The force generated by a muscle depends on the number of actin and myosin cross-bridges formed; a larger number of cross-bridges results in a larger amount of force. However, cross-bridge formation is not immediate, so if myofilaments slide over each other at a faster rate the ability to form cross bridges and resultant force are both reduced.
At maximum velocity no cross-bridges can form, so no force is generated, resulting in the production of zero power right edge of graph. The reverse is true for stretching of muscle. Although the force of the muscle is increased, there is no velocity of contraction and zero power is generated left edge of graph. Maximum power is generated at approximately one-third of maximum shortening velocity.
Skeletal muscle contractions can be broadly separated into twitch and tetanic contractions. In a twitch contraction, a short burst of stimulation causes the muscle to contract, but the duration is so brief that the muscle begins relaxing before reaching peak force. If another contraction occurs before complete relaxation of a muscle twitch, then the next twitch will simply sum onto the previous twitch, a phenomenon called summation.
If the stimulation is long enough, the muscle reaches peak force and plateaus at this level, resulting in a tetanic contraction. Force-Velocity Relationship: As velocity increases force and power produced is reduced. However, cross-bridge formation is not immediate and if myofilaments slide over each other at a faster rate, their ability to form cross-bridges and subsequent force are both reduced.
At a maximum velocity no cross-bridges can form so no force is generated, resulting in the production of zero power right edge of graph. The reverse is true for stretching of muscle; although the force of the muscle is increased, there is no velocity of contraction and zero power is generated left edge of graph. Twitch contractions are short in duration. Though they have high velocity, they begin resting before reaching peak force. Tetanic contractions, which are long in duration, reach peak force and plateau.
The motor unit is the functional unit of muscle contraction and includes the motor nerve fiber and the muscle fibers it innervates. A motor unit consists of the motor neuron and the grouping of muscle fibers innervated by the neuron.
Precision is inversely proportional to the size of the motor unit. Thus, small motor units can exercise greater precision of movement compared to larger motor units. For instance, thigh muscles, responsible for large powerful movements, can have a thousand fibers in each unit, while eye muscles, requiring small precise movements, might only have ten.
Groups of motor units are innervated to coordinate contraction of a whole muscle and generate appropriate movement; all of the motor units within a muscle are considered a motor pool. There are often multiple sizes of motor unit within a motor pool as a means of modulating the precision and force produced by a single muscle. For example, a small motor unit in the biceps can be activated for small precise movements, while a larger motor unit can be activated to facilitate more forceful actions.
These multiple motor units of different sizes within a motor pool allow for very fine control of force either spatially or temporally.
Even when at rest, muscle fibers are at least partially contracted, possessing a small degree of tension which is termed muscle tone or tonus. Muscle tone is controlled by neuronal impulses and influenced by receptors found in the muscle and tendons. This influence leads to the generation of reflexes in the spinal cord, such as the immediately obvious knee jerk reaction but also including key functions such as the posture maintenance and proper digestive system function..
Sliding Filament Model of Contraction : Muscle fibers in relaxed and contracted positions. Muscle tone ensures that even when at rest the muscle is at least partially contracted. The main regulator of muscle tone is the muscle spindle, a small sensory unit that is closely associated with and lies parallel to a muscle.
Connecting to the endomysium of a muscle fiber, muscle spindles are composed of nuclear bag fibers and nuclear chain fibers. Both are similar to muscle fibers in that they contain actin and myosin myofilaments that allow them to stretch with the muscle. However, unlike skeletal muscle fibers where the nuclei are spread out and located at the periphery of the cell, in nuclear bag and nuclear chain fibers the nuclei are located in a central region which is enlarged in nuclear bag fibers.
Both cells of the muscle spindle contain sensory neurons. When stretched, muscle spindles become activated, triggering impulses to the spinal cord that can generate an immediate reflex. Spindles can also trigger impulses to the cerebral cortex providing information about the degree of stretch within the muscle. To maintain tone, spindles also operate a feedback loop by directly triggering motor neurons linked to their associated muscles. If tone decreases and the muscle stretches the spindle, an impulse results in a muscle contraction.
