Is muscle powered by springs or motors?

In the sarcomere, the structural unit of the skeletal and cardiac muscle, the giant protein titin, which spans from the boundary between sarcomeres to the centre of the sarcomere, has been conventionally identified as the elastic element responsible for the passive tension developed at long sarcomere length. However, in a recent paper (Rivas-Pardo et al., Cell Reports 14:1339, 2016) a role is claimed for titin also in active contraction, following the demonstration that in vitro titin is able to shorten against 6 pN force by 10 nm refolding of the Immunoglobulin (Ig) domains at a rate of 2/s. The idea is refuted on the basis of simple kinetics and energetics considerations showing that the contribution in terms of power per half-thick filament of the Ig domain refolding is 2 orders of magnitude smaller than that of the array of myosin motors.