Beetroot Juice Increases Skeletal Muscle Force Production in Recreationally Active Subjects.

Abstract

Supplementation with dietary inorganic nitrate (NO3-) in the form of beetroot juice (BRJ) reduces whole-body oxygen consumption during submaximal exercise. There are currently two proposed mechanisms to explain this decrease; 1) improved mitochondrial function and 2) improved mechanical efficiency. Previous research has shown that mitochondrial bioenergetics are not improved following 7-d of BRJ supplementation, however there was an increase in H2O2 emission. As reactive oxygen species (ROS) have been shown to increase skeletal muscle (SkM) contractile force, this presents a possible mechanism for the ergogenic effects of BRJ. PURPOSE: To examine the effects of 7-d of BRJ supplementation on SkM contractile characteristics and function. METHODS: Young recreationally active males (n=8) underwent transcutaneous electrical muscle stimulation of the vastus lateralis for evaluation of SkM contractile characteristics pre and post 7-d of BRJ supplementation (280 ml/day, ~26 mmol NO3-). Subjects performed 2 isometric maximal voluntary contractions (MVC), which were used for determination of a voltage that elicited ~40% MVC at 100 Hz. This was subsequently used for performance of a force frequency (FF) curve (10, 20, 30, 50, 100 Hz). Muscle twitches were evoked using a single supramaximal (~150 V, 50 μs duration) impulse twice prior to and twice following performing the FF curve. RESULTS: Following supplementation, there was no change in force produced during MVC (602.1 ± 50.4 vs. 596.6 ± 56.4 N), however peak twitch tension was increased (132.6 ± 3.3 vs. 153.5 ± 6.0 N, p<0.01). While there was no change in twitch half-relaxation time, the maximal rates of force development and relaxation were increased (3529 ± 251 vs. 2455 ± 135 and -2676 ± 283 vs. -2010 ± 168 N/s, respectively, p<0.05). Force was also increased during the FF curve at 10 Hz (41.1 ± 2.3 vs. 37.6 ± 2.4 % of peak force, p<0.05), but was not different at other frequencies. CONCLUSION: Following 7-d of BRJ supplementation, force production at low stimulation frequencies is increased in human SkM. In addition to the increased rates of force development and relaxation, these results suggest a possible mechanism of action for BRJ is via modification of calcium handling or calcium sensitivity of the contractile apparatus, possibly due to ROS. Supported by NSERC, Canada.