Biomedical subjects

P J Griffiths

Publications and source records attributed to P J Griffiths.

At least 19 recordsLinked to original sources

PubMed2005

Effects of the number of actin-bound S1 and axial force on X-ray patterns of intact skeletal muscle.

Effects of the number of actin-bound S1 and of axial tension on x-ray patterns from tetanized, intact skeletal muscle fibers were investigated. The muscle relaxant, BDM, reduced tetanic M3 meridional x-ray reflection intensity (I(M3)), M3 spacing (d(M3)), and the equatorial I(11)/I(10) ratio in a manner consistent with a reduction in the fraction of S1 bound to actin rather than by generation of low-force S1-actin isomers. At complete force suppression, I(M3) was 78% of its relaxed value. BDM distorted dynamic I(M3) responses to sinusoidal length oscillations in a manner consistent with an increased cross-bridge contribution to total sarcomere compliance, rather than a changed S1 lever orientation in BDM. When the number of actin-bound S1 was varied by altering myofilament overlap, tetanic I(M3) at low overlap was similar to that in high [BDM] (79% of relaxed I(M3)). Tetanic d(M3) dependence on active tension in overlap experiments differed from that observed with BDM. At high BDM, tetanic d(M3) approached its relaxed value (14.34 nm), whereas tetanic d(M3) at low overlap was 14.50 nm, close to its value at full overlap (14.56 nm). This difference in tetanic d(M3) behavior was explicable by a nonlinear thick filament compliance which is extended by both active and passive tension.

Actin Cytoskeleton↗

PubMed2005

Myosin lever disposition during length oscillations when power stroke tilting is reduced.

M3 reflection intensity (I(M3)) from tetanized, intact skeletal muscle fiber bundles was measured during sinusoidal length oscillations at 2.8 kHz, a frequency at which the myosin motor's power stroke is greatly reduced. I(M3) signals were approximately sinusoidal, but showed a "double peak" distortion previously observed only at lower oscillation frequencies. A tilting lever arm model simulated this distortion, where I(M3) was calculated from the molecular structure of myosin subfragment 1 (S1). Simulations showed an isometric lever arm disposition close to normal to the filament axis at isometric tension, similar to that found using lower oscillation frequencies, where the power stroke contributes more toward total S1 movement. Inclusion of a second detached S1 in each actin-bound myosin dimer increased simulated I(M3) signal amplitude and improved agreement with the experimental data. The best agreement was obtained when detached heads have a fixed orientation, insensitive to length changes, and similar to that of attached heads at tetanus plateau. This configuration also accounts for the variations in relative intensity of the two main peaks of the M3 reflection substructure after a length change. This evidence of an I(M3) signal distortion when power stroke tilting is suppressed, provided that a large enough amplitude of length oscillation is used, is consistent with the tilting lever arm model of the power stroke.

Animals↗

PubMed2001

Time-resolved X-ray diffraction by skinned skeletal muscle fibers during activation and shortening.

Force, sarcomere length, and equatorial x-ray reflections (using synchrotron radiation) were studied in chemically skinned bundles of fibers from Rana temporaria sartorius muscle, activated by UV flash photolysis of a new photolabile calcium chelator, NP-EGTA. Experiments were performed with or without compression by 3% dextran at 4 degrees C. Isometric tension developed at a similar rate (t(1/2) = 40 +/- 5 ms) to the development of tetanic tension measured in other studies (Cecchi et al., 1991). Changes in intensity of equatorial reflections (I(11) t(1/2), 15-19 ms; I(10) t(1/2), 24-26 ms) led isometric tension development and were faster than for tetanus. During shortening at 0.14P(o), I(10) and I(11) changes were partially reversed (18% and 30%, respectively, compressed lattice), in agreement with intact cell data. In zero dextran, activation caused a compression of A-band lattice spacing by 0.7 nm. In 3% dextran, activation caused an expansion of 1.4 nm, consistent with an equilibrium spacing of 45 nm. But, in both cases, discharge of isometric tension by shortening caused a rapid lattice expansion of 1.0-1.1 nm, suggesting discharge of a compressive cross-bridge force, with or without compression by dextran, and the development of an additional expansive force during activation. In contrast to I(10) and I(11) data, these findings for lattice spacing did not resemble intact fiber data.

Animals↗

PubMed2001

Frequency-dependent distortion of meridional intensity changes during sinusoidal length oscillations of activated skeletal muscle.

