Muscle Physiology - Functional Properties
This relationship between force, energy use, and the velocity of contraction has important implications for understanding muscle efficiency, but. to this curve (see p. ). A later investigation into the heat production during isotonic contraction. (Hill, ) showed that the shape of the force-velocity curve . The force velocity relationship is the observation that muscle force and contraction velocity are inversely related. A key concept for athletic performance!.
All these factors can combine to allow the heart to develop more force when required. At any given length the velocity of contraction is a function of the load lifted, with the velocity decreasing as the load is increased.
Response of the heart to stress Demands on the heart vary from moment to moment and from day to day. In moving from rest to exercisethe cardiac output may be increased tenfold. Other increases in demand are seen when the heart must pump blood against a high pressure such as that seen in hypertensive heart disease.
Each of these stresses requires special adjustments. Short-term increases in demand on the heart e. These changes are mediated by increases in sympathetic nervous system activity, an increase in the frequency of contraction, and changes in muscle length. The response to long-term stress hypertension and thyrotoxicosis results in an increase in the mass of the heart hypertrophyproviding more heart muscle to pump the blood, which helps meet the increase in demand.
In addition, subtle intracellular changes affect the performance of the muscle cells. In the pressure-overload type of hypertrophy hypertensive heart diseasethe pumping system of the sarcoplasmic reticulum responsible for calcium removal is slowed while the contractile protein myosin shifts toward slower cross-bridge cycling. The outcome is a slower, more economical heart that can meet the demand for pumping against an increase in pressure. At the molecular level the slowing of calcium uptake is caused by a reduction in the number of calcium pumps in the sarcoplasmic reticulum.
The change in the maximum velocity of shortening and economy of force development occur because each myosin cross-bridge head cycles more slowly and remains in the attached force-producing state for a longer period of time.
In the thyrotoxic type of hypertrophy, calcium is removed more quickly while there is a shift in myosin. At the molecular level there are more sarcoplasmic reticular calcium pumps, while the myosin cross-bridge head cycles more rapidly and remains attached in the force-producing state for a shorter period of time. As the Huxley contraction model Figure 3 only describes distribution of myosin head attached to actin filament during constant velocity shortening, Podolsky and Nolan [ 10 ] proposed another contraction model to account for the isotonic velocity transients Figure 9.
In contrast with the Huxley contraction model, the Podolsky-Nolan model assumes large values of f and a very small value of g in the positive x region, so that all myosin heads passing through this region form A-M links irrespective the velocity of filament sliding. In the negative x region, g remains to be very small over a distance from the equilibrium position 0 and then increases to a large value Figure 9.
As the result, the mode of distribution of A-M link under various loads is markedly different from that in the Huxley contraction model Figure By some additional assumptions, the Podolsky-Nolan model can explain not only the isotonic velocity transients, but also other muscle contraction characteristics and also heat measurement results.
Podolsky-Nolan contraction model constructed to explain the isotonic velocity transient. Upper and middle panel show f and g, i. Lower panel shows dependence of elastic constant k of A-M link on x. Values of P are given at the left of diagrams. In each diagram, A-M link distribution immediately after quick changes in load is given by shaded area, while the subsequent steady A-M link distribution is given by solid line.
Schematic drawing of experimental arrangement to apply quick changes in load in two arbitrary steps. A single fiber P is mounted between force transducer T and lever L with clips C1 and C2, and stimulated maximally with Pt wire electrodes.
Lever L is pivoted at A and loaded by spring F1, which is hooked to lever L and another lever K, so that the length of F1 can be changed by micromanipulator G1 carrying K. Long arms of L and K are restrained by pairs of electromagnetically controlled stops S1 and S2 and S3 and S4, respectively.
Short arm of L serves as a vane interrupting light bean C directed towards photodiode not shown to serve as displacement transducer recording fiber length changes.
With a pair of additional springs F2 and F3, whose lengths are adjusted by microman3 ipulators G2 and G3, the length of F1 can be changed quickly when S3 and S4 are removed to produce movement of K. Oscillation of K is damped with Y shaped dashpot device H. After the fiber develops the maximum isometric force Po, stops restraining lever motion are removed in various sequences, so that the amount of load on the fiber can be changed in two arbitrary steps, as shown in Figure 12 and 13 From ref.
The early time course of isotonic shortening was similar to the isotonic velocity transients reported by Civan and Podolsky [ 9 ], while the early time course of isotonic lengthening was variable. The values of P are expressed relative to Po on the left of each record. Records of experiments, in which the load on the fiber was increased quickly after a period of isotonic shortening under a load of 0.
Note marked oscillatory length changes with alternate lengthening and shortening. To account for the marked fiber length oscillations with alternate fibre lengthening and shortening shown in Figure 13, Sugi and Tsuchiya [ 11 ] constructed a contraction model shown in Figure The dependence of the values of f and g, i. Sugi-Tsuchiya contraction model constructed to explain the marked oscillatory fiber length changes following quick increases in load.
Force velocity relationship | S&C Research
The values of rate constants f and g for the formation and the breaking of A-M links, respectively, are shown as functions of distance from the myosin head equilibrium position 0. As described above, no definite conclusion can be reached about what is actually taking place in muscle, though various contraction models have been presented to explain mechanical responses of muscle fibers in terms of changes in the A-M link distribution.
