Reliability of isolated isometric function measures in rat muscles composed of different fibre types

The present study investigated the absolute reliability (R-Ab) of isometric measures of time-to-peak tension (TTP), half-rise time (1/2RT), half-fall time (1/2FT), twitch force (TWf) tetanic force (TETf) and the sag ratio as applied to the slow soleus (SOL) and the fast-twitch extensor digitorum longus (EDL) and medial gastrocnemius (MG) muscles of the rat hindlimb. In addition, the relationship of each individual isometric measure was examined with regard to the pattern of myosin heavy chain (MHC) isoform expression. Measures of TTP, 1/2RT, 1/2FT and sag ratio were negatively correlated with MHCIId(x) and MHCIIb (P < 0.0001), and positively correlated with MHCI (P < 0.0001). TWf and TETf were negatively correlated with MHCI content (P < 0.0001) and positively with MHCIId(x) (P < 0.0001) and MHCIIb (P < 0.001). Comparisons of isometric measures using a paired Student's t test revealed that they were not different between the right and left legs; all measures displayed high correlations between the left and right legs (r= 0.71-0.85, P < 0.0001). In contrast to standard tests of statistical significance, these functional measures exhibited a considerable range of R-Ab when individual muscles were studied in only one hindlimb. When averaged across all muscles, however, the 1/2FT, 1/2RT, TWf and TTP measures possessed high overall reliability; measures of TETf and sag ratio were moderately reliable. The results of this study show that the isometric measures studied possess significant predictive value with regard to MHC isoform content; the left and right legs are interchangeable but display a considerable range of reliability when only one hindlimb is studied.