With this contraction, the spindle is no longer stretched. This modified version represents the force-velocity relationship of the isolated sartorius muscle of a frog. During the following decades, more studies were conducted in order to confirm the results reported by Hill Katz conducted a series of experiments in which the shortening and lengthening velocity of muscle was evaluated at forces both lower and higher than P 0.
Of note, heat changes during lengthening contractions were found to be too small to be accurately captured Hill, In line with this observation, decreased muscle excitation levels were observed during active muscle lengthening Bigland and Lippold, , demonstrating physiological differences between eccentric and concentric muscle actions.
The authors reported that the F-V relationship was best represented by a curvilinear function, but these results were affected by apparent effects of fatigue. Had only the best attempts i. Figure 3. Force torque -velocity angular velocity relationship.
Data were obtained from Figure 8 in Dern et al. P 0 denotes the maximal isometric torque. The attempts with no apparent effect of fatigue were selected. The next work to address the F-V relationship was a unique study performed with amputee men with cineplastic 2 tunnels through various muscles of the upper extremity, making it possible to evaluate the in vivo F-V relationship of isolated, yet voluntarily contracting human muscles Ralston et al.
However, post hoc analyses of the respective data performed by us showed that the F-V data by Ralston et al. These analyses negate a purely hyperbolic F-V relation. Figure 4. Force-velocity relationship. Data were obtained from Figure 1 in Ralston et al. This modified version represents the force-velocity relationship of the in vivo human pectoralis major muscle. By contrast, Wilkie reported a Hill-type load-velocity relationship for the human elbow flexor muscles. While the linearity of the F-V relationship at high forces was not clearly evident in the studies conducted by Hill and Wilkie , an explanation might be provided by a later study by Abbott and Wilkie In their experiments, after completion of the dynamic contractions, the authors noted not only a decrease in maximal isometric force but also a shift of the optimal length to longer muscle lengths Abbott and Wilkie, A similar observation was reported by Ritchie , who recorded isometric force in isolated rat muscle during every third or fourth isotonic contraction, and found that isometric force decreased toward the end of the series of tests performed with increasing loads.
In some previous studies, the isometric force was not evaluated Gasser and Hill, ; Levin and Wyman, ; Fenn and Marsh, or it was only measured at the beginning of the experiments Hill, ; Katz, In addition, in experiments performed in isolated muscles, the isometric force was measured at the optimal length, while velocity values during dynamic actions were registered after the muscle had shortened a certain distance Hill, ; Katz, Thus, isometric force and dynamic F-V data were collected at slightly different muscle lengths, leading to an overestimation of P 0.
Consequently, Ritchie and Wilkie proposed that previous studies might have missed deviations from the rectangular hyperbola because of the considerable uncertainty of the appropriate value of the isometric tension. Figure 5. In addition, despite the generally satisfactory fitting of the hyperbolic F-V function with experimental F-V data obtained below a certain level of force, evidence from studies conducted either in isolated muscle Katz, ; Ralston et al.
Even Hill himself pointed out that the F-V data might deviate from a hyperbola in the high-force region of the F-V relationship Hill, , pp — The main limitation of studies performed at that time was that none had specifically investigated the observed deviations from the rectangular hyperbola.
In addition, few studies had described the eccentric portion of the F-V relationship Levin and Wyman, ; Katz, Indeed, most studies did not evaluate the F-V relationship in the high-force range i.
The experiments were performed in single isolated frog muscle fibers and also in bundles of frog muscle fibers, and a sufficient number of experimental data points over the whole F-V relationship was obtained. Importantly, the deviation from the hyperbolic F-V relationship was found to be independent from the mode of activation or the time interval between tetani 1, 3, or 60 min Edman et al. Thus, the deviation from the hyperbola was found not to be caused by muscle fatigue.
Therefore, the F-V relationship was found to be better characterized by a double-hyperbolic F-V equation Edman, a :. Importantly, the fact that the double-hyperbolic F-V relationship was observed in both the fiber as a whole and a short segment of the same fiber suggests that this pattern represented the contractile behavior at the sarcomere level.
