Effect of stimulation frequency on force, net power output, and fatigue in mouse soleus muscle in vitro.

The effects of electrical stimulation frequency on force, work loop power output, and fatigue of mouse soleus muscle were investigated in vitro at 35 degrees C. Increasing stimulation frequency did not significantly affect maximal isometric tetanic stress (overall mean +/- SD, 205 +/- 16.6 kN.m-2 between 70 and 160 Hz) but did significantly increase the rate of force generation. The maximal net power output during work loops significantly increased with stimulation frequency: 18.2 +/- 3.7, 22.5 +/- 3.3, 26.8 +/- 3.7, and 28.6 +/- 3.4 W.kg(-1) at 70, 100, 130, and 160 Hz, respectively. The stimulation frequency that was used affected the pattern of fatigue observed during work loop studies. At stimulation frequencies of 100 and 130 Hz, there were periods of mean net negative work during the fatigue tests due to a slowing of relaxation rate. In contrast, mean net work remained positive throughout the fatigue test when stimulation frequencies of 70 and 160 Hz were used. The highest cumulative work during the fatigue test was performed at 70 and 160 Hz, followed by 130 Hz, then 100 Hz. Therefore, stimulation frequency affects power output and the pattern of fatigue in mouse soleus muscle.

Interindividual variation of isolated muscle performance and fibre-type composition in the toad Bufo viridus.

Interindividual analyses of physiological performance represent one of the most powerful tools for identifying functional positive and negative linkages between various performance traits. In this study we investigated functional linkages in the whole-gastrocnemius performance of juvenile Bufo viridis by examining interindividual variation in in vitro muscle performance and muscle fibre-type composition. We used the work-loop technique to investigate the maximum in vitro power output and fatigue resistance of the gastrocnemius muscle during repeated sets of three cycles at the cycle frequency of 5 Hz, simulating an intermittent style of locomotion. We found several significant correlations between different measures of in vitro muscle performance, including a negative correlation between maximum net power output and fatigue resistance of power, indicating functional trade-offs between these performance traits. We also investigated the extent of individual variation in the proportions of different fibre types, and tested for correlations between individual variation in muscle fibre-type composition and the previously measured isolated muscle performance. Fast glycolytic fibres represented 84.0 +/- 3.4% of the muscle, while the combined slow oxidative and fast oxidative-glycolytic fibres represented 16 +/- 3.4%. We found no significant correlations between measures of in vitro muscle performance and the proportion of different fibre types in the gastrocnemius muscle. However, despite this lack of correlation between whole-muscle performance and muscle fibre-type composition data, we suggest the functional linkages detected between different measures of in vitro muscular performance have important ecological and evolutionary consequences.

Mechanical properties of the latissimus dorsi muscle after cyclic training.

Cardiomyoplasty is a procedure developed to improve heart performance in patients suffering from congestive heart failure. The latissimus dorsi (LD) muscle is surgically wrapped around the failing ventricles and stimulated to contract in synchrony with the heart. The LD muscle is easily fatigued and as a result is unsuitable for cardiomyoplasty. For useful operation as a cardiac-assist device, the fatigue resistance of the LD muscle must be improved while retaining a high power output. The LD muscle of rabbits was subjected to a training regime in which cyclic work was performed. Training transformed the fiber-type composition from approximately equal proportions of fast oxidative glycolytic (FOG) and fast glycolytic (FG) fibers to one composed of almost entirely of FOG with no FG, which increased fatigue resistance while retaining rapid contraction kinetics. Muscle mass and cross-sectional area increased but power output decreased, relative to control muscles. This training regime represents a significant improvement in terms of preserving muscle mass and power compared with other training regimes, while enhancing fatigue resistance, although some fiber damage occurred. The power output of the trained LD muscle was calculated to be sufficient to deliver a significant level of assistance to a failing heart during cardiomyoplasty.