the T-F relationship of the dorsiflexors in 13 and 12 healthy participants, respectively. To assess PAP, a resting twitch was evoked via femoral nerve stimulation before and 2-300 s after a 10-s maximal voluntary contraction (MVC). A T-F relationship was obtained by stimulating the common fibular nerve with a single pulse and 1-s trains between 5 and 100 Hz. During hypoxia, peripheral oxygen saturation decreased by ∼18% from 98.0 ± 0.8% at baseline (<i>P</i> < 0.001). MVC force and voluntary activation (VA) of the knee extensors were lower than baseline throughout hypoxia (e.g., ∼8% and ∼5%, respectively, at H2; <i>P</i> ≤ 0.027); however, the magnitude of PAP was not altered by hypoxia (<i>P</i> ≥ 0.711). Surprisingly, PAP did increase with time across the control day (<i>P</i> ≤ 0.012). MVC torque and VA of the dorsiflexors were unaffected by hypoxia (<i>P</i> ≥ 0.127), but the estimated frequency required to evoke 50% of 100 Hz torque increased by ∼1.2 Hz at H2 (<i>P</i> ≤ 0.021). These results imply that 2 h of normobaric hypoxia were sufficient to <i>1</i>) impair neural drive to the knee extensors but not the mechanism(s) responsible for PAP and <i>2</i>) lead to a rightward shift of the T-F relationship for the dorsiflexors.<b>NEW & NOTEWORTHY</b> Postactivation potentiation of the knee extensors was unaffected by 4 h of normobaric hypoxia exposure but may be confounded by hypoxia-related impairments to the conditioning contraction. In the dorsiflexors, contractile rates increased in hypoxia, which led to a rightward shift of the torque-frequency relationship, such that a higher frequency was required to obtain 50% of maximal torque. These results expand our understanding of the acute effects of hypoxia on skeletal muscle function.</p>","PeriodicalId":15160,"journal":{"name":"Journal of applied physiology","volume":" ","pages":"1389-1396"},"PeriodicalIF":3.3000,"publicationDate":"2024-11-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":"0","resultStr":null,"platform":"Semanticscholar","paperid":null,"PeriodicalName":"Journal of applied physiology","FirstCategoryId":"3","ListUrlMain":"https://doi.org/10.1152/japplphysiol.00378.2024","RegionNum":3,"RegionCategory":"医学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":null,"EPubDate":"2024/9/19 0:00:00","PubModel":"Epub","JCR":"Q1","JCRName":"PHYSIOLOGY","Score":null,"Total":0}
引用次数: 0
Abstract
Low fractions of inspired oxygen ([Formula: see text]; i.e., hypoxia) affect aspects of skeletal muscle contractility in humans, but it remains unclear if postactivation potentiation (PAP) and the torque-frequency (T-F) relationship are altered. We investigated the effects of 2 (H2) and 4 (H4) h of normobaric hypoxia ([Formula: see text] = 0.11 ± 0.47) on the magnitude of PAP of the knee extensors and the T-F relationship of the dorsiflexors in 13 and 12 healthy participants, respectively. To assess PAP, a resting twitch was evoked via femoral nerve stimulation before and 2-300 s after a 10-s maximal voluntary contraction (MVC). A T-F relationship was obtained by stimulating the common fibular nerve with a single pulse and 1-s trains between 5 and 100 Hz. During hypoxia, peripheral oxygen saturation decreased by ∼18% from 98.0 ± 0.8% at baseline (P < 0.001). MVC force and voluntary activation (VA) of the knee extensors were lower than baseline throughout hypoxia (e.g., ∼8% and ∼5%, respectively, at H2; P ≤ 0.027); however, the magnitude of PAP was not altered by hypoxia (P ≥ 0.711). Surprisingly, PAP did increase with time across the control day (P ≤ 0.012). MVC torque and VA of the dorsiflexors were unaffected by hypoxia (P ≥ 0.127), but the estimated frequency required to evoke 50% of 100 Hz torque increased by ∼1.2 Hz at H2 (P ≤ 0.021). These results imply that 2 h of normobaric hypoxia were sufficient to 1) impair neural drive to the knee extensors but not the mechanism(s) responsible for PAP and 2) lead to a rightward shift of the T-F relationship for the dorsiflexors.NEW & NOTEWORTHY Postactivation potentiation of the knee extensors was unaffected by 4 h of normobaric hypoxia exposure but may be confounded by hypoxia-related impairments to the conditioning contraction. In the dorsiflexors, contractile rates increased in hypoxia, which led to a rightward shift of the torque-frequency relationship, such that a higher frequency was required to obtain 50% of maximal torque. These results expand our understanding of the acute effects of hypoxia on skeletal muscle function.
期刊介绍:
The Journal of Applied Physiology publishes the highest quality original research and reviews that examine novel adaptive and integrative physiological mechanisms in humans and animals that advance the field. The journal encourages the submission of manuscripts that examine the acute and adaptive responses of various organs, tissues, cells and/or molecular pathways to environmental, physiological and/or pathophysiological stressors. As an applied physiology journal, topics of interest are not limited to a particular organ system. The journal, therefore, considers a wide array of integrative and translational research topics examining the mechanisms involved in disease processes and mitigation strategies, as well as the promotion of health and well-being throughout the lifespan. Priority is given to manuscripts that provide mechanistic insight deemed to exert an impact on the field.