In addition, the F-V relationship was also investigated at forces that exceeded the isometric force 1. The F-V relation formed a smooth sigmoidal function with inflection at P 0 , and was observed to be nearly flat between 0. By contrast, changes in velocity were progressively greater at forces between 1. Figure 6.
A Double-hyperbolic force-velocity relationship. Data were obtained from Figure 2 in Edman a using specialized software ImageJ 1. This modified version represents the force-velocity relationship of a single muscle fiber from the anterior tibialis muscle of a frog. Data were obtained from Figure 7 in Edman a using specialized software ImageJ 1.
This modified version represents the eccentric and concentric force-velocity relationship of a single muscle fiber from the anterior tibialis muscle of a frog. Note the drastic differences in force around the isometric force open square 0. The double-hyperbolic shape of the F-V relationship was confirmed in intact frog single muscle fibers Iwamoto et al.
The first study evaluating whether the double-hyperbolic F-V relation might also be found in whole mammalian skeletal muscle in situ was that conducted by Devrome and MacIntosh The sciatic nerve was electrically stimulated to elicit maximal muscle contractions against different loads ranging from nearly unloaded to maximal isometric force. After a fatiguing protocol, both the P 0 and V max values were significantly decreased, though the double-hyperbolic nature of the F-V relation was maintained.
With fatigue the breakpoint was located at the same relative value of P 0 , but increased to 8. Nonetheless, deviations from the rectangular hyperbola have also been found at very high contraction velocities Edman, ; Claflin and Faulkner, Maximum shortening velocity or the velocity of unloaded shortening i.
V 0 is an important measure in that it reflects the kinetic properties of actomyosin interactions Schiaffino and Reggiani, ; Bottinelli and Reggiani, However, V 0 has been found to be greater than V max in whole muscles Edman, ; Claflin and Faulkner, due to the fact that V 0 is a measure of the maximal unloaded shortening velocity of the fastest muscle fibers, whereas V max is a function of the F-V relationship of all muscle fibers, provided that it is estimated from F-V data obtained at moderate loads.
Thus, V max and V 0 are similar when measurement data are obtained at sufficiently low forces Edman, , while substantial differences may be found when V max is obtained from F-V points relatively far from V 0 Claflin and Faulkner, Furthermore, high fiber type heterogeneity may augment the differences between V max and V 0 values because the presence of slower fibers would influence V max estimations while not having any effect on V 0 values Josephson and Edman, It is important to note that, in contrast to in vitro studies of isolated single muscle fibers or whole muscles, several factors other than cross-bridge kinetics influence the observed F-V relationship under in vivo conditions.
These factors include neural activation, the mechanical properties of in-series elastic components, lateral force transmission between neighboring muscle fibers, muscle architecture, lever arms of joints, coordination of agonist and antagonist muscles and other possible factors that might be outside of our current understanding on muscle contraction and function.
For example, it has been demonstrated that muscle moment arm length influences the torque-velocity relationship Nagano and Komura, A longer moment arm requires muscle shortening velocity to be greater at any given joint angular velocity, thus forcing the muscle to act in a lower region of its F-V relationship. This detrimental effect on muscle force is compensated by the longer moment arm during slow joint angular velocities, and consequently greater joint moments were observed at slow joint angular velocities compared with having a shorter moment arm.
In contrast, the decreased muscle force could not be compensated by the longer moment arm during fast joint angular velocities, resulting in lower joint moments Nagano and Komura, These considerations notwithstanding, the evaluation of the in vivo F-V relationship is still of great relevance for muscle and exercise physiology, as the F-V curve reflects human performance Dorel et al. Deviations from the rectangular hyperbola are not unusual in the in vivo F-V relationship in humans during either single- Komi, ; Thorstensson et al.
In both cases, the reason for the deviations was speculated to be a central inhibitory mechanism Perrine and Edgerton, ; Wickiewicz et al. This hypothesis was tested by several studies evaluating the in vivo human F-V relationship during isokinetic knee extensions elicited by maximal voluntary muscle actions versus neuromuscular electrical stimulation or superimposed electrical stimulation.
