Dietary Nitrate Supplementation Improves Exercise Tolerance by Reducing Muscle Fatigue and Perceptual Responses

Abstract

The present study was designed to provide further insight into the mechanistic basis for the improved exercise tolerance following dietary nitrate supplementation. In a randomized, double-blind, crossover design, twelve recreationally active males completed a dynamic time-to-exhaustion test of the knee extensors after 5 days of consuming both nitrate-rich (NITRATE) and nitrate-depleted beetroot juice (PLACEBO). Participants who improved their time-to-exhaustion following NITRATE performed a time-matched trial corresponding to the PLACEBO exercise duration with another 5 days of dietary nitrate supplementation. This procedure was performed to obtain time-matched exercise trials with (NITRATE_tm) and without dietary nitrate supplementation (PLACEBO). Neuromuscular tests were performed before and after each time-matched condition. Muscle fatigue was quantified as percentage change in maximal voluntary torque from pre- to post-exercise (ΔMVT). Changes in voluntary activation (ΔVA) and quadriceps twitch torque (ΔPS100) were used to quantify central and peripheral factors of muscle fatigue, respectively. Muscle oxygen saturation, quadriceps muscle activity as well as perceptual data (i.e., perception of effort and leg muscle pain) were recorded during exercise. Time-to-exhaustion was improved with NITRATE (12:41 ± 07:18 min) compared to PLACEBO (09:03 ± 04:18 min; P = 0.010). NITRATE_tm resulted in both lower ΔMVT and ΔPS100 compared to PLACEBO (P = 0.002; P = 0.001, respectively). ΔVA was not different between conditions (P = 0.308). NITRATE_tm resulted in reduced perception of effort and leg muscle pain. Our findings extend the mechanistic basis for the improved exercise tolerance by showing that dietary nitrate supplementation (i) attenuated the development of muscle fatigue by reducing the exercise-induced impairments in contractile muscle function; and (ii) lowered the perception of both effort and leg muscle pain during exercise.

Keywords: beetroot juice; central fatigue; contractile function; muscle pain; performance fatigability; peripheral fatigue.

FIGURE 1

(A) Illustration of the experimental design. A time-to-exhaustion test of the knee extensors was performed after 5 days of dietary nitrate (NITRATE) and PLACEBO supplementation. Neuromuscular function of the quadriceps muscle was assessed before and immediately after both PLACEBO and time-matched dietary nitrate condition (NITRATE_tm). Effort perception and leg muscle pain were recorded every min during exercise. Electromyography (EMG) and near-infrared spectroscopy (NIRS) data were continuously recorded during exercise. (B) The neuromuscular testing procedure comprised isometric MVC of the knee extensors combined with electrical stimulation to assess maximal voluntary torque (MVT), voluntary activation (via the interpolated twitch technique), and quadriceps twitch torques in response to paired electrical stimuli at 100 Hz (PS100) and at 10 Hz (PS10) as well as single stimuli (SS). A representative torque-time curve of the neuromuscular assessment procedure can be found in a previous publication from our group (Husmann et al., 2018).

FIGURE 2

Mean values and individual data for time-to-exhaustion (s) between experimental conditions. Please note that eight out of twelve participants improved their performance over a range from ∼9 to ∼51%. Significantly different between conditions: ^∗P ≤ 0.05.

FIGURE 3

Percentage change from pre-exercise values for maximal voluntary torque (MVT), twitch torque in response to paired (PS100) and single (SS) electrical stimuli, PS10 ⋅ PS100^-1 ratio, and voluntary activation (%VA) across both PLACEBO and time-matched dietary nitrate condition (NITRATE_tm). Values are presented as mean ± SD. Significantly different between conditions: ^∗∗P ≤ 0.01, ^∗∗∗P ≤ 0.001.

FIGURE 4

Percentage increase from baseline for the normalized muscle activity of the quadriceps muscle (Q RMS ⋅ M^-1). To estimate the total muscle activity of the quadriceps muscle, RMS ⋅ M^-1 was averaged across vastus medialis, rectus femoris, and vastus lateralis for five contractions at 25, 50, 75, and 100% of each trial, respectively. Baseline was defined as the average of the first five contractions. Values are presented as mean ± SD. NITRATE_tm, time-matched dietary nitrate condition; RMS ⋅ M^-1, the root mean square of the EMG signal normalized to M_max. Significantly different between conditions: ^∗∗P ≤ 0.01, ^∗∗∗P ≤ 0.001.