Posterior Digastric Myosin Heavy Chain Isoform Percentages per Age Group. There was a main effect for age in percentage of MHC isoform composition in type I, IIx, and IIb (F1,29= 38.82, p<0.0001; F1,29= 49.56, p<0.0001; F1,29= 49.75, p=<0.0001 respectively).This demonstrates a general shift from faster contracting fiber types to slower contracting types. Standard error is denoted in error bars.