No alterations in voluntary activation were observed during isometric or concentric knee extensions Dudley et al. These observations conflict with the hypothesis of neural inhibition being the factor explaining the deviation of force values from the hyperbolic F-V relationship at low concentric velocities.
The F-V data presented by Harris and Dudley , from both voluntary and electrically stimulated muscle actions, might correspond well with a double-hyperbolic F-V relationship. Evidence of a double-hyperbolic F-V relationship during single-joint muscle actions can also be inferred from other studies Dudley et al.
By contrast, the in vivo human F-V relationship during multi-joint muscle actions has been reported to follow a strictly linear pattern Bobbert, Indeed, Hahn et al. The authors based their conclusion on the observation of individual R 2 values ranging from 0. By a careful analysis of the data from one subject presented in Figure 1 of that study Cuevas-Aburto et al.
These discrepancies are likely to be much higher in those individuals showing inferior R 2 values from the linear model i. Therefore, although linear models might be adequate in some individuals because of their feasibility and similar output results compared with hyperbolic models, the F-V relationship is in fact curvilinear in the range of moderate-to-low forces.
Another factor that may influence the shape of F-V relation during in vivo measurements is the joint angle at which F-V data are obtained. Secondly, it is important to note that due to the influence of the in series elastic component of the muscle-tendon complex Reeves and Narici, it is not possible to infer identical muscle length from equivalent joint angles when measurements are recorded at different contraction velocities.
Tendons are visco-elastic structures that exhibit both rate-dependent viscous and rate-independent elastic properties. Several ultrasound studies Hauraix et al.
When muscle force decays tendons shorten releasing the elastic energy previously stored Finni et al. This mechanism of tendon recoil enables muscle fascicles to shorten at lower velocities at given muscle-tendon unit velocities, which enhances force production. Muscle-tendon interaction also implies that muscle length at a given joint angle is shorter under higher forces i. Other structures within muscles exhibiting spring-like properties may amplify this effect e.
Fortunately, ultrasound studies have shown that peak torques during concentric knee extensions and plantar flexions at different angular velocities occurred when vastus lateralis and medial gastrocnemius fascicle lengths, respectively, were close to their optimal fascicle lengths Ichinose et al. Thus, collecting F-V data at the point of peak torque may allow for the effects of velocity to be studied in isolation.
In any case, the F-V relationship obtained from peak values or angle-specific values has been reported to display essentially the same shape with minor differences in curvature , and differ only in magnitude Wyatt and Edwards, ; Yates and Kamon, ; Westing et al. With the advent of modern imaging techniques, increased efforts have been made to study the F-V relationship in vivo through the combined use of ultrasound and dynamometry Ichinose et al.
However, the estimation of fascicle force from external joint torque relies on several important assumptions. First, moment arms and the relative contribution of the target muscle to external force must be assumed to be constant across subjects, although recent research points to substantial inter-individual differences in these parameters Massey et al.
Even within subjects, the relative contribution of individual agonist muscles or muscle fascicles to external force production at different contraction velocities might not be the same, because their F-V properties may differ due to distinct characteristics of muscle architecture and ATPase activity Barany, ; Spector et al. The activity of antagonist muscles lowering joint torque is usually not considered.
Moreover, muscle architecture and fascicle behavior along the muscle are presumed to be uniform, in spite of reports demonstrating great intramuscular heterogeneity Trezise et al.
These differences can even be magnified by the arrangement of muscles around the joints Lieber and Friden, In addition, it should be noted that force produced by a fascicle cannot be directly inferred from the degree of its shortening: if the muscle fibers under investigation were shortening at their maximal unloaded velocity they would not transmit forces to their myotendinous junctions.
Indeed, studies in prepared frog muscles suggest that single fibers might even be shortening at velocities much greater than their unloaded contraction velocities, due to assisting force provided by the fastest fibers Edman, In the light of these methodological challenges, the validity of muscle fascicle F-V relationships as estimated from ultrasound measurements must be doubted.
