Tetanizing stimulation of the muscle causes a sustained forceful contraction due to:

"Tetanizing stimulus" means high-frequency stimulation. When stimuli are applied in quick succession, there is release of Ca++, with each stimulus, from the SR into the sarcoplasm. For the muscle to relax, this Ca++ should be pumped back into the SR, which fails to happen due to the rapid succession of stimuli. The muscle remains in a sustained state of contraction, called as "tetanus".