A

Tetanus occurs when successive stimuli arrive at intervals shorter than the muscle’s contraction phase; the critical-fusion (tetanizing) frequency is therefore approximately the reciprocal of contraction time (1 / 0.04 s ≈ 25 Hz). Experimental work shows that muscles with longer contraction times fuse at proportionally lower frequencies (e.g., a 74 ms canine muscle tetanizes at ~30 Hz) [Spurgeon, 1978, PMID 749569], supporting this inverse relationship.