The inability of ultrasound measurements to capture out-of-plane movements of muscle fascicles or the lack of a fixed frame of reference should also be considered Karamanidis et al. On the other hand, the effect of different muscle lengths on the double-hyperbolic F-V relationship has not been thoroughly studied yet, although Hahn et al. This may be due to the history dependence of muscle contraction Rassier and Herzog, , by which force depression is observed after active muscle shortening Edman et al.
This effect is more pronounced when greater mechanical work is performed Herzog et al. The eccentric portion of the F-V relationship has not been studied as extensively as the concentric part. However, eccentric muscle function is vital during various activities of daily living such as absorbing energy when landing from a jump or lowering an object or body mass, or for proper antagonist muscle function. Early studies conducted in animal muscles showed that the eccentric portion of the F-V relation follows a convex upward curve with force values rising substantially above isometric levels in the range of low negative contraction velocities, while force values remain practically unchanged in the range of moderate-to-high negative contraction velocities Levin and Wyman, ; Katz, Edman a described the transition from concentric to eccentric forces as a sigmoidal function with inflection at P 0 , and noted that force values increased steeply at low negative velocities up to 1.
These findings were confirmed by other studies Lannergren, ; Stienen et al. A specific, albeit rarely used, hyperbolic equation has been proposed to describe the eccentric F-V relationship in the range of forces between 1.
Other hyperbolic equations for modeling the eccentric F-V relationship with an asymptote set at 1. To our knowledge, the first study evaluating the eccentric portion of the F-V relationship in humans was that conducted by Komi using an isokinetic dynamometer that measured force during eccentric and concentric elbow flexions.
This enhanced eccentric force response was accompanied by similar EMG values being recorded in agonist and antagonist muscles at the different concentric and eccentric velocities Komi, Similar results were reported by Rodgers and Berger soon after the study by Komi. Studying both men Westing et al. To elucidate such mechanisms, various investigations have studied the F-V relationship using electrical muscle stimulation applied either in isolation or superimposed on voluntary eccentric contractions.
Discrepant torque augmentation through electrical stimulation observed in elite athletes and sedentary subjects Amiridis et al. Confirmatively, neuromuscular activation Aagaard et al. In addition to learning effects, the preceding state i. Indeed, when muscle actions started from a resting state or a low preload, the muscle first shortened presumably stretching the tendon, even if joint rotation indicated the onset of the eccentric contraction Hahn, In such contractions, eccentric forces may very well be similar or even lower than the maximal isometric force, since they actually correspond to a concentric muscle action.
Another methodological consideration to keep in mind is that, while registering angle-specific force values during eccentric contractions may help to acquire data at similar muscle length, this may differ from the muscle length recorded at the same joint angle during isometric and concentric contractions Reeves and Narici, Another explanation that may contribute to the lower forces observed in in vivo as compared to in vitro studies is related to temperature effects.
In addition, eccentric torque enhancement may also be related to the amplitude of muscle stretch. A positive relationship between stretch amplitude and eccentric torque enhancement has been demonstrated for electrically evoked Cook and McDonagh, and maximal voluntary muscle actions Hahn et al. Finally, maximal eccentric forces appear to level off or decrease beyond a certain level of lengthening velocity Dudley et al. The sliding filament theory is a physico-chemical theory accounting for the mechanical, chemical, and structural features of skeletal muscle that was formulated by Huxley The theory was inspired by previous evidence showing the microscopic structure of skeletal muscle and chemical reactions observed in glycerinated muscle preparations Szent-Gyorgyi, , and the relationship observed between force, velocity, and heat production Hill, This leads to the movement of actin relative to myosin filaments, until the link is broken due to a chemical reaction Huxley, According to the theory, the decrease in force observed at increasing contraction velocity is caused by: 1 the increasing likeliness of pairs of actin and myosin myofilaments passing each other without cross-bridges being formed; and 2 the increasing proportion of links formed between actin and myosin that will not be disassociated in time, generating a force in the opposite sense of muscle shortening.
Then, the maximal velocity of unloaded shortening is found at the point where negative forces equal positive forces, and net force is zero Huxley, The agreement between the experimental F-V data reported by Hill and the sliding filament theory Huxley, was decisive for the acceptance of the theory.
