rAAV.micro-dystrophin gene transfer improves force generation measurements significantly. (A) rAAV8.micro-dystrophin and rAAV6.micro-dystrophin treated mdx extensor digitorum longus (EDL) muscles (8 week time point) demonstrated significant maximum specific force improvement (mN/mm2) over mdx-untreated muscles (p < 0.05) but did not reach levels of age-matched C57BL/10 wild type (WT) controls (P < 0.05). Values are presented as the means ± SEM, (B) EDL muscles were subjected to 10 cycles of isometric stimulation at 150 Hz with a 10% lengthening to induce damage during the last 100 ms of each contraction. Force values are represented as fractions of the first contraction. rAAV8.micro-dystrophin and rAAV6.micro-dystrophin treated EDL muscles exhibited significant protection from damage compared to mdx-untreated during the first two cycles (p < 0.05), while thereafter all 4 groups showed such robust injury that differences could no longer be determined. There was no significant difference between rAAV8- or rAAV6-treated and WT. (C) The ratio of force (ECC2/ECC1) after one lengthening contraction of rAAV-treated versus the initial contraction of WT is not different, but is significantly improved compared to mdx-untreated (n = 6–8 per group; P < 0.05).