Bundles of intact, tetanized skeletal muscle fibers from Rana temporaria were subjected to sinusoidal length oscillations in the frequency domain 100 Hz to 3 kHz while measuring force and sarcomere length. Simultaneously, intensity of the third-order x-ray reflection of the axial myosin unit cell (I(M3)) was measured using synchrotron radiation. At oscillation frequencies <1 kHz, I(M3) was distorted during the shortening phase of the sinusoid (i.e., where bundle length was less than rest length). Otherwise, during the stretch phase of oscillations at all frequencies, during the shortening phase of oscillations above 1 kHz, and for bundles in the rigor state, I(M3) was approximately sinusoidal in form. Mean I(M3) during oscillations was reduced by 20% compared to the isometric value, suggesting a possible change in S1 disposition during oscillations. However, the amplitude of length change required to produce distortion (estimated from the phase angle at which distortion was first evident) corresponded to that of a step release sufficient to reach the maximum I(M3), indicating a mean S1 disposition during oscillations close to that during an isometric tetanus. The mechanical properties of the bundle during oscillations were also consistent with an unaltered S1 disposition during oscillations.

Actins↗

PubMed1999

Time-resolved equatorial X-ray diffraction studies of skinned muscle fibres during stretch and release.

Equatorial X-ray diffraction patterns were recorded from small bundles of one to three chemically skinned frog sartorius muscle fibres (time resolution 250 microseconds) during rapid stretch and subsequent release. In the relaxed state, the dynamic A-band lattice spacing change as a result of a 2 % step stretch (determined from the positions of the 10 and 11 reflections) resulted in a 21 % increase in lattice volume, while static studies of spacing and sarcomere length indicated than an increase in volume of >/=50 % for the same length change. In rigor, stretch caused a lattice volume decrease which was reversed by a subsequent release. In activated fibres (pCa 4.5) exposed to 10 mM 2,3-butanedione 2-monoxime (BDM), stretch was accompanied by a lattice compression exceeding that of constant volume behaviour, but during tension recovery, compression was partially reversed to leave a net spacing change close to that observed in the relaxed fibre. In the relaxed state, spacing changes were correlated with the amplitude of the length step, while in rigor and BDM states, spacing changes correlated more closely with axial force. This behaviour is explicable in terms of two components of radial force, one due to structural constraints as seen in the relaxed state, and an additional component arising from cross-bridge formation. The ratio of axial to radial force for a single thick filament resulting from a length step was four in rigor and BDM, but close to unity for the relaxed state.

Animals↗

PubMed1999

Submillisecond changes in myosin lattice spacing resulting from rapid length changes.

The speed of the myofilament lattice spacing response to rapid changes in load or length of single, intact muscle fibres of the frog, was investigated during isometric tetani. During ramp releases at close to Vmax and during step length changes (completed within 250 microseconds), lattice spacing was calculated from the equatorial X-ray diffraction pattern (sampled at 250 microseconds time resolution using synchrotron radiation). Ramp releases (total shortening=1.39 %) caused a spacing increase, described with an exponential function (alpha=271 s-1, amplitude=1.15 nm) plus an elastic component having the time course of discharge of axial tension (amplitude 0.28 nm). For a step release (amplitude=0.87%), lattice expansion could be described with an exponential (alpha =1005 s-1, amplitude=0.56 nm) plus an elastic component of 0.25 nm amplitude. Lattice compression was associated with a step stretch (amplitude=0.62 %), and was also quasi-exponential (alpha=367 s-1, amplitude=0.74 nm), with an elastic component of 0.28 nm. The spacing change time course for length steps resembled that of the accompanying quick recovery of axial tension and the associated change in the meridional 14.5 nm reflection intensity, which are both believed to be determined by the kinetics of the molecular power stroke. Therefore, this shows that lattice spacing changes, arising from radial forces exerted by attached crossbridges, are fast enough to occur during the power stroke event.

Animals↗

PubMed1998

Studies on the 14.5 nm meridional X-ray diffraction reflection during length changes of intact frog muscle fibres.