For that purpose, the rate constants f rate constant for the formation of cross-bridges and g rate constant for the detachment of cross-bridges were given specific values that varied depending on the distance between the active site on the actin filament and the equilibrium position of the sliding element on the myosin filament Huxley, ; Gordon et al.
Thus, it is currently accepted that muscle contraction results from the relative sliding of two sets of filaments arranged in parallel in each sarcomere: the thick filament in skeletal muscles mainly composed of the motor protein myosin II and the thin filament containing actin filaments.
Upon activation, myosin heads repeatedly attach to actin, stroke and then detach again, determining muscle performance i. However, as shown previously, deviations from the rectangular hyperbola in different portions of the F-V relation have been found. These deviations may be due to the violation of one or more of three central assumptions regarding the kinetics of cross-bridge formation Seow, : 1 the detachment rate is linearly proportional to the shortening velocity; 2 the attachment rate is independent of shortening velocity; and 3 force per cross-bridge declines linearly with shortening velocity.
Piazzesi et al. They found that the detachment rate decreased linearly with decreasing shortening velocity, but never dropped to zero Figure 7A Piazzesi et al. Finally, the force exerted per cross-bridge expected to decrease with shortening velocity was found to be nearly constant at intermediate and low velocities Figure 7B Piazzesi et al.
These observations gave support to the double-hyperbolic F-V relationship. Figure 7. Molecular mechanisms accounting for the double-hyperbolic shape of the F-V relationship. Data were obtained from Figure 4 in Piazzesi et al. In this modified version: A Cross-bridge kinetics detachment — solid line — and attachment — dashed line — rate constants are presented as a function of velocity.
B The number of attached motors solid line varies with velocity, following a nearly hyperbolic shape, while the average force per attached motor discontinuous line decreases below a certain velocity threshold. To test whether the double-hyperbolic F-V relation observed in skeletal muscles and single muscle fibers Edman, a ; Devrome and MacIntosh, was also evident at the single sarcomere level, an in vitro motility assay system was developed to investigate the steady-state F-V relation derived from the interaction between actin and myosin molecules Chaen et al.
F-V data resembled a hyperbola at forces lower than 0. The F-V relationship was found to be analogous in shape to the double-hyperbolic F-V relationship observed in single muscle fibers Edman, a. Conflicting with this hypothesis, other studies have provided evidence to show that the shape of the F-V relation is rather influenced by the change in the number of attached cross-bridges and the force created by each of them. Edman conducted a series of experiments in which fiber force and stiffness were recorded while fibers shortened at various velocities during tetanic contraction.
Since stiffness indicates the proportion of active cross-bridges within the fiber Ford et al. These findings were later confirmed in both rested and moderately fatigued intact single fibers Curtin and Edman, This modulation may explain the bend of the F-V relationship at high forces and its appearance as a double-hyperbolic curve. In line with this, Mansson presented a four-state cross-bridge model that considered both the velocity-dependent attachment rate and the variation of the proportion of cross-bridges attached to the actin filaments at different force-generating states.
The proportion of cross-bridges in a low force-generating state decreased, while that of cross-bridges in a high force-generating state increased between 0. On the other hand, enhanced force production during eccentric contractions may be due to an increased number of cross-bridges attached to actin, increased force per cross-bridge, or a combination of these Rassier, Several studies have reported an increased number of myosin heads attached to actin during eccentric compared with isometric contractions Linari et al.
This would be facilitated by the attachment of the second motor domain of the myosin, which remains inactive under isometric and concentric conditions, but is activated with stretch.
It is interesting to note that, despite the increased number of attached cross-bridges, ATP consumption is actually lower during eccentric contractions compared with isometric contractions Linari et al. This is because myosin heads detach from actin by a strain-dependent process that does not require ATP splitting Piazzesi et al. In support of the theory on multiple force-generating states of cross-bridges, myosin heads that are only weakly bound to actin pre-power stroke state under isometric conditions have been reported to switch to a strong binding state with stretch Getz et al.
