Effects of fatigue and training on sarcoplasmic reticulum Ca²⁺regulation in human skeletal muscle

Abstract

Little is known about fatigue and training effects on sarcoplasmic reticulum (SR) function in human muscle, and we therefore investigated this in eight untrained controls (UT), eight endurance-trained (ET), and eight resistance-trained athletes (RT). Muscle biopsies (vastus lateralis) taken at rest and after 50 maximal quadriceps contractions (180 degrees/s, 0.5 Hz) were analyzed for fiber composition, metabolites and maximal SR Ca(2+) release, Ca(2+) uptake, and Ca(2+)-ATPase activity. Fatigue reduced (P < 0.05) Ca(2+) release (42.1 +/- 3.8%, 43.4 +/- 3.9%, 31.3 +/- 6.1%), Ca(2+) uptake (43.0 +/- 5.2%, 34.1 +/- 4.6%, 28.4 +/- 2.8%), and Ca(2+)-ATPase activity (38.6 +/- 4.2%, 48.5 +/- 5.7%, 29.6 +/- 5.0%), in UT, RT, and ET, respectively. These decreases were correlated with fatigability and with type II fiber proportion (P < 0.05). Resting SR measures were correlated with type II proportion (r > or = 0.51, P < 0.05). ET had lower resting Ca(2+) release, Ca(2+) uptake, and Ca(2+)-ATPase (P < 0.05) than UT and RT (P < 0.05), probably because of their lower type II proportion; only minor effects were found in RT. Thus SR function is markedly depressed with fatigue in controls and in athletes, is dependent on fiber type, and appears to be minimally affected by chronic training status.

Decreased central fatigue in multiple sclerosis patients after 8 weeks of surface functional electrical stimulation

“…The quotient of the torques after a fatigue protocol is presented as the Fatigue Index, which is commonly used as an index of a muscle's ability to resist fatigue [29][30]. The decrease in MVC forces after fatigue exercise represents both peripheral and suprasegmental parts of fatigue [27][28]31].…”

Adaptations in human muscle sarcoplasmic reticulum to prolonged submaximal training

“…Although the reasons underlying the contradictory findings remain unclear, particularly with regard to our laboratory's initial study in this area, the fact that previously trained volunteers were used by Madsen et al (21) might explain the lack of an effect with additional training. In a recent study (19), selected SR properties in the vastus lateralis muscle were compared between untrained control subjects and endurance-trained and resistancetrained athletes. Compared with untrained control subjects, endurance-trained athletes demonstrated lower Ca 2ϩ uptake and Ca 2ϩ release.…”

Skeletal Muscle Fatigue: Cellular Mechanisms

“…A number of studies have reported that Ca 2ϩ release from isolated SR is reduced by ϳ20 -40% following prolonged or intense exercise in humans (210,284,285) and rodents (161), although some studies found no change (111,396) or a reduction in slow-twitch but not fast-twitch muscle (161,221). It is unclear how long this effect persists after exercise, as this was only examined in one study to date, and the results were somewhat equivocal at the one recovery time examined (3.5 h) (210 ] i also greatly prolongs PCD in Xenopus fast-twitch fibers (69,70,269).…”