Neuromuscular changes associated with superior fatigue resistance in African runners.

BACKGROUND: An enhanced ability to resist fatigue has been proposed as one of African runners' performance superiority sources, although their fatigue resistance during sustained non-running-specific activities remain unclear. This study aimed to compare fatigue resistance during sustained isometric exercise between performance-matched African and European runners. METHODS: Thirty long-distance runners (16 African, 14 European) performed submaximal fatiguing sustained isometric knee extensions. Rectus femoris electromyographic (EMG) activity was measured, and the muscle was electrically stimulated to contract at the beginning and end of the test. RESULTS: Time to task failure was greater for African than European runners (269±115 vs. 193±52 s, P=0.002; effect size [ES]=0.85, large effect). During the test, EMG amplitude increased less over time (P=0.031), and the left shift in the EMG frequency spectrum was less over time for the African runners (P<0.001). In addition, there was a lower relative reduction in stimulated force output from the first to the second stimulation in African runners (17.75±14.95 vs. 37.89±14.78%, P=0.006; ES=1.35; large effect). CONCLUSIONS: These findings of greater fatigue resistance during non-running-specific activity and the associated muscle recruitment profile may contribute to the understanding of the physiology underlying endurance performance in African runners.

The effect of induced alkalosis and submaximal cycling on neuromuscular response during sustained isometric contraction.

The aim of this study was to determine if inducing metabolic alkalosis would alter neuromuscular control after 50 min of standardized submaximal cycling. Eight trained male cyclists (mean age 32 years, s = 7; [Vdot]O(2max) 62 ml . kg(-1) x min(-1), s = 8) ingested capsules containing either CaCO(3) (placebo) or NaHCO(3) (0.3 g x kg(-1) body mass) in eight doses over 2 h on two separate occasions, commencing 3 h before exercise. Participants performed three maximal isometric voluntary contractions (MVC) of the knee extensors while determining the central activation ratio by superimposing electrical stimulation both pre-ingestion and post-exercise, followed by a 50-s sustained maximal contraction in which force, EMG amplitude, and muscle fibre conduction velocity were assessed. Plasma pH, blood base excess, and plasma HCO(3) were higher (P < 0.01) during the NaHCO(3) trial. After cycling, muscle fibre conduction velocity was higher (P < 0.05) during the 50-s sustained maximal contraction with NaHCO(3) than with placebo (5.1 m x s(-1), s = 0.4 vs. 4.2 m x s(-1), s = 0.4) while the EMG amplitude remained the same. Force decline rate was less (P < 0.05) during alkalosis-sustained maximal contraction and no differences were shown in central activation ratio. These data indicate that induced metabolic alkalosis can increase muscle fibre conduction velocity following prolonged submaximal cycling.

Agonist-antagonist common drive during fatiguing knee extension efforts using surface electromyography.

AIM: This study examined the electromyographic (EMG) activity of knee extensor agonists and a knee extensor antagonist muscle during fatiguing isometric extensions across a range of force levels. METHODS: Five female subjects performed isometric knee extensions at 25%, 50%, 75% and 100% of their maximal voluntary contraction (MVC) with the knee flexed to 75 degrees. Surface EMG (SEMG) was recorded with bipolar electrodes from the vastus lateralis (VL), vastus medialis (VM), rectus femoris (RF) and biceps femoris (BF) and the root-mean-squared (RMS) amplitude and the percentage frequency compression of these recordings were calculated. Commonality and cross talk between recordings were also examined. RESULTS: Cross talk between recordings was deemed negligible despite significant levels of commonality between the agonist and antagonist SEMG, which was attributed to common drive. SEMG RMS amplitude increased significantly for all muscles during the 25%, 50%, 75% MVC knee extensions until task failure, and decreased significantly for 100% MVC. The frequency spectrum of the SEMG compressed significantly for all muscles and % MVC levels. The VM, VL and BF SEMG recordings responded similarly to fatigue. The RF's frequency spectrum compressed to a significantly higher degree. CONCLUSIONS: The VM, VL, RF, and BF fatigue in parallel, with high similarity between VM, VL and BF, giving support to the concept of a shared agonist-antagonist motoneuron pool.