Notably, having a larger fraction of myosin heads in a weak binding state under isometric conditions has been related to a greater force production during active muscle lengthening, which depended on the myosin heavy chain isoform Linari et al.
Regardless the cross-bridge-related mechanism, force enhancement in lengthening contractions has been described to occur in two phases Getz et al. Consequently, detached cross-bridges must reattach rapidly to actin to keep producing force. Fortunately, reattachment during muscle lengthening occurs at a higher rate compared with reattachment under isometric or concentric conditions Lombardi and Piazzesi, By contrast, force exerted during the second phase as a consequence of cross-bridge reattachment decreases as a function of lengthening velocity attachment rate-dependent.
Thus, the higher the ability of the myosin heads to rapidly detach from actin, the higher the maximal unloaded shortening velocity. However, contrary to the expectation of high metabolic costs that would result from this mechanism, the rate of ATP splitting is low during rapid or unloaded muscle shortening Kushmerick et al. It has recently become clear that the thick filaments have a second mechanosensing mechanism that is distinct from that of the individual myosin heads, and that is also independent from the thin filament-based calcium-dependent regulatory mechanism of muscle contraction Irving, Thick filaments remain in a structural and functional OFF state at rest, with the two heads of the myosin locked in a conformation in which they can neither bind to actin nor hydrolyze ATP even in the presence of high intracellular calcium concentration Irving, Thick filaments are progressively switched ON i.
This thick filament-based muscle contraction mechanism would be especially important to control the metabolic cost of muscle contraction. The exact mechanism of this thick filament mechanosensing is poorly understood, since scant data of the molecular structure of the thick filament exist. Other proteins such as titin and myosin binding protein-C have been suggested as potential regulators of mechanosensitivity Irving, The mechanosensing of the thick filament is likely to be associated with the deviation of F-V data from the rectangular hyperbola at forces below 0.
To better understand this relationship, it is important to be aware of motor unit recruitment patterns during voluntary muscle actions. Thus, motor units are orderly recruited i.
However, in situations where rapid force development is required i. Ballistic muscle actions are characterized by initially high firing frequencies Oishi et al. This event allows an earlier recruitment of most motor units even before force production can be detected, with some reversals in the order of recruitment between smaller and larger motor units Desmedt and Godaux, ; Yoneda et al.
Since larger motor units possess faster conduction velocities, they may produce force earlier than smaller motor units even if recruited at a slightly later time Desmedt and Godaux, The compression of the range of motor unit recruitment thresholds and the early activation of faster motor units would be an advantage during very fast ballistic muscle actions that are so brief for the motor unit recruitment pattern to be amenable to modification through proprioceptive feedback.
Under low loads or unloaded conditions i. By contrast, the slower muscle fibers would not perceive any mechanical stress, and thus their thick filaments would remain in an OFF state. Therefore, this mechanism would allow the fastest muscle fibers to contract under very low loads or unloaded conditions with no or little resistive forces coming from the slower muscle fibers.
This hypothesis would explain both the discrepancies found in the literature between V max and V 0 values Edman, ; Claflin and Faulkner, , and the ability of single muscle fibers to shorten above their maximal unloaded shortening velocity without being damaged Edman, One further factor that might influence the shape of the F-V relationship is the history-dependent behavior of muscle contraction Rassier and Herzog, : isometric force at a given muscle length is lower when the contraction is preceded by muscle shortening force depression after active muscle shortening Abbott and Aubert, ; while isometric force at a given muscle length is higher when the contraction is preceded by muscle lengthening residual force enhancement after active muscle lengthening Edman et al.
These aspects are rarely considered in cross-bridge models of muscle contraction. To explain the force depression, several hypotheses have been proposed that are related to the non-uniformity of sarcomere length, the accumulation of metabolites or the stress-induced inhibition of cross-bridge attachment, with the latter hypothesis being most supported by scientific evidence Rassier and Herzog, The diminution of attached cross-bridges Sugi and Tsuchiya, ; Lee and Herzog, is expected to be caused by the stress that they impose on the portion of myofilaments that is initially not yet inside the overlap zone, but that will reach that zone with the advancement of muscle shortening Marechal and Plaghki, In addition, the PEVK region of titin might attach to actin filaments, thus inhibiting cross-bridge formation and leading to force depression Rode et al.
