Locomotion And Movement

Muscle contraction is explained by the sliding filament theory, in which the contractile unit, the sarcomere, shortens without any change in the lengths of the individual filaments. As detailed in NCERT Class 11, Chapter 20 (Locomotion and Movement), the thin actin filaments slide over the thick myosin filaments toward the centre of the sarcomere when myosin heads form cross-bridges and pull them inward, drawing the Z lines closer together. The shortening of individual myosin molecules is incorrect because the filaments themselves do not contract in length; only their relative sliding produces shortening. Breakdown of actin into amino acids does not occur during contraction; actin remains intact and is reused in every cycle, so that option is biochemically wrong. Lengthening of the A band is also incorrect, since the A band, which corresponds to the length of the thick filaments, stays constant during contraction while the I band and H zone shorten. Hence thin filaments sliding over thick filaments is the answer. A structural plausibility check confirms it: a constant A band coupled with shortening I bands is only consistent with filaments sliding past one another rather than shrinking.