This relationship between force and velocity is what may have prompted some to suggest that the voluntary muscle action should be carried out. The isometric length-tension curve represents the force a muscle is The general form of this relationship is shown in the graph below. On the. In , Hill described the relation between force (P=load) and initial velocity of shortening (V) in contracting whole frog muscles as part of rectangular.
Arguments for purposefully slow PS training Muscle force: While PS proponents vary in their reasoning for suggesting this method, the basic premise is that when the weight is moving quickly, the muscles will not be able to exert as much force and thus the training effect will be diminished Brzycki, ; Wescott, While true that the muscles will not produce as much force at the higher velocities during maximum effort velocity-controlled actions, the previous statement ignores the requisite force to initiate high velocity movements for a given load in an isoinertial condition.
In addition, the aforementioned F-V relationship was derived under conditions of maximal acceleration maximal voluntary muscle activationand thus differs from intentionally slow movements. An attempt to reduce the speed of motion subsequently reduces the force expressed Keogh et al. Modifications to any one of these metabolic factors during exercise may alter signal transduction pathways and hence modify gene transcription for muscle growth Rennie et al.
Potential strength adaptations due to acute metabolic stimuli have recently been reviewed elsewhere Crewther et al. The metabolic hypothesis has not yet been examined in conjunction with PS training studies; therefore these ideas are currently speculative for this type of training.
Force-Velocity Relation: Its Implications about Molecular Mechanism of Muscle Contraction
Movements performed at low velocities prolong the time of contraction in each repetition for a given range of motion time-under-tension; TUT.
Proponents of PS training regard this increased time as a positive characteristic to stimulate training adaptation Wescott et al. TUT can be considered a manner by which to prescribe a dose of resistance exercise Tran and Docherty,which is crucial as the optimal dose for weight training is subject to tremendous debate Carpinelli and Otto, ; Stone et al. PS advocates suggest that this time dose or TUT is of greater importance than the actual load lifted, which could be related to the fact that perceived effort in PS and normal training session have been shown to be similar Egan et al.
This rationale originates from the hypothesis of a direct relationship between the duration of contraction and metabolic stimulus, but this hypothesis has not been supported in studies examining PS exercise Gentil et al.
A potential caveat of increased TUT is that the load must be decreased to perform a successful s concentric contraction as compared to a maximal acceleration repetition i. This is concerning as the load, or mechanical stimuli, has been suggested to be of critical importance for inducing adaptation Dudley et al.
However, the reduced load advocated by PS might be less effective for hypertrophy due to the load constraints. This reduction in load is seen by PS advocates as inconsequential to the ultimate physiological effects. However, a basic premise of tissue adaptation i. Wolff's and Davis' Laws Biewener and Bertram, is that a minimum threshold of force is required to elicit adaptation. The notion that load is peripheral in its importance is in direct opposition to other authors' demonstrating the magnitude of mechanical stress i.
Please note that although related, load and muscle force are not equal, as propulsive forces can differ. Increasing TUT for an exercise session can be accomplished by simply increasing the number of total repetitions of maximal-acceleration exercises increased volume-load; Tran and Docherty, This would ultimately increase the time that the muscle has been under tension for that session, but the force output of the muscle will have been greater due to the relatively larger loads.
The complex relationship between load and TUT requires further investigation. Forms of resistance training fall within a continuum from slow to fast velocities. Resistance training such as powerlifting relatively slow and weightlifting relatively fast are quite far apart on this continuum. Weightlifting WL is the sport by which athletes attempt to lift maximal weight in the snatch and clean and jerk Chiu and Schilling, WL is characterized by high accelerations and fast velocities due to the inherent nature of the sport by which a loaded barbell is moved from the ground at an initial velocity of '0' to an eventual overhead position.
Successful performances of these lifts necessitate great velocities and thus great power Garhammer, ; When tension at each length is plotted against length, a relationship such as that shown below is obtained. While a general description of this relationship was established early in the history of biologic science, the precise structural basis for the length-tension relationship in skeletal muscle was not elucidated until the sophisticated mechanical experiments of the early s were performed Gordon et al.
In its most basic form, the length-tension relationship states that isometric tension generation in skeletal muscle is a function of the magnitude of overlap between actin and myosin filaments. Force-velocity Relationship The force generated by a muscle is a function of its velocity. Historically, the force-velocity relationship has been used to define the dynamic properties of the cross-bridges which cycle during muscle contraction.
The force-velocity relationship, like the length-tension relationship, is a curve that actually represents the results of many experiments plotted on the same graph. Experimentally, a muscle is allowed to shorten against a constant load. P-V relations under various auxotonic loads in maximally stimulated single frog muscle fibers.
Broken line indicates the double hyperbolic P-V relation obtained from isotonic shortening experiments. Inset shows length upper traces and force lower traces changes when a muscle fiber generating the maximum isometric force Po is first made slack, and then subjected to auxotonic loads with different compliance values.
Dotted lines in both length and force records show the development of isometric force when the fiber length is kept at the slackened length. Note that, after being made to slack, the fiber starts shortening against auxotonic load with extremely small velocities compared to the velocity of subsequent shortening.
P-V relation during auxotonic shortening after normalization of forces P during auxotonic shortening. A Diagram showing method of normalization of P relative to Piso during isomeric force development at the same time t after the beginning of auxotonic shortening and force development. Isotonic Velocity Transients and Its Explanation in Terms of Attached Myosin Head Distribution Experiments with whole frog muscle are inadequate to record time course of muscle shortening at its early phase, because of mechanical vibrations when muscle starts shortening against a massive load [ 8 ].
To make this point clear, Civan and Podolsky [ 9 ] performed experiments, in which the early phase of isotonic shortening of isolated single frog muscle fibers was recorded. To avoid mechanical vibrations at the beginning of shortening, they used a steel spring with a length much longer than the distance of fiber shortening, so that the fibers shortened by pulling the long spring, so that the fiber shortening took place under practically constant load.
By this method, they could successfully record the early phase of isotonic fiber shortening. They found that, at the beginning of isotonic shortening, the fiber first showed non steady shortening resembling damped sine waves.
As can be seen in Figure 8, the fiber shortens initially at a velocity higher than the subsequent constant velocity.
The velocity then slows down and again increase until it reaches a constant value. Upper and lower records show fiber shortening and step changes in load, respectively. The magnitude of step changes in load is given as fractions of Po. 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.
Muscle Physiology - Functional Properties
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.
File:Muscle Force Velocity relationship.png
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. 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 ].