Trading force for speed: Why superfast crossbridge kinetics leads to superlow forces

Superfast muscles power high-frequency motions such as sound production and visual tracking. As a class, these muscles also generate low forces. Using the toadfish swimbladder muscle, the fastest known. vertebrate muscle, we examined the crossbridge kinetic rates responsible for high contraction rat es and how these might affect force generation. Swimbladder fibers have evo lved a 10-fold faster crossbridge detachment rate than fast-twitch locomoto ry fibers, but surprisingly the crossbridge attachment rate has remained un changed. These kinetics result in very few crossbridges being attached duri ng contraction of superfast fibers (only approximate to 1/6 of that in loco motory fibers) and thus low force. This imbalance between attachment and de tachment rates is likely to be a general mechanism that imposes a tradeoff of force for speed in all superfast fibers.