Contractile characteristics of sarcomeres arranged in series or mechanically isolated from myofibrils.

The mechanisms of contraction are intrinsically connected to -sarcomere mechanics during muscle activation and relaxation. This chapter presents two sets of experiments performed with (1) myofibrils, in which individual sarcomeres in series can be evaluated during contractions, and (2) mechanically isolated sarcomeres. When activated at optimal length (approximately 2.0-2.4 μm), myofibrils and sarcomeres produce similar forces. However, their dependence on length differs: sarcomeres in series in a myofibril are able to produce similar forces at distinct lengths, while isolated sarcomeres show a force-length relation that resembles that obtained in original studies performed with single muscle fibers. Although force in isolated sarcomeres is rapidly stabilized during activation, significant movements of A-band are present when the contraction is produced at optimal lengths, which leads to different dynamics in the two half-sarcomeres. A-band movements decrease linearly with increasing lengths between 2.6 and 3.6 μm. Myofibrils and sarcomeres represent reliable techniques to evaluate contractile mechanisms at the most basic level of muscle organization. However, they present different mechanical characteristics that must be taken into account when scientists evaluate mechanisms of contraction.