Skeletal muscle weakness is a comorbidity in patients with rheumatoid arthritis (RA), which impairs the ability to work and leads to reduced quality of life for the afflicted patients. However, little molecular insight is available on RA-induced muscle weakness. The reactive oxygen/nitrogen species peroxynitrite (ONOO-) can induce oxidative post-translational protein modifications by nitrating tyrosine residues (3-NT) and facilitating of malondialdehyde (MDA) adduct formation on basic amino acids, e.g. histidine. Increased 3-NT and MDA levels have been observed in rodent models with arthritis and in patients with RA. The skeletal muscles depend on interaction between actin and myosin as the main constituents for force production. Previous work in rodents by others, and us, indicate that arthritis-induced muscle weakness is associated with higher levels of oxidative modifications on actin. However, it has not yet been known how oxidative modification interferers with actin and the force producing machinery in RA. Here we show that oxidative post-translational modifications directly introduced on the contractile machinery and actin lead to impaired actin polymerization and reduced force production. Using mass spectrometry (MS), we identified which actin residues that were targeted by 3-NT or MDA modifications in weakened skeletal muscle from mice with arthritis (3-NT 3; MDA 10) and patients with RA (3-NT 2; MDA 9). The residues were primarily located to three hotspots within the three-dimensional structure of actin. Intriguingly, the identified actin residues from mice with arthritis matched the ones identified from patients with RA. Moreover, structural analysis together with molecular dynamic simulations provided atomistic details on actin and highlighted that four of the residues were sited at locations important for filament stability, and intersubdomain and myosin interaction. In summary, we have identified specific skeletal muscle actin residues that are 3-NT or MDA modified in weakened RA patients and provided molecular insight how this promotes muscle weakness.