Muscle pain after electric shock is due to violent muscle contraction during electric shock.
Electricity stimulates the muscle, this is why when electric current passes through the body it causes the muscle withing the pathway of the its flow to contract violently. The extent of the contraction is directly proportional to the intensity of the current and the type of muscle it travels through.
When a current above 10mA travels through flexor muscles, such as the ones in our forearms that close the fingers, it causes a sustained contraction; at this point, the victim may be unable to let go of the source of the current, making the duration of the contact longer and increasing the severity of the shock.
Also;
When a current above 10mA travels through extensor muscles, it causes a violent spasm. If the muscles affected are the hip extensors that lengthen the limbs away from the body, the victim may be propelled, sometimes many metres away!
Muscles, ligaments and tendons may tear as a result of the sudden contraction caused by an electric shock.
This violent contraction of the muscles during electric shock is the reason behind muscle pain after electric shock.
Further Reading
7 Ways of preventing electric shock
Electric Shock – What determines the severity and first aid measures
GFCI (Ground fault circuit interrupter) electrical safety concept
15 necessary precaution to ensure safe use of electrical equipment