Reports of the length dependence of fatigue are greatly exaggerated

The Relationship Between Length and Active Force for Submaximal Skeletal Muscle Contractions: a Review

The force-length relationship is usually obtained for isometric contractions with maximal activation, but less is known about how sarcomere length affects force during submaximal activation. During submaximal activation, length-dependent alterations in calcium sensitivity, owing to changes in cross-bridge kinetics (rate of attachment and/or detachment), result in an activation-dependent shift in optimal length to longer sarcomere lengths. It is known that sarcomere length, as well as temperature and phosphorylation of the regulatory light chains of myosin, can modify Ca2⁺ sensitivity by altering the probability of cross-bridge interaction. This altered calcium sensitivity is particularly important for submaximal force levels, as it can change the shape of the length dependence of force, with peak force occurring at sarcomere lengths longer than those associated with maximal filament overlap. In athletic contexts, contractions typically do not reach maximal intensity. Therefore, understanding that the ability to produce force under both maximal and submaximal conditions can differ, and that peak force can be generated at different lengths, could influence the development of targeted training regimens optimal for each sport.

Enhanced capacity for CaMKII signaling mitigates calcium release related contractile fatigue with high intensity exercise

BACKGROUND

We tested whether enhancing the capacity for calcium/calmodulin-dependent protein kinase type II (CaMKII) signaling would delay fatigue of excitation-induced calcium release and improve contractile characteristics of skeletal muscle during fatiguing exercise.

METHODS

Fast and slow type muscle, gastrocnemius medialis (GM) and soleus (SOL), of rats and mouse interosseus (IO) muscle fibers, were transfected with pcDNA3-based plasmids for rat α and β CaMKII or empty controls. Levels of CaMKII, its T287-phosphorylation (pT287-CaMKII), and phosphorylation of components of calcium release and re-uptake, ryanodine receptor 1 (pS2843-RyR1) and phospholamban (pT17-PLN), were quantified biochemically. Sarcoplasmic calcium in transfected muscle fibers was monitored microscopically during trains of electrical excitation based on Fluo-4 FF fluorescence (n = 5-7). Effects of low- (n = 6) and high- (n = 8) intensity exercise on pT287-CaMKII and contractile characteristics were studied in situ.

RESULTS

Co-transfection with αCaMKII-pcDNA3/βCaMKII-pcDNA3 increased α and βCaMKII levels in SOL (+45.8 %, +250.5 %) and GM (+40.4 %, +89.9 %) muscle fibers compared to control transfection. High-intensity exercise increased pT287-βCaMKII and pS2843-RyR1 levels in SOL (+269 %, +151 %) and GM (+354 %, +119 %), but decreased pT287-αCaMKII and p17-PLN levels in GM compared to SOL (-76 % vs. +166 %; 0 % vs. +128 %). α/β CaMKII overexpression attenuated the decline of calcium release in muscle fibers with repeated excitation, and mitigated exercise-induced deterioration of rates in force production, and passive force, in a muscle-dependent manner, in correlation with pS2843-RyR1 and pT17-PLN levels (|r| > 0.7).

CONCLUSION

Enhanced capacity for α/β CaMKII signaling improves fatigue-resistance of active and passive contractile muscle properties in association with RyR1- and PLN-related improvements in sarcoplasmic calcium release.

Potentiation of force by extracellular potassium is not dependent on muscle length in mouse EDL muscle

Increases in myofiber extracellular potassium with prolonged contractile activity can potentiate twitch force. Activity-dependent potentiation, another mechanism of force increase in skeletal muscle, has a strong dependence on muscle or sarcomere length. Thus, potassium-mediated twitch potentiation could also be length-dependent. However, this has not been previously investigated. To this end, we used isolated C57BL/6 mouse extensor digitorum longus (EDL) muscles and elicited twitches at 0.9 Lo, Lo, and 1.1 Lo (Lo refers to optimal length) in normal (5 mM) and high (10 mM) potassium solutions. Potentiation magnitude was similar to previous observations and was not significantly different between lengths (0.9 Lo: 12.3 ± 4.4%, Lo: 12.2 ± 3.6%, 1.1 Lo: 11.8 ± 4.8%, values are means ± SD). Exposure to dantrolene sodium, a compound that attenuates calcium release, reduced twitch force across lengths by ∼70%. When dantrolene-affected muscles were subsequently exposed to high potassium, potentiation was similar to that observed in the absence of the former. In total, these findings provide novel information on potassium-mediated twitch potentiation.NEW & NOTEWORTHY Here, we investigated the length-dependence of twitch force potentiation by extracellular potassium in mouse extensor digitorum longus (EDL) in vitro, at 25°C. Potentiation magnitude did not display a statistically significant difference between the examined muscle lengths. These results describe, for the first time, the relationship of this form of potentiation with muscle length, thus furthering the understanding of how it is integrated in in vivo muscle function.

