]o relationships for the twitch and tetanus were the same in nonfatigued extensor digitorum longus and soleus muscles: force was maintained over a large range of [Na ϩ ]o and then decreased abruptly over a much smaller range. However, fatigue was significantly exacerbated at a lowered [Na ϩ ]o that had little effect in nonfatigued soleus muscle. This finding suggests that substantial differences exist in the Na ϩ effect on force between nonfatigued and fatigued muscle. The reduced contractility in nonfatigued muscles at lowered [Na ϩ ]o was largely due to 1) an increased number of inexcitable fibers and threshold for action potentials, 2) a reduction of action potential amplitude, and 3) a reduced capacity to generate action potentials throughout trains. sodium gradient; muscle contraction; action potential train; extensor digitorum longus; soleus DURING REPETITIVE ACTIVATION of skeletal muscle, a net influx of sodium ions (Na ϩ