The contractile response during steady lengthening of stimulated frog muscle fibres.

Abstract

SUMMARY1. Steady lengthenings at different velocities (0-025-1.2 ,tm/s per half-sarcomere; temperature 2-5-5°C) were imposed on isolated frog muscle fibres at the isometric tetanus plateau by means of a loudspeaker motor. The lengthening at the sarcomere level was measured by means of a striation follower either in fixed-end or in lengthclamp mode. The force response was measured by a capacitance gauge transducer (resonance frequency 50 kHz). Preparations showing gross non-homogeneity during lengthening were excluded.2. A steady tension was in all cases reached after about 20 nm per half-sarcomere of lengthening. Tension during this steady phase rose with speed of elongation up to 0-25-0-4 #sm/s per half-sarcomere, when tension was 1-9-2 times isometric tetanic force (TO). Further increase in speed produced only very little increase in the steady tension.3. During the transitory phase, before steady tension was reached, the tension rose monotonically if speed of lengthening was less than 0-25-0-3 ,um/s per halfsarcomere; at higher speed the tension rose above the steady level, reaching a peak when extension was 10-14 nm per half-sarcomere, and then fell to the steady level. Tension at the peak continued to rise with speed of lengthening above 0 3 ,um/s per half-sarcomere.4. During the tension rise within the transitory phase of force response the segment elongated at a speed 15-20% lower than that imposed on the whole fibre, as a consequence of-tendon compliance.5. During the steady phase, non-homogeneity of lengthening speed began above a speed of lengthening which varied from fibre to fibre. At speeds below this value, segments elongated at the same speed as that imposed on the fibre.6. Tension responses to large step stretches (up to 12 nm per half-sarcomere), applied at the plateau of isometric tetanus, showed that the instantaneous elasticity of contractile machinery is not responsible for the limit in force attained with highspeed lengthening.7. Instantaneous stiffness was determined during the steady state of force response by superposing small steps (< 15 nm per half-sarcomere) on steady lengthening at different velocities. Stiffness was 10-20% larger during lengthening MS 8214 V. LOMBARDI AND G. PIAZZESI than at the plateau of isometric tetanus and remained practically constant, independent of lengthening velocity, in the range of velocities used.8. The results indicate that steady lengthening of a tetanized fibre induces a crossbridge cycle characterized by fast detachment of the cross-bridge extended beyond a critical level. Reattachment of cross-bridges detached in this way is also very rapid. 9. A model of contraction, including separate steps for cross-bridge attachment, force generation and detachment, was found to be compatible with the experimental force-and stiffness-velocity relations. In the model, detachment of extended crossbridges occurs at an early stage of the cycle. The rate constant of this detachment process increases sharply beyond a critical amount of cross-bridge strain; reattac...

The torque–velocity relationship in large human muscles: Maximum voluntary versus electrically stimulated behaviour

“…The fitted ratio values for stimulated flexors and extensors of 1.5 − 1.7 were higher than the value of 1.4 reported by Dudley et al (1990) and may be due to the inclusion of measured torque at higher eccentric velocities and / or the fitting procedure giving an extrapolated eccentric maximum value. Indeed, the fitted ratio values in this study fall more in line with the measured in vitro tetanic values reported in the literature of 1.6 − 2.0 (Katz, 1939;Délèze, 1961;Edmann, 1988;Harry et al, 1990;Lombardi and Piazzesi, 1990;Krylow and Sandercock, 1997).…”

“…The lower end of this range is similar to values reported in previous in vivo stimulation studies for the quadriceps (Dudley et al, 1990;Westing et al, 1991) soleus (Pinniger et al, 2000) and adductor pollicis (De Ruiter et al, 2000;Lee and Herzog, 2002). While the upper end is similar to values found in previous in vivo stimulation studies for the first dorsal interosseus (Cook and McDonagh, 1995) and in vitro studies for mouse and frog muscle fibres (Katz, 1939;Délèze, 1961;Edmann, 1988;Lombardi and Piazzesi, 1990;Krylow and Sandercock, 1997).…”

Characterization of actomyosin bond properties in intact skeletal muscle by force spectroscopy

“…Force response of activated muscles or single muscle fibers to ramp stretches has been extensively studied in the past by a number of authors (27)(28)(29)(30), starting from the pioneering experiments of Katz (31). Most of these studies were performed with slow stretching speeds and mainly aimed at investigating the mechanism of the steady force increase occurring during stretching.…”

Preservation of eccentric strength in older adults: Evidence, mechanisms and implications for training and rehabilitation

“…It has been suggested that the stretch produced during eccentric contractions might shift myosin heads into a strongly bound state (Lombardi and Piazzesi, 1990). This, in turn, would reduce during eccentric contractions the age-related force deficits commonly observed during either isometric or concentric contractions (Phillips et al, 1991).…”