Titin, with a massive single chain of 3-4 MDa and multiple modular motifs,
spans the half-sarcomere of skeletal and cardiac muscles and serves importa
nt, multifaceted functions. In recent years, titin has become a favored sub
ject of single molecule observations by atomic force microscopy (AFM) and l
aser optical trap (LOT). Here we review these single titin molecule extensi
on studies with an emphasis on understanding their relevance to titin elast
icity in muscle function. Some fundamental aspects of the methods for singl
e titin molecule investigations, including the application of dynamic force
, the elasticity models for filamentous titin motifs, the technical foundat
ions and calibrations of AFM and LOT, and titin sample preparations are pro
vided. A chronological review of major publications on recent single titin
extension observations is presented. This is followed by summary evaluation
s of titin domain folding/unfolding results and of elastic properties of fi
lamentous titin motifs. Implications of these single titin measurements for
muscle physiology/pathology are discussed and forthcoming advances in sing
le titin studies are anticipated. Published by Elsevier Science Ltd.