**Core Concept**
The resting membrane potential in muscle cells, also known as the myoplasm, is primarily determined by the balance of ions across the cell membrane. The isoelectric potential, also known as the Nernst potential, is a measure of the electrical potential difference that would exist across a cell membrane if it were in equilibrium with a particular ion.

**Why the Correct Answer is Right**
The resting membrane potential in muscle cells is equal to the isoelectric potential of potassium ions (K+). This is because the muscle cell membrane is highly permeable to K+ ions, allowing them to diffuse out of the cell and establish an equilibrium potential. The Nernst equation calculates this equilibrium potential for K+ ions, resulting in a value close to -90mV, which is the resting membrane potential in muscle cells.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the isoelectric potential of sodium ions (Na+) is typically more positive than the resting membrane potential, around +60mV.
**Option B:** This option is incorrect because the isoelectric potential of chloride ions (Cl-) is typically close to the resting membrane potential, but it is not the primary determinant of the resting membrane potential in muscle cells.
**Option C:** This option is incorrect because the isoelectric potential of calcium ions (Ca2+) is typically more positive than the resting membrane potential, around +100mV.

**Clinical Pearl / High-Yield Fact**
Remember that the resting membrane potential in muscle cells is primarily determined by the balance of potassium ions (K+) across the cell membrane, which is essential for muscle contraction and relaxation.

**Correct Answer:** K+