Duchenne muscular dystrophy (DMD) is characterized by progressive wasting of skeletal muscle. Previous work from our laboratory, using the extensor digitorium longus (EDL) muscle from adult (>6 mth) mdx mice, suggests that the branched fibres formed during repeated bouts of regeneration could be responsible for the terminal phase of muscle wasting [1-3]. To further test this hypothesis, fast-twitch EDL and slow-twitch soleus (SOL) muscles from mdx mice and littermate controls (6-9 wks, “young”; 17-28 months, “old”) were attached to a force transducer. After contractile analysis single muscle fibres were enzymatically isolated and suspended in a relaxing solution in order to view individual fibres. In old controls, branching was found in <7% of the 426 fibres counted. In young mdx EDL muscles, 59% of 669 fibres counted were branched, while in old mdx EDL muscles, 98% of 672 fibres counted were branched. In young mdx EDL muscles, 63% of the branched fibres had only one branch, while in old mdx EDL muscles, 66% of the branched fibres contained 4 or more branches. In young mdx SOL muscles, 40% of 172 fibres counted were branched, while in old mdx SOL muscles, 79% of 462 fibres counted were branched. In young mdx SOL muscles, 73% of the branched fibres had only one branch, and in old mdx SOL muscles, only 17% of the branched fibres contained 4 or more branches. Muscles were subjected to a series of 10 isometric contractions (1-2 s with 1 min rest). For the EDL, muscles from young control and young mdx mice lost <5% of force over the 10 contractions. In old mice, the force loss was 10.3 ± 1.0% (n = 15) for control and 33.8 ± 4.1% (n = 11) for mdx (p < 0.0001). For the SOL, force loss was negligible in control and mdx muscles of both age groups. Muscles were subjected to a series of eccentric contractions. In old mdx EDL muscles, the first contraction caused so much force loss that subsequent contractions resembled passive stretches. The first contraction at 10% strain produced a force loss of 5.5 ± 1.6% (n = 5) in young controls and 14.4 ± 1.5% (n = 6) in young mdx (p < 0.01). In old mice, the force loss was 1.7 ± 1.7% (n = 11) in controls and 63.1 ± 2.1% (n = 5) in mdx (p < 0.0001). For the age-genotype interaction, p < 0.0001. In SOL muscles, eccentric contractions produced negligible force losses in mdx and controls. We have shown that the marked aberrations in cytoarchitecture present in virtually all EDL fibres of very old mdx mice have a distinct association with the increased vulnerability to isometric and eccentric damage of their muscles. The association between degree of branching and degree of damage is further confirmed by the internal control of the SOL muscle, which shows much less complex branching than the EDL and virtually no damage from isometric and eccentric contractions.