Calculating Set-Volume for the Limb Muscles with the Performance of Multi-Joint Exercises: Implications for Resistance Training Prescription

Resistance training volume, determined by the number of sets performed (set-volume) is considered one of the key variables in promoting muscle hypertrophy. To better guide resistance exercise prescription for weekly per-muscle training volume, the purpose of this paper is to provide evidence-based considerations for set-volume ratios between multi-joint (MJ) and single-joint (SJ) exercises so that practitioners can better manage prescription of training volume in program design. We analyzed this topic from three primary areas of focus: (1) biomechanical and physiological factors; (2) acute research; and (3) longitudinal research. From a biomechanical and physiological standpoint, when considering force production of different muscle groups, the moment arm of a given muscle, “motor abundance”, the link between biomechanics and exercise-induced fatigue, as well as the amount of time in voluntary muscle activation, a logical rationale can be made for SJ exercises producing greater hypertrophy of the limb muscles than MJ exercises (at least from specific exercises and under certain conditions). This would mean that sets for a MJ exercise should be counted fractionally for select muscles compared to an SJ exercise (i.e., less than a 1:1 ratio) when prescribing set-volumes for given muscles. When considering results from acute studies that measured muscle activation during the performance of SJ and MJ exercises, it seems that MJ exercises are not sufficient to maximize muscle activation of specific muscles. For example, during performance of the leg press and squat, muscle activation of the hamstrings is markedly lower than that of the quadriceps. These results suggest that a 1:1 ratio cannot be assumed. Current longitudinal research comparing the effects of training with MJ vs. SJ or MJ + SJ exercises is limited to the elbow flexors and the evidence is somewhat conflicting. Until more research is conducted to derive stronger conclusions on the topic, we propose the best advice would be to view set-volume prescription on a 1:1 basis, and then use logical rationale and personal expertise to make determinations on program design. Future research should focus on investigating longitudinal hypertrophic changes between MJ and SJ in a variety of populations, particularly resistance-trained individuals, while using site-specific measures of muscle growth to more systematically and precisely compute effective individualized set-volumes.

Force-velocity relationship during isometric and isotonic fatiguing contractions

Fatiguing contractions change the force-velocity relationship, but assessment of this relationship in fatigue has usually been obtained after isometric contractions. We studied fatigue caused by isometric or isotonic contractions, by assessment of the force-velocity relationship while the contractions maintaining fatigue were continued. This approach allowed determination of the force-velocity relationship during a steady condition of fatigue. We used the in situ rat medial gastrocnemius muscle, a physiologically relevant preparation. Intermittent (1/s) stimulation at 170 Hz for 100 ms resulted in decreased isometric force to ~35% of initial or decreased peak velocity of shortening in dynamic contractions to ~45% of initial. Dynamic contractions resulted in a transient initial increase in velocity, followed by a rapid decline until a reasonably steady level was maintained. Data were fit to the classic Hill equation for determination of the force-velocity relationship. Isometric and dynamic contractions resulted in similar decreases in maximal isometric force and peak power. Only V_max was different between the types of contraction ( P < 0.005) with greater decrease in V_max during isotonic contractions to 171.7 ± 7.3 mm/s than during isometric contractions to 208.8 mm/s. Curvature indicated by a/Po (constants from fit to Hill equation) changed from 0.45 ± 0.04 to 0.71 ± 0.11 during isometric contractions and from 0.51 ± 0.04 to 0.85 ± 0.18 during isotonic contractions. Recovery was incomplete 45 min after stopping the intermittent contractions. At this time, recovery of low-frequency isometric force was substantially less after isometric contractions, implicating force during intermittent contractions as a determining factor with this measure of fatigue. NEW & NOTEWORTHY The force-velocity relationship was captured while fatigue was maintained at a constant level during isometric and dynamic contractions. The curvature of the force-velocity relationship was less curved during fatigue than prefatigued, but within 45 min this recovered. Low-frequency fatigue persisted with greater depression of low-frequency force after isometric contractions, possibly because of higher force contractions during intermittent contractions.

