Fig. 5

Proximal (designated with cross signs) and distal titin segment lengths (dots) in single sarcomeres of a myofibril stretched while it is in an activated state. Note that the proximal and distal titin segments initially elongate linearly with the elongation of the sarcomere, but after a short stretch, the proximal segment stops elongating while the distal segment accommodates the entire sarcomere stretch. We interpret this result as an attachment of the proximal titin segment to actin after a short stretch distance, thereby only leaving the short and stiff distal segment to accommodate the sarcomere elongation. If correct, this binding of titin to actin (predicted theoretically to occur in the middle of the so-called PEVK segment of titin [33]) would increase titin’s stiffness dramatically, thereby causing increased titin forces in actively compared to passively stretched sarcomeres. When myofibrils are stretched passively, the proximal and distal segments are stretched throughout the entire stretch phase in the same manner as indicated in this figure prior to titin attachment to actin, indicating that titin to actin binding does not take place in passively stretched muscles (results not shown)