Skeletal muscle has six special properties: summation, staircase, tetanization, post-tetanic potentiation, length-tension and load-velocity relationships.
Summation and Staircase Phenomenon
Summation of contraction: a second stimulus during the first twitch's contraction phase adds its Ca2+ release to the first, giving a higher combined response.
Staircase phenomenon (treppe): rapid suprathreshold stimuli below tetanizing frequency progressively raise contraction force until a maximal uniform tension is reached.
Skeletal muscle's action potential (1 to 4 msec) is brief relative to twitch duration, allowing summation.
Contractile elements have no refractory period and do not follow the all-or-none law, unlike the action potential.
In treppe, incompletely relaxed muscle retains cytoplasmic Ca2+ not yet pumped back into the SR, adding to the next stimulus's Ca2+.
Heat from the preceding contraction also lowers sarcoplasmic viscosity and aids enzymatic activity, raising twitch height until Ca2+ saturates.
Treppe was first described in frog heart by Henry Pickering Bowditch (1871); it also occurs in cardiac muscle.