Properties of Muscles on Electrical Stimulation

When a skeletal muscle is stimulated electrically, it displays several characteristic properties:

A. Single muscle twitch — a contraction triggered by a single, brief stimulus. It occurs in three stages: a latent period (no visible contraction yet), a contraction period, and a relaxation period.

B. Summation — if a second stimulus arrives before the twitch produced by the first has finished, the tensions add up (summate) and produce greater overall tension. Repeated stimuli in this way produce a step-wise increase in the strength of contraction, called the staircase phenomenon.

C. Tetanus — if stimulation is rapid and frequent enough that the muscle has no time to relax between successive stimuli, it remains continuously in a state of contraction; this sustained contracted state is called tetanus.

D. Refractory period — immediately after one stimulus, a muscle fibre cannot respond to another stimulus for a brief resting (refractory) period, roughly 0.02 seconds.

E. Threshold stimulus — the minimum strength or intensity of stimulus needed for a muscle fibre to contract at all.

F. All-or-none principle — a stimulus below threshold produces no contraction; a stimulus at or above threshold produces full contraction leading to maximum force, and increasing the stimulus further does not increase that force. In other words, a single muscle fibre contracts either fully or not at all. All types of muscle fibres and nerve fibres obey this law.

G. Oxygen debt — during strenuous exercise, the muscle's oxygen supply becomes insufficient to keep up with the demand of oxidative phosphorylation of the respiratory substrate. The muscle then contracts anaerobically, and lactic acid accumulates from anaerobic glycolysis; lactic acid yields less ATP than aerobic respiration and is toxic, causing tiredness, pain and muscle cramps. During recovery, the muscle's oxygen consumption far exceeds its resting level, since this extra oxygen is used to oxidise the accumulated lactic acid and to restore the depleted creatine phosphate and ATP stores — this extra oxygen consumed during recovery is the muscle's oxygen debt. …