(A) Isometric force versus in vivo stimulation frequency of the gastrocnemius muscle showed significantly reduced force in IL-10^tm mice (aged control, n = 7; IL-10^tm, n = 8). (B) Force normalized to gastrocnemius weight showed no difference in total force. (C) Total body and gastrocnemius weight are lower in IL-10^tm mice as compared with those of age-matched control. (D and E) Kinetics of contraction assessed at 80 Hz showed the maximal rate of contraction (+dF/dt) was significantly lower in the IL-10^tm mice (D), while the maximal rate of relaxation (–dF/dT) was unchanged (E). (F) Morphology of the neuromuscular junction (NMJ) was quantified with β-3 tubulin (green) to denote the presynaptic area and bungarotoxin (red) to visualize the postsynaptic area. Representative confocal image of an NMJ in old WT B6 mice (control) reveals the presynaptic neural structure (green) opposed to a prototypical postsynaptic NMJ end-plate. Adjacent is a representative NMJ from old IL-10^tm mice showing the absence of presynaptic neural connectivity (green) and disorganized postsynaptic structure (red). (G) Semiquantitative analysis of NMJ morphology. The first panel shows the ratio of presynapatic to postsynapatic junctional area is significantly reduced in IL-10^tm mice (P < 0.0001). The second panel shows the distribution of the ratio between control old B6 mice versus IL-10^tm mice. (H and I) Cross section of whole gastrocnemius muscle with Wheat Germ Agglutinin (WGA) labeling to detect muscle fiber boundaries (fluorescence image inverted for clarity). (H) Muscle fibers from old control mice show rounded muscle fibers of variable size. Magnification, 10×. (I) Myofibers of IL-10^tm mouse are smaller in diameter than control with many being angular in shape. (J) Quantification of muscle fiber cross sectional area (CSA) shows a significant (P < 0.0001) reduction in IL-10^tm mice compared with age matched control.