During an electrical shock, which factor often causes the muscles to contract?

The factor that often causes muscles to contract during an electrical shock is electrical stimulation. When an electrical current passes through the body, it disrupts the normal electrical signals that control muscle movement. The electrical energy from the shock can cause involuntary muscle contractions, leading to the phenomenon known as tetany, where muscles contract uncontrollably. This can result in the victim being unable to let go of the source of electricity, thereby prolonging the exposure to the shock.

The other options provided do not directly account for the mechanism of muscle contraction in the context of electrical shocks. Physical injury may result from the consequences of a shock but does not explain the involuntary muscle contraction itself. Pressure on the body can cause discomfort or injury, but it is not the underlying cause of muscle contraction during a shock. Heat generation might occur as a result of electrical resistance but does not directly lead to the muscle contractions experienced during an electrical shock. Thus, electrical stimulation is the correct factor associated with this phenomenon.