Abstract
This study was conducted to investigate the effect of concentrated beetroot juice on isometric strength and knee extensor muscle endurance in healthy adults. We conducted a randomized cross-over, double-blind experiment in which participants (18 healthy, physically active adults, 9 men, 9 women) consumed either concentrated beetroot juice (140 mL) or low-nitrate control supplement 2.5 h before the measurement. Isometric maximum strength (peak torque), explosive strength (isometric rate of torque development), and strength endurance at 50% of peak torque were measured on an isometric dynamometer. The results showed that concentrated beetroot juice had no effect on the maximum voluntary isometric strength and rate of torque development of the knee extensors. The only exception was the maximum rate of torque development, for which a positive influence was demonstrated only in men. As for the endurance of the knee extensors, the supplement had a positive effect in men (endurance time increased from 86.4 ± 46.1 s to 103.4 ± 53.7 s; p = 0.022), but not in women. The absence of effect on maximal voluntary strength is consistent with previous research. One the other hand, improvements in endurance and rate of torque development in men only point to an important aspect of a previously under-researched area of sex-specific responses to nitrate supplementation.