The intensity of the 14.5 nm meridional reflection (M3) from activated skeletal muscle fibres was studied in both single fibres and fibre bundles during the imposition of length changes. During shortening at small load, the intensity of the reflection decreased within 2 ms to less than 20% of isometric intensity, then recovered partially during the remainder of the shortening. When shortening was terminated, recovery of intensity was delayed. Small shortening steps (0.5% fibre length) produced a fall in M3 intensity (IM3) delayed by ca. 250 microseconds compared to the fall in tension. For larger step releases (1% fibre length), the fall in IM3 was not delayed. The fall in IM3 could be almost completely reversed by a subsequent restretch applied within 1.5 ms. Beyond 10 ms after the initial release, the restretch caused a further fall in intensity. A rapid step stretch (0.5% fibre length) also caused a fall in IM3 without delay, which was partially reversed by a release applied within 10 ms. A second small release applied 3 ms (or less) after the first caused a second fall in M3 intensity, but without delay and with faster time course. Small amplitude sinusoidal length oscillations (0.15-0.2% sarcomere; 1 kHz) caused a sinusoidal change in M3 intensity, which was 180 degrees out of phase with the force oscillations, and lacked distortion during its release phase.

Animals↗

PubMed1998

Volume changes of the myosin lattice resulting from repetitive stimulation of single muscle fibers.

Single muscle fibers at 1 degreesC were subjected to brief tetani (20 Hz) at intervals of between 20 s and 300 s over a period of up to 2 h. A band lattice spacing increased during this period at a rate inversely dependent on the rest interval between tetani. Spacing increased rapidly during the first 10 tetani at a rate equivalent to the production of 0.04 mOsmol.liter-1 of osmolyte per contraction, then continued to expand at a much slower rate. For short rest intervals, where lattice expansion was largest, spacing increased to a limiting value between 46 and 47 nm (sarcomere length 2.2 micrometer), corresponding to accumulation of 30 mOsmol.liter-1 of osmolytes, where it remained constant until repetitive stimulation was terminated. At this limiting spacing, force was reduced by up to 30%. The effect of lattice swelling on the lattice compression that accompanies isometric force recovery from unloaded shortening was to increase the compression, similar to that observed in hypotonic media at a similar spacing. During recovery from repetitive stimulation, spacing recompressed to its original value with a half-time of 15-30 min. These findings suggest that mechanical activity produces an increase in osmotic pressure within the cell as a result of product accumulation from cross-bridge and sarcoplasmic reticulum ATPases and glycolysis.

Animals↗

PubMed1994

Lattice spacing changes accompanying isometric tension development in intact single muscle fibers.

The myosin lattice spacing of single intact muscle fibers of the frog, Rana temporaria, was studied in Ringer's solution (standard osmolarity 230 mOsm) and hyper- and hypotonic salines (1.4 and 0.8 times standard osmolarity respectively) in the relaxed state, during "fixed end" tetani, and during shortening, using synchrotron radiation. At standard tonicity, a tetanus was associated with an initial brief lattice expansion (and a small amount of sarcomere shortening), followed by a slow compression (unaccompanied by sarcomere length changes). In hypertonic saline (myosin lattice compressed by 8.1%), these spacing changes were suppressed, in hypotonic saline (lattice spacing increased by 7.5%), they were enhanced. During unloaded shortening of activated fibers, a rapid lattice expansion occurred at all tonicities, but became larger as tonicity was reduced. This expansion was caused in part by the change in length of the preparation, but also by a recoil of a stressed radial compliance associated with axial force. The lattice spacing during unloaded shortening was equal to or occasionally greater than predicted for a relaxed fiber at that sarcomere length, indicating that the lattice compression associated with activation is rapidly reversed upon loss of axial force. Lattice recompression occurred upon termination of shortening under standard and hypotonic conditions, but was almost absent under hypertonic conditions. These observations indicate that axial cross-bridge tension is associated with a compressive radial force in intact muscle fibers at full overlap; however, this radial force exhibits a much greater sensitivity to lattice spacing than does the axial force.

Animals↗

PubMed1993

Time-resolved equatorial X-ray diffraction measurements in single intact muscle fibres.

Equatorial X-ray diffraction techniques have been successfully applied to the intact single muscle fibre preparation under length clamp and "fixed end" conditions. 10 and 11 intensity changes and stiffness have been measured in the same preparation. Under isometric conditions, equatorial signals and stiffness led force by 14-20ms during the rise of tetanic tension. During relaxation, stiffness and equatorial signals lagged force. The time course of the intensity changes suggests a low force crossbridge state is present to a greater extent during the rise of tetanic tension and during relaxation than at the tetanus plateau. During isotonic shortening at Vmax, stiffness fell to 30% of its isometric level, while equatorial signals fell to 60%. Since stiffness and equatorial signals are thought to detect attached crossbridges, either the average stiffness per attached bridge measured at 4kHz during shortening is less than at the plateau, or the relation between equatorial intensities and the proportion of attached crossbridges during isotonic shortening differs from that measured under isometric conditions. Active tension also affects the lattice spacing. The myosin lattice was compressed during the development of longitudinal force. This implies a radial component of crossbridge tension. The lattice compression was smaller in a compressed lattice and larger in an expanded lattice.

