Fatigue Etiology At Exhaustion When Cycling Above Vs At Or Below Maximal Lactate Steady-state Threshold.

Abstract

Exercising above maximal lactate steady state (MLSS) has detrimental effects compared to exercising at MLSS. However, neuromuscular fatigue (NMF) to exercise above MLSS is unknown. PURPOSE: To evaluate NMF during exercise slightly below, at and above MLSS. METHODS: Nine men (24±3 yrs.; 76±7 kg; V̇O2max, 3.55±0.19 L·min-1) exercised to exhaustion at MLSS, 10 W below (-10W) and above (+10W) MLSS. NMF was evaluated before (B), at min 10 (10’), 30 (30’) and at exhaustion, by maximal voluntary contractions (MVC). Voluntary activation (VA) and contractile function of knee extensors [peak twitch torque (TwPt)] were tested using electrically-evoked contractions. Oxygen uptake (V̇O2) and blood lactate concentration ([Lac]) were measured. RESULTS: Time to exhaustion (TTE) for -10W (66±14 min) and +10W (44±8 min) were longer and shorter than MLSS (53±9 min) (p<0.05). V̇O2 was stable for -10W (10’, 2.9±0.19; 30’, 2.9±0.28; TTE, 2.9±0.25 L·min-1) and MLSS (10’, 3.0±0.21; 30’, 3.0±0.29; TTE, 3.1±0.23 L·min-1) (p>0.05), but increased at +10W (10’, 3.0±0.21; 30’, 3.2±0.32; TTE, 3.3±0.39 L·min-1; p<0.05). [Lac] was stable for -10W (10’, 4.1±1.1; 30’, 4.4±1.2; TTE, 4.0±1.0 mM; p>0.05). MLSS showed an increase at TTE (10’, 5.3±1.4; 30’, 5.8±1.4; TTE, 6.1±1.8 mM; p<0.05). +10W showed a constant increase (10’, 5.3±1.2; 30’, 8.0±1.6; TTE, 8.6±1.6 mM; p<0.05). MVC dropped from baseline for -10W (B, 561±178; 10’, 436±128; 30’, 420±131; TTE, 395±120 N; p<0.05), MLSS (B, 518±130; 10’, 405±96; 30’, 361±75; TTE, 344±73 N; p<0.05) and +10W (B, 517±172; 10’, 392±124; 30’, 384±146; TTE, 349±88 N; p<0.05), and was not different amongst conditions. VA dropped from baseline for -10W (B, 98±2; 10’, 90±6; 30’, 89±6; TTE, 89±6 %; p<0.05), MLSS (B, 97±2; 10’, 91±5; 30’, 91±5; TTE, 88±5 N; p<0.05) and +10W (B, 98±2; 10’, 94±5; 30’, 90±7; TTE, 87±7 %; p<0.05), and was not different amongst conditions. TwPt dropped from baseline for -10W (B, 173±30; 10’, 129±22; 30’, 119±17; TTE, 112±17 N; p<0.05), MLSS (B, 172±31; 10’, 129±28; 30’, 119±24; TTE, 111±22 N; p<0.05) and +10W (B, 167±33; 10’, 113±25; 30’, 103±27; TTE, 97±26 N; p<0.05). +10W showed greater decline throughout the bout compared to the other conditions. CONCLUSION: Despite different TTE amongst conditions, peripheral NMF was only different exercising slightly above MLSS compared to the other conditions.