A-band length, striation spacing and tension change on stretch of active muscle.

1. Single muscle fibres of the frog were stretched during tetanus and filmed down the interference microscope. 2. Sarcomere lengths and A- and I-band widths were measured from cine films taken of the experimental sequences. 3. When a fibre was stretched during a tetanus, its A-band widths remained constant while its I-band widths changed according to the extent of the stretch. This behaviour is identical with that of a fibre stretched at rest. 4. Sarcomere lengths remained uniform along the fibre during a tetanus plus stretch, increasing as the I-band widths increased when the fibre was stretched. 5. At sarcomere lengths longer than LPmax (the length corresponding to greatest isometric tetanus tension) where the isometric tension diminishes as the fibre is lengthened, if the stretch was imposed during a tetanus the tension was higher after the stretch than before, showing a plateau which lasted for the duration of the tetanus. 6. This effect increased with longer sarcomere lengths and was not due to increased resting tension.