Abstract
Dietary nitrate (NO3−) supplementation, which can enhance performance in exercise settings involving repeated high-intensity efforts, has been linked to improved skeletal muscle contractile function. Although muscular strength is an important component of explosive movements and sport-specific skills, few studies have quantified indices of muscular strength following NO3− supplementation, particularly isokinetic assessments at different angular velocities. We performed a systematic review and meta-analysis to determine whether dietary NO3− supplementation improves peak torque, as assessed by the gold standard method of isokinetic dynamometry, and if this effect was linked to the angular velocity imposed during the assessment. Dialnet, Directory of Open Access Journals, MEDLINE, PubMed, SciELO, Scopus, and SPORTDiscus were searched for articles using the following search strategy: (nitrate OR beet*) AND (supplement* OR nutr* OR diet*) AND (isokinetic OR strength OR “resistance exercise” OR “resistance training” OR “muscular power”). The meta-analysis of data from 5 studies with 60 participants revealed an overall effect size of −0.01 for the effect of nitrate supplementation on isokinetic peak torque, whereas trivial effect sizes ranging from −0.11 to 0.16 were observed for independent velocity-specific (90°/s, 180°/s, 270°/s, and 360°/s) isokinetic peak torque. Four of the five studies indicated that dietary NO3− supplementation is not likely to influence voluntary knee extensor isokinetic torque across a variety of angular velocities. These results suggest that NO3− supplementation does not influence isokinetic peak torque, but further work is required to elucidate the potential of NO3− supplementation to influence other indices of muscular strength, given the dearth of experimental evidence on this topic.
Highlights
Performance outcome in a variety of sports is determined by the ability to perform a range of rapid, dynamic, and explosive movements such as sprinting, changing direction, jumping, and exhibiting sport-specific skills
Isokinetic dynamometry was used in all studies to assess voluntary peak torque production at various angular velocities ranging from stationary (0◦ /s) to fast movement (360◦ /s)
Knee extension and flexion were performed at 60◦ /s and 240◦ /s [40], respectively, while in the remaining studies, knee extensions were perfomed at 0◦ /s, 90◦ /s, 180◦ /s, 270◦ /s, and 360◦ /s [30,31,32,48]
Summary
Performance outcome in a variety of sports is determined by the ability to perform a range of rapid, dynamic, and explosive movements such as sprinting, changing direction, jumping, and exhibiting sport-specific skills (e.g., kicking, throwing, or hitting a ball). It is important to note that there are a variety of methods to quantify force production (i.e., strength), such as isokinetic dynamometry, isometric assessments, resistance exercise protocols (e.g., one-repetition maximum (1RM), 3–10RM), and field tests (e.g., hand-grip and hand-held dynamometry, and body mass muscle testing such as vertical jump height) [3,4]. Isokinetic strength measures have limited translation to sport-specific tasks, since these are rarely completed at an isokinetic pace [6], isokinetic assessments have several important advantages. These conducting assessments against a maximal resistance throughout the complete range of motion of a joint at different movement velocities [7], and the possibility of measuring torque [8]. Isokinetic dynamometry has been proposed as a gold standard method of strength assessment [11,12,13,14]
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