In muscle cells, resting membrane potential is equal to the isoelectric potential of
The isoelectric potential of Cl- is -70mV and of K+ is -90mV.
According to ‘Ganong’ and ‘Principles of Medical Physiology’, the actual measured RMP of a nerve fibre is -70mV rather than the calculated value of -90mV. 
So in nerve fibres, the RMP is equal to or closer to the isoelectric potential of Cl-.
Whereas in muscle cells, the RMP is -90mV, which is equal to the isoelectric potential of K+.
Na+
Cl-
K+
Mg++