This occurs when a muscle's motor unit is stimulated by multiple impulses at a sufficiently high frequency.
If stimuli are delivered at high frequency, the twitches will overlap, resulting in tetanic contraction.
[5] Fused tetanus is when there is no relaxation of the muscle fibers between stimuli and it occurs during a high rate of stimulation.
In comparison with tetanic contraction in an isometric state (such as holding up a heavy box for several minutes), it differs only in the percentage of motor units participating at any moment and the frequency of neural signals; but the low percentage and low frequency in healthy tone are the key factors defining it as healthy (and not tetanic).
On the moderate to severe parts of the spectrum are dystonia, trismus, pathologic tetanus, and other movement disorders featuring involuntary sustained strong contractions of skeletal muscle.