It is customary to measure the contractile properties of muscle in terms of either the twitch responses to single shocks or less frequently from tetanic responses to high frequency volleys. Both measures are unsatisfactory. Twitch properties vary widely, and depend heavily on the muscle's activation history. Initially they may potentiate up to 3 times their initial value, or not change.
Tetanic responses are more stable but only if examined infrequently.
They fatigue rapidly. Both twitch and tetanic responses are also foreign to most types of normal voluntary use. A better measurement is needed.