Animals↗

PubMed1993

Barnacle muscle: Ca2+, activation and mechanics.

In this review, aspects of the ways in which Ca2+ is transported and regulated within muscle cells have been considered, with particular reference to crustacean muscle fibres. The large size of these fibres permits easy access to the internal environment of the cell, allowing it to be altered by microinjection or microperfusion. At rest, Ca2+ is not in equilibrium across the cell membrane, it enters the cell down a steep electrochemical gradient. The free [Ca2+] at rest is maintained at a value close to 200 nM by a combination of internal buffering systems, mainly the SR, mitochondria, and the fixed and diffusible Ca(2+)-binding proteins, as well as by an energy-dependent extrusion system operating across the external cell membrane. This system relies upon the inward movement of Na+ down its own electrochemical gradient to provide the energy for the extrusion of Ca2+ ions. As a result of electrical excitation, voltage-sensitive channels for Ca2+ are activated and permit Ca2+ to enter the cell more rapidly than at rest. It has been possible to determine both the amount of Ca2+ entering by this step, and what part this externally derived Ca2+ plays in the development of force as well as in the free Ca2+ change. The latter can be determined directly by Ca(2+)-sensitive indicators introduced into the cell sarcoplasm. A combination of techniques, allowing both the total and free Ca2+ changes to be assessed during electrical excitation, has provided valuable information as to how muscle cells buffer their Ca2+ in order to regulate the extent of the change in the free Ca2+ concentration. The data indicate that the entering Ca2+ can only make a small direct contribution to the force developed by the cell. The implication here is that the major source of Ca2+ for contraction must be derived from the internal Ca2+ storage sites within the SR system, a view reinforced by caged Ca2+ methods. The ability to measure the free Ca2+ concentration changes within a single cell during activation has also provided the opportunity to analyse, in detail, the likely relations between free Ca2+ and the process of force development in muscle. The fact that the free Ca2+ change precedes the development of force implies that there are delays in the mechanism, either at the site of Ca2+ attachment on the myofibril, or at some later stage in the process of force development that were not previously anticipated.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗

PubMed1993

Cross-bridge attachment and stiffness during isotonic shortening of intact single muscle fibers.

Equatorial x-ray diffraction pattern intensities (I10 and I11), fiber stiffness and sarcomere length were measured in single, intact muscle fibers under isometric conditions and during constant velocity (ramp) shortening. At the velocity of unloaded shortening (Vmax) the I10 change accompanying activation was reduced to 50.8% of its isometric value, I11 reduced to 60.7%. If the roughly linear relation between numbers of attached bridges and equatorial signals in the isometric state also applies during shortening, this would predict 51-61% attachment. Stiffness (measured using 4 kHz sinusoidal length oscillations), another putative measure of bridge attachment, was 30% of its isometric value at Vmax. When small step length changes were applied to the preparation (such as used for construction of T1 curves), no equatorial intensity changes could be detected with our present time resolution (5 ms). Therefore, unlike the isometric situation, stiffness and equatorial signals obtained during ramp shortening are not in agreement. This may be a result of a changed crossbridge spatial orientation during shortening, a different average stiffness per attached crossbridge, or a higher proportion of single headed crossbridges during shortening.

Actins↗

PubMed1991

Time-resolved changes in equatorial x-ray diffraction and stiffness during rise of tetanic tension in intact length-clamped single muscle fibers.

We report the first time-resolved x-ray diffraction studies on tetanized intact single muscle fibers of the frog. The 10, 11, 20, 21, 30, and Z equatorial reflections were clearly resolved in the relaxed fiber. The preparation readily withstood 100 1-s duration (0.4-s beam exposure) tetani at 4 degrees C (less than 4% decline of force and no deterioration in the 10, 11 equatorial intensity ratio at rest or during activation). Equatorial intensity changes (10 and 11) and fiber stiffness led tension (t1/2 lead 20 ms at 4 degrees C) during the tetanus rise and lagged during the isometric phase of relaxation. These findings support the existence of a low force cross-bridge state during the rise of tetanic tension and isometric relaxation that is not evident at the tetanus plateau. In "fixed end" tetani lattice expansion occurred with a time course similar to stiffness during the tetanus rise. During relaxation, lattice spacing increased slightly, while the sarcomere length remained isometric, but underwent large changes after the "shoulder" of tension. Under length clamp control, lattice expansion during the tetanus rise was reduced or abolished, and compression (2%) of the lattice was observed. A lattice compression is predicted by certain cross-bridge models of force generation (Schoenberg, M. 1980. Biophys. J. 30:51-68; Schoenberg, M. 1980. Biophys. J. 30:69-78).

