Contraction force generated by tarsal joint flexion and extension in dogs with golden retriever muscular dystrophy

Force generated due to torque caused by tarsal joint flexion and extension was measured noninvasively at 3, 4.5, 6, and 12 months of age in dogs with the Duchenne homologue, golden retriever muscular dystrophy (GRMD). Absolute and body-weight-corrected GRMD twitch and tetanic force values were lower than normal at all ages (P<0.01 for most). Tarsal flexion and extension were differentially affected. Flexion values were especially low at 3 months, whereas extension was affected more at later ages. Several other GRMD findings differed from normal. The twitch/tetany ratio was generally lower; post-tetanic potentiation for flexion values was less marked; and extension relaxation and contraction times were longer. The consistency of GRMD values was studied to determine which measurements will be most useful in evaluating treatment outcome. Standard deviation was proportionally greater for GRMD versus normal recordings. More consistent values were seen for tetany versus twitch and for flexion versus extension. Left and right limb tetanic flexion values did not differ in GRMD; extension values were more variable. These results suggest that measurement of tarsal tetanic force should be most useful to document therapeutic benefit in GRMD dogs.