Model-based analysis of fatigued human knee extensors : Effects of isometrically induced fatigue on Hill-type model parameters and ballistic contractions

This study investigated the effect of isometrically induced fatigue on Hill-type muscle model parameters and related task-dependent effects. Parameter identification methods were used to extract fatigue-related parameter trends from isometric and ballistic dynamic maximum voluntary knee extensions. Nine subjects, who completed ten fatiguing sets, each consisting of nine 3 s isometric maximum voluntary contractions with 3 s rest plus two ballistic contractions with different loads, were analyzed. Only at the isometric task, the identified optimized model parameter values of muscle activation rate and maximum force generating capacity of the contractile element decreased from [Formula: see text] to [Formula: see text] Hz and from [Formula: see text] to [Formula: see text] N, respectively. For all tasks, the maximum efficiency of the contractile element, mathematically related to the curvature of the force-velocity relation, increased from [Formula: see text] to [Formula: see text]. The model parameter maximum contraction velocity decreased from [Formula: see text] to [Formula: see text] m/s and the stiffness of the serial elastic element from [Formula: see text] to [Formula: see text] N/mm. Thus, models of fatigue should consider fatigue dependencies in active as well as in passive elements, and muscle activation dynamics should account for the task dependency of fatigue.

Procedures for rat in situ skeletal muscle contractile properties

There are many circumstances where it is desirable to obtain the contractile response of skeletal muscle under physiological circumstances: normal circulation, intact whole muscle, at body temperature. This includes the study of contractile responses like posttetanic potentiation, staircase and fatigue. Furthermore, the consequences of disease, disuse, injury, training and drug treatment can be of interest. This video demonstrates appropriate procedures to set up and use this valuable muscle preparation. To set up this preparation, the animal must be anesthetized, and the medial gastrocnemius muscle is surgically isolated, with the origin intact. Care must be taken to maintain the blood and nerve supplies. A long section of the sciatic nerve is cleared of connective tissue, and severed proximally. All branches of the distal stump that do not innervate the medial gastrocnemius muscle are severed. The distal nerve stump is inserted into a cuff lined with stainless steel stimulating wires. The calcaneus is severed, leaving a small piece of bone still attached to the Achilles tendon. Sonometric crystals and/or electrodes for electromyography can be inserted. Immobilization by metal probes in the femur and tibia prevents movement of the muscle origin. The Achilles tendon is attached to the force transducer and the loosened skin is pulled up at the sides to form a container that is filled with warmed paraffin oil. The oil distributes heat evenly and minimizes evaporative heat loss. A heat lamp is directed on the muscle, and the muscle and rat are allowed to warm up to 37°C. While it is warming, maximal voltage and optimal length can be determined. These are important initial conditions for any experiment on intact whole muscle. The experiment may include determination of standard contractile properties, like the force-frequency relationship, force-length relationship, and force-velocity relationship. With care in surgical isolation, immobilization of the origin of the muscle and alignment of the muscle-tendon unit with the force transducer, and proper data analysis, high quality measurements can be obtained with this muscle preparation.

Force-frequency and force-length properties in skeletal muscle following unilateral focal ischaemic insult in a rat model

AIM

Our purpose was to quantify skeletal muscle properties following unilateral focal ischaemic insult (stroke) in a rat model.

METHODS

Male rats were divided into two groups: stroke and 2 weeks recovery (n = 8) and control group (n = 7). Stroke was induced in the area of the motor neocortex containing hind limb corticospinal neurones. Contractile properties of the medial gastrocnemius muscle were measured in situ in both limbs. Force-length and force-frequency properties were measured before and 35 min after 5 min fatiguing stimulation.

RESULTS

Stroke resulted in bilateral tetanic fade during 200 Hz stimulation. When normalized to 100 Hz contractions, force at 200 Hz was 95.4 +/- 0.9% for the paretic muscles, 96.7 +/- 1.7% for non-paretic muscles and 102.2 +/- 1.0% for muscles of control rats (P = 0.006). Prior to fatiguing contractions, there was no difference in the length dependence of force. During repetitive contractions, active force fell significantly to 19 +/- 4 and 25 +/- 5% of initial force in paretic and non-paretic muscles of animals with a stroke respectively. In control animals active force fell to 37 +/- 5%. During repetitive contractions, fusion index increased in muscles of stroke animals to 1.0 +/- 0 but in control animals it was 0.95 +/- 0.02. There was selective force depression at short lengths for fatigued paretic muscle (significant difference at muscle lengths less than reference length -2 mm).

CONCLUSION

The tetanic fade at high stimulation frequencies indicates that there may be activation failure following focal ischaemic insult. The greater magnitude of fatigue and selective depression at short lengths following repetitive contractions should be investigated further.

Low-frequency fatigue at maximal and submaximal muscle contractions

Skeletal muscle force production following repetitive contractions is preferentially reduced when muscle is evaluated with low-frequency stimulation. This selective impairment in force generation is called low-frequency fatigue (LFF) and could be dependent on the contraction type. The purpose of this study was to compare LFF after concentric and eccentric maximal and submaximal contractions of