Much more experimental work is needed for the full understanding of contraction mechanism. Force-Velocity Relation in Single Skinned Muscle Fibers To obtain information about the molecular mechanism of muscle contraction, the use of intact muscle fibers has serious limitations resulting from difficulties in precisely control chemical and ionic conditions around the myofilaments.
The difficulties can be overcome by the use of skinned muscle fibers, from with surface membrane is removed by mechanical or chemical means. To eliminate the gap between contraction characteristic of intact muscle or muscle fibers and biochemical studies on actomyosin ATPase reaction steps in solution, where the three-dimensional myofilament lattice is destroyed, skinned fibers are widely used and their characteristics including the force-pCa relation and MgATP concentration dependence of force development and shortening velocity have been obtained [ 1213 ].
Another great advantage in the use of skinned fibers is that the rate of ATP hydrolysis by the contractile system can be measured simultaneously with mechanical experiments by measuring the rate of ADP production by NADH fluorescence [ 14 ]. Due to structural instability of skinned fibers, however, it is difficult to obtain enough data points to obtain P-V relations, since fiber deterioration slowly proceeds in each contraction-relaxation cycle by the application of contracting and relaxing solutions [ 12 ].
Until the early s, it was necessary for us to use hand-made mechanical or electronic elements to construct such an experimental setup such as shown in Figure 9, with enormous technical skill and patience, because the performance of commercially available electronic devices at that time were not satisfactory for our purpose.
Fortunately, it is now possible to perform sophisticated mechanical experiments including multi-step load changes and ramp decreases in force with instruments commercially available Aurora Scientific Inc.
They found that, in the presence of the antibody 1. In response to ramp decreases in force, skinned fibers shortened with velocities increasing with decreasing force, reaching a maximum at zero force.
The P-V relations thus obtained are presented in Figure Despite the decreased steady isometric force, the value of Vmax remained unchanged Figure 16Aand when the P-V curves were found to be identical if they were normalized with respect to the maximum steady force attained Figure 16B.
These results indicate that the decrease in isomeric force by the antibody results from decrease in the number of myosin heads involved in force generation, while all myosin heads hydrolyze ATP irrespective of whether they generate force or do not generate force; in other words, individual myosin molecules in myosin filaments no longer generate force if the antibody attach to their subfragment-2 region, while their ATPase activity remains unchanged Figures 17 and Effect of antibody to myosin subfragment-2 on the isometric force development and the MgATPase activity in skinned muscle fibers.
The records were obtained before application Aand after min B and min C after application of antibody to myosin subfragment-2 1. Times of application of contracting and relaxing solutions are indicated by upward and downward arrows, respectively.
Upper and lower records show fiber length and force changes in the fiber, respectively. Records A were taken before, and records B, C and D were taken at 30, 60 and 90 min after application of the antibody 1. A P-V relations obtained before and at 30, 60 and 90 min after application of the antibody. Note the value of Vmax remains unchanged despite decrease in the maximum force attained.
Force-Velocity Relation: Its Implications about Molecular Mechanism of Muscle Contraction
B P-V relations normalized with respect to the maximum force attained. Note that the curves are identical. On the other hand, it has been known that the magnitude of maximum isometric force Po in skinned frog muscle fibers increases markedly at low ionic strength [ 1718 ], though the results were somewhat complicated due to gradual deterioration of the fibers. We examined the effect of low ionic strength on contraction characteristics and MgATPase activity of skinned rabbit psoas muscle fibers [ 19 ].
We obtained P-V relations at various ionic strengths, and found that the value of Vmax remained the same irrespective of the magnitude of maximum isometric force, which increased with decreasing ionic strength, and if the P-V curves are normalized with respect to the maximum force, the P-V curves are also found to be identical in shape.
Not that the value of Vmax does not change appreciably despite decrease of steady isometric force with increasing KCl concentration. B The P-V relations normalized with respect to the steady forces attained. Not that the P-V curves are identical in shape. As described above, P-V relation is extremely useful in obtaining insights into the molecular mechanism, with which various factors affect contraction characteristics and MgATPase activity of skinned muscle fibers.
Conclusion In this article, we presented an overview about research work on the P-V relation in whole muscles, single intact muscle fibers, and single skinned muscle fibers. This fundamental property of contracting muscle or muscle fiber has been explained in terms of distribution of A-M link at both sides of the equilibrium position of myosin head based on assumptions of rate constants f and g. Up to the present time, however, no contraction models can explain all the experimental results obtained from contracting muscle or muscle fibers.
Muscle contraction is still filled with a number of mysteries.
In our opinion, experiments on skinned muscle fibers are most important to reach full understanding on the molecular mechanism of muscle contraction, despite their inherent problems.
Acknowledgements We wish to thank Drs. Teizo Tsuchiya, Takakazu Kobayashi and Hiroyuki Iwamoto for their contribution to the experiments described in this article. Our thanks are also due to Mr. Matthew Brokowski of Aurora Scientific Inc. References Hill AV The heat of shortening and the dynamic constants of muscle. Proc Roy Soc B Podolsky RJ The nature of the contractile mechanism in muscle. In Biophysics of Physiological and Pharmacological Actions. Am Ass Adv 1Sci