Answer
At this length ( 2.0um-2.4um) there is maximal overlap between the parts of the thick filaments with the myosin heads and the thin filaments. At lower levels of overlap, contraction and sarcomere shortening is limited.
Work Step by Step
Myosin heads must bind to actin filaments and form cross-bridges for contraction of muscle fibers to occur. A length of 2.2um is close to the average resting length of sarcomeres. At this length the zone of overlap between thick filaments and thin filaments is optimal . Maximal tension is possible when the zone of overlap between a tick filament and a thin filament extends from the end of the H zone to reach the edges of the thin filaments. As the sarcomere gets longer the zone of overlap is reduced and the number of myosin heads that can contact thin filaments is reduced. The consequence is that potential tension decreases. If the sarcomere is stretched to over 170% of its resting length, then there is no overlap ; no myosin heads can bind to thin filaments ; muscles cannot contract and zero tension is possible.
