Final answer:
Magnesium acts as a smooth muscle relaxant, helping to reduce muscle cramps and tension without directly strengthening or contracting muscles.
Step-by-step explanation:
The effect of magnesium (Mag.) on muscles is that it functions as a smooth muscle relaxant. Specifically, magnesium can help diminish muscle cramps and tension by relaxing the muscles. This is particularly beneficial in conditions such as asthma where bronchial smooth muscle relaxation can help alleviate symptoms. It's important to note that magnesium doesn't strengthen or contract muscles directly, nor does it have no effect; its role in muscle physiology is more about modulation and maintaining proper function.