title: Age-related effects of fatigue and recovery from fatigue in rat medial gastrocnemius muscle.
abstract: Force-velocity, power-velocity and unloaded shortening data were obtained from in situ medial gastrocnemius muscle-tendon complexes (stimulated at 60 Hz) with intact circulation of mature male rats (approximately 125 days old). Measurements were carried out at the end of a long (15 s) contraction (fatigued muscles) or with a short (1 s) contraction either in the fresh state (fresh muscles) or in muscles which had recovered for 15 min after a long contraction. Compared to the fresh state fatigue reduced isometric force by 57%, maximal shortening velocity by approximately 40% and maximal power output by 81%. These reductions were similar to data previously obtained with younger rats (40 days old). However, the velocity data of the muscles which had recovered for 15 min after a long contraction showed a greater reduction in the mature rats. This difference between the two age groups together with a difference in the changes in the initial parts of the isometric force time curves suggest an age-dependent response of the fast-fatigable fibre population of these mixed muscles. In a separate series of experiments the underlying mechanism of the recovery from fatigue was studied in a group of young rats. Fatigue was induced with five long (15 s) contractions (each at 5 min intervals). The recovery of isometric force and power output was monitored with short contractions which indicated a plateau of recovery but the absolute values were still reduced after 60 min (85 and 71% of prefatigue values, respectively). Phosphocreatine concentration recovered rapidly, whereas the ATP concentration was still markedly reduced after 1 h of recovery. The time courses of recovery of inosine-5'-monophosphate (IMP) and lactate concentrations resembled those of force and power output. Thus it is possible that age-dependent differences in IMP and/or lactate production may play a role in fatigue and recovery from fatigue.
authors.affiliation: Department of Exercise Physiology and Health Science, Faculty of Human Movement Sciences, Vrije Universiteit, The Netherlands.
authors.forename: A
authors.initials: A
authors.lastname: de Haan
authors.forename: M A
authors.initials: MA
authors.lastname: Lodder
authors.forename: A J
authors.initials: AJ
authors.lastname: Sargeant
mesh_terms: D000249:Adenosine Monophosphate / Q000378:metabolism; D000818:Animals; D005221:Fatigue / Q000503:physiopathology*; D007291:Inosine Monophosphate / Q000378:metabolism; D007537:Isometric Contraction; D007773:Lactates / Q000378:metabolism; D019344:Lactic Acid; D008297:Male; D009119:Muscle Contraction; D010725:Phosphocreatine / Q000378:metabolism; D051381:Rats; D011919:Rats, Inbred Strains; D013997:Time Factors
chemical_list: D007773:Lactates; D010725:Phosphocreatine; D007291:Inosine Monophosphate; D019344:Lactic Acid; D000249:Adenosine Monophosphate