Cellular movements are produced by forces. Typically, cytoskeletal proteins such as microtubules and actin
filaments generate forces via polymerization or in conjunction with molecular motors. However, the fertilization of a Limulus
polyphemus egg involves a third type of actin-based cellular engine—a biological spring. During the acrosome reaction, a
60-mm long coiled and twisted bundle of actin filaments straightens and extends from a sperm cell, penetrating the vitelline layer
surrounding the egg. A subtle overtwist of 0.2/subunit underlies the mechanochemical basis for the extension of this actin
spring. Upon calcium activation, this conformational strain energy is converted to mechanical work, generating the force required to extend the bundle through the vitelline layer. In this article, we stall the extension of the acrosome bundle in agarose
gels of different concentrations. From the stall forces, we estimate a maximum force of 2 nN and a puncturing pressure of 1.6
MPa. We show the maximum force of extension is three times larger than the force required to puncture the vitelline layer. Thus,
the elastic strain energy stored in the acrosome bundle is more than sufficient to power the acrosome reaction through the
egg envelope.