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Department of Anesthesia Research Laboratories, Harvard Medical School, Brigham and Women's Hospital, Boston, MA 02115-6195, USA. david.morgan@monash.edu.au

1. A series of contractions with stretches (eccentric contractions) beyond the optimal length for tension generation (optimum) were shown to induce a shift in that optimum in single muscle fibres of frog, as has been previously reported for whole muscles. Shifts averaging 0.129 micron (sarcomere)-1 or 6% were found, without apparent damage to the fibre. 2. The stiffness of fibres was found to fall during a stretch, even though tension was rising. In addition, the isometric stiffness fell as a result of a series of eccentric contractions. 3. Calcium-sensitive fluorescent dyes indicated that such contractions did not reduce the amplitude of the intracellular calcium transient, but did increase its duration. A rise in resting [Ca2+] was found to accompany damage, but not necessarily the shift in optimum. 4. The twitch potentiator nitrate was shown to increase myoplasmic [Ca2+] during twitch and tetani, but not to reverse the shift in optimum length due to eccentric contractions. Both eccentric contractions and twitch potentiation reduced the maximum stimulation rate to which a fibre could respond with propagated action potentials. 5. These results exclude reduced myoplasmic [Ca2+] as the cause of the shift in optimum length in this preparation.