Hence, higher forces i. On the other hand, sarcomere length non-uniformity and instability, an increase in the proportion of attached cross-bridges, or an engagement of a passive element have been proposed as possible contributors to residual force enhancement after active muscle lengthening Rassier and Herzog, Due to the breadth of the topic, we were forced to omit several studies on the F-V relationship that we considered less relevant for the overall topic of this review.
We apologize to all authors whose works we were not able to include. Further reviews focusing on other aspects of the F-V relationship or muscle contraction during shortening and lengthening can be found in the literature, among others: Gulch, ; Gordon et al.
Knowledge about the shape of the F-V relationship in skeletal muscles has evolved substantially over the last years. However, the present review reveals that significant discrepancies regarding the shape of the eccentric and concentric F-V relationship still exist in the current literature.
The deviations of F-V values from the original hyperbolic F-V relationship at both low and very high positive concentric velocities may be due to the fact that the F-V relationship is actually double-hyperbolic Edman, a and to differences in the proportion and characteristics of single muscle fibers contributing to force production at different shortening velocities Josephson and Edman, , respectively.
This bi-phasic relationship has been confirmed in various muscle preparations Mansson et al. The double-hyperbolic F-V relationship is presumably caused by velocity-specific cross-bridge kinetics Piazzesi et al. A double-hyperbolic F-V relationship is functionally plausible, since it is expected to improve the mechanical stability of the myofilament system at high forces Edman et al.
This makes the muscle an efficient braking system when stretched. For the eccentric part of the F-V relationship, several human studies have shown a reduction, no change or an increase in eccentric force values compared with maximal isometric force. The evidence has demonstrated that these discrepancies may be caused by a lack of familiarization Hahn, , insufficient muscle conditioning Amiridis et al. In any case, maximum eccentric force values should generally be found to be greater than isometric force up to 1.
Neural factors may account for decreases in eccentric force production at high lengthening velocities Duchateau and Enoka, Based on the present literature overview, the following methodological recommendations should be considered to improve standardization of the assessment of F-V data in humans:. Although further research is necessary, a minimum of two sessions for the concentric Alcazar et al.
Until studies addressing this issue are conducted, we encourage researchers to assess the F-V relation in the highest possible ranges of forces and velocities. The implementation of ultrasonography for controlling muscle length or the collection of F-V data at the point of peak force may help minimize possible interactions between the F-V and the force-muscle length relationships Ichinose et al.
JA and RC wrote the manuscript based on relevant investigations for the overall topic of this work. IA and LA supervised and made substantial contributions to the final version of the manuscript. All authors listed approved the manuscript for publication. The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
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A Biol. The double-hyperbolic force-velocity relationship in humans. Acta Physiol. Oxford, England [Epub ahead of print]. PubMed Abstract Google Scholar. Force-velocity profiling in older adults: an adequate tool for the management of functional trajectories with aging. The force-velocity relationship in older people: reliability and validity of a systematic procedure.
Sports Med. Allen, P. At maximum velocity no cross-bridges can form, so no force is generated, resulting in the production of zero power right edge of graph. The reverse is true for stretching of muscle. Although the force of the muscle is increased, there is no velocity of contraction and zero power is generated left edge of graph. Maximum power is generated at approximately one-third of maximum shortening velocity.
Learning Objectives Differentiate between force-length and force-velocity of muscle contraction. Key Points The force -length relationship indicates that muscles generate the greatest force when at their resting ideal length, and the least amount of force when shortened or stretched relative to the resting length.
The force-velocity relationship demonstrates that power produced is controlled by the velocity and force of muscle contraction, with an optimum power output at one third of maximum velocity. Key Terms force : Any influence that causes an object to undergo a certain change concerning its movement, direction, or geometrical construction.
Force-Length Relationship : The relationship between sarcomere length and force produced in the muscle, modulated by actin and myosin myofilament overlap. Muscle Force Generation The force a muscle generates is dependent on the length of the muscle and its shortening velocity.
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