Interhemispheric inhibitory interactions (IHI) operate between homologous distal hand representations in primary motor cortex (M1). It is not known if proximal arm representations exhibit comparable effects on their homologous counterparts. We studied IHI in different arm representations, targeting triceps brachii (TB,n=13), first dorsal interosseous (FDI,n=13) and biceps brachii (BB,n=7) muscles in healthy volunteers. TMS test stimuli (TS) were delivered to M1 contralateral to the target muscle preceded 10ms by a conditioning stimulus (CS) to the opposite M1 at 110-150% resting motor threshold (RMT). IHI was calculated as the ratio between MEP amplitudes in conditioned relative to unconditioned trials. Mean RMTs were 38.9, 46.9 and 46.0% of stimulator output in FDI, TB and BB muscles respectively. IHI was 0.45±0.41(FDI), 0.78±0.38(TB) and 0.52±0.32(BB, p<0.01) when test MEP amplitudes were matched and 0.28±0.17(FDI) and 0.85±0.31(TB, p<0.05) when TS intensities expressed as % RMT were matched. Significant IHI (p<0.05) was identified with minimal CS intensities (expressed as % stimulator output) in the 30's for FDI, 60's for TB and 40's for BB. Additionally, a CS of ~120% RMT suppressed the test MEP but not a test H-reflex in BB, suggesting IHI observed in BB is likely mediated by a supraspinal mechanism. We conclude that IHI differs between different arm muscle representations, comparable between BB and FDI but lesser for TB. This finding suggests the amount of IHI between different arm representations does not strictly follow a proximal to distal gradient, but may be related to the role of each muscle in functional movement synergies.