Animals↗

PubMed1990

Detection of radial crossbridge force by lattice spacing changes in intact single muscle fibers.

Time-resolved lattice spacing changes were measured (10-millisecond time resolution) by x-ray diffraction of synchrotron radiation in single intact muscle fibers of the frog Rana temporaria undergoing electrically stimulated tension development during application of stretches and releases. Ramp releases, which decreased fiber length at constant speed, caused a lattice expansion. After the ramp, increasing tension during recovery was accompanied by lattice compression. Ramp stretches caused a compression of the lattice. While the fiber was held at a constant length after the stretch, tension decreased and lattice spacing increased. These observations demonstrate the existence of a previously undetected radial component of the force generated by a cycling crossbridge. At sarcomere lengths of 2.05 to 2.2 micrometers, the radial force compresses the myofilament lattice. Hence, the myofilament lattice does not maintain a constant volume during changes in force.

Animals↗

PubMed1990

Mechanical characteristics of skinned and intact muscle fibres from the giant barnacle, Balanus nubilus.

Intact muscle fibres from Balanus nubilus develop tensions of up to 600 kN.m-2 during electrical stimulation. The rise of tension occurs with a half-time (177 ms at 12 degrees C) about fivefold longer than that of tetanised frog muscle at the same temperature. The response of myofibrillar bundles to a rapid stretch resembles that of frog muscle but has a yo value (i.e. the size of an instantaneous release necessary to just discharge tension) which is ca. 2.5 times smaller, and phase 2 of the tension transient (the "quick phase") occurs at a rate comparable to that of frog muscle. In contrast, the ATPase activity (0.018 mmoles.kg wet weight-1.s-1) of this preparation and its maximum shortening velocity (0.15-0.16 muscle lengths.s-1) are both at least fivefold slower than frog muscle. These findings can be accounted for by a cross-bridge cycle in barnacle muscle in which events prior and subsequent to the tension generating step(s) occur at a rate at least fivefold slower than comparable steps in frog muscle, but the step(s) associated with tension development occur at similar rates in the two preparations. Since the rate of mechanical relaxation in barnacle muscle is modified in the presence of intracellular calcium buffers and by depolarisation-induced elevation of the free calcium during the relaxation phase, it is proposed that the time course of relaxation is not determined exclusively by the kinetics of the cross-bridge cycle, but is also dependent on the free calcium concentration during relaxation.

Adenosine Triphosphatases↗

PubMed1988

Transient kinetics and time-resolved X-ray diffraction studies in isolated single muscle fibres.

The timing of events associated with the contraction and relaxation of the force cycle is described in isolated single arthropod muscle fibers using the fluorescently labelled derivatives of the Ca2+ binding sub-unit of troponin TnC. The kinetics of the subtracted fluorescence (490-410 nm) response from injected TnCDANZ, labelled at the Ca2+ specific sites, shows a rapid rise which is some 90% complete at 50% force consistent with rapid Ca2+ binding to this sub-unit. Subsequently the TnCDANZ fluorescence decays 2x more slowly, at 12 degrees C, than force consistent with a slower release of this bound Ca2+. In fibers injected with both aequorin and TnCDANZ, the aequorin kinetics are essentially unaltered compared to control fibers in the presence of 10-100 microM TnCDANZ. The peak of the aequorin response occurs some 150-170 msec in front of the TnCDANZ peak and the T 1/2 for light decay is faster than either force or TnCDANZ decay, but there is a 'tail' to the aequorin light response (elevated free Ca2+) well into the relaxation phase, seen both in cannulated and intact muscle fibers. The kinetics of the fluorescence of TnCIAANS, labelled of the Ca2+-Mg2+ sites, shows a slow decrease (T 1/2 1.8 sec) and subsequent increase (T 1/2 2.5 sec) in fluorescence consistent with a slow loading and unloading of these sites with Ca2+ during a tetanus. Time resolved X-ray diffraction from intact muscle fibers indicate that forces of up to 600 kN/m2 can be developed at sarcomere lengths of 8-10 micron. Force shows a marked sarcomere dependency while the aequorin response is relatively insensitive. At these high forces, there is a marked change in intensity of the first actin layer line (A2 at 38 nm), consistent with S1 (cross-bridge) attachment, which has a T 1/2 for rise of 125-150 msec.

