The pronators and supinators play a key role in the medio-lateral stability of the ankle joint complex (i.e. talo-crural and subtalar joints). We hypothesized that each shank muscle has a specific activation pattern determined by its anatomical course around the axes of the subtalar and talo-crural joints. A secondary objective was to examine the effect of foot posture on these activation patterns. Forty-nine young adults (25 normal-arched feet, 24 flat-arched feet) performed maximum voluntary isometric contractions against manual resistance in four movement directions: plantarflexion (PF), dorsiflexion (DF), pronation (PRO) and supination (SUP). Electromyographic activity was recorded from tibialis posterior (TP) and peroneus longus (PL) with intramuscular electrodes, and gastrocnemius medialis (GM) and tibialis anterior (TA) with surface electrodes. When compared to their agonist function, all muscles were co-activated at significantly lower levels in their synergistic function (GM: 23% during SUP, TA: 72% during SUP; TP: 42% during PF, PL: 52% during PF) (p<0.001). A significant interaction between foot posture and contraction type was evident for TA. During isometric contractions, the electromyographic activity of the shank muscles is geared to their biomechanical advantage according to their position relative to the subtalar and talo-crural joint axes.