Muscle performance following fatigue induced by isotonic and quasi-isometric contractions of rat extensor digitorum longus and soleus muscles in vitro

Aim: To study the effect of contraction mode on fatigue development. Methods: Muscle fatigue was induced by isotonic and quasi-isometric contractions in rat soleus (SOL) and extensor digitorum longus (EDL) muscles, using identical stimulation protocol (60 Hz, 400 ms s(-1)) for 100 s in SOL and 60 s in EDL. Fatigue was quantified as the decline in peak values of shortening, shortening velocity, relaxation and work during the isotonic contractions, and, correspondingly, of force, rate of force development, relaxation and work during the quasi-isometric contractions. Maximal test contractions (60 Hz, 1.5 s) performed before and after fatigue were analysed for decline in force development (F-max), rate of force development (dF/dt(max)) and relaxation (-dF/dt(max)). Results: F-max declined to significantly lower values after isotonic than after quasi-isometric fatiguing contractions (fatigued in percentage of unfatigued): 58.5+/-6.4% vs. 64.4+/-7.0% in SOL, and 30.4+/-4.1% vs. 33.3+/-3.6% in EDL, respectively. The same pattern was seen for dF/dt(max) which decreased to: 46.3+/-9.9% vs. 52.3+/-8.5% in SOL, and 19.1+/-4.3% vs. 22.3+/-3.2% in EDL after isotonic and quasi-isometric contractions, respectively. Similarly, when comparing fatigue development during the two contraction modes, the respective fatigue variables decreased more rapidly and to lower levels during isotonic vs. quasi-isometric contractions. During maximal test contractions, the dynamic fatigue variables (+/-dF/dt(max)) declined to significantly lower levels than F-max . Conclusions: Fatigue development was significantly larger during isotonic vs. quasi-isometric contractions. The use of force as the only experimental fatigue variable may underestimate the functional impairment of fatigued muscle, neglecting the fatigue effect on time and length dimensions.