Actins↗

PubMed1986

An examination of the ability of inositol 1,4,5-trisphosphate to induce calcium release and tension development in skinned skeletal muscle fibres of frog and crustacea.

We have examined the ability of inositol 1,4,5-trisphosphate (InsP3) to cause contractions of mechanically skinned muscle fibres of frog and barnacle. InsP3 (10-500 microM) did not cause any tension development in 25 frog skinned fibres and 26 barnacle myofibrillar bundles, although contractions could be readily evoked by caffeine and by replacement of an impermeant anion by Cl-, treatments known to release calcium from the sarcoplasmic reticulum (SR). Four barnacle bundles did give responses to InsP3. InsP3 did not modify responses to caffeine or calcium-induced calcium release. Free Mg2+ was lowered to 40 microM and 15 mM D-2,3-diphosphoglycerate was added in order to inhibit the possible breakdown of InsP3 by inositol trisphosphatase. Neither measure revealed a response to InsP3. Arsenazo III absorbance measurements failed to detect any binding of Mg2+ (0-0.5 mM) by 0.35 mM InsP3 in our solutions. Inhibitors of SR calcium uptake (cadium, quercetin, furosemide), omission of EGTA from the solution and varying the temperature from 4 degrees to 22 degrees C also failed to reveal a response of frog skinned fibres to InsP3. The nucleotide GTP, which has been reported to enhance InsP3-induced calcium release from rat liver microsomes, had no effect at 50 microM on the response of frog fibres to InsP3. It is concluded that under conditions in which other calcium release mechanisms operate well, InsP3 is relatively ineffective at releasing calcium from the SR in amounts sufficient to induce contraction. Although we have been unable to find evidence to support the proposed role of InsP3 as an essential link in excitation-contraction coupling of skeletal muscle, we cannot entirely reject its role if essential cofactors are lost in the skinned preparations.

2,3-Diphosphoglycerate↗

PubMed1986

Stiffness and force in activated frog skeletal muscle fibers.

Single fibers, isolated intact from frog skeletal muscles, were held firmly very near to each end by stiff metal clasps fastened to the tendons. The fibers were then placed horizontally between two steel hooks inserted in eyelets of the tendon clasps. One hook was attached to a capacitance gauge force transducer (resonance frequency up to approximately 50 kHz) and the other was attached to a moving-coil length changer. This allowed us to impose small, rapid releases (complete in less than 0.15 ms) and high frequency oscillations (up to 13 kHz) to one end of a resting or contracting fiber and measure the consequences at the other end with fast time resolution at 4 to 6 degrees C. The stiffness of short fibers (1.8-2.6 mm) was determined directly from the ratio of force to length variations produced by the length changer. The resonance frequency of short fibers was so high (approximately 40 kHz) that intrinsic oscillations were not detectably excited. The stiffness of long fibers, on the other hand, was calculated from measurement of the mechanical resonance frequency of a fiber. Using both short and long fibers, we measured the sinusoids of force at one end of a contracting fiber that were produced by relatively small sinusoidal length changes at the other end. The amplitudes of the sinusoidal length changes were small compared with the size of step changes that produce nonlinear force-extension relations. The sinusoids of force from long fibers changed amplitude and shifted phase with changes in oscillation frequency in a manner expected of a transmission line composed of mass, compliance, and viscosity, similar to that modelled by (Ford, L. E., A. F. Huxley, and R. M. Simmons, 1981, J. Physiol. (Lond.), 311:219-249). A rapid release during the plateau of tetanic tension in short fibers caused a fall in force and stiffness, a relative change in stiffness that putatively was much smaller than that of force. Our results are, for the most part, consistent with the cross-bridge model of force generation proposed by Huxley, A. F., and R. M. Simmons (1971, Nature (Lond.), 213:533-538). However, stiffness in short fibers developed markedly faster than force during the tetanus rise. Thus our findings show the presence of one or more noteworthy cross-bridge states at the onset and during the rise of active tension towards a plateau in that attachment apparently is followed by a relatively long delay before force generation occurs. A set of equations is given in the Appendix that describes the frequency dependence of the applied sinusoid and its response. This model predicts that frequency dependent changes can be used as a measure of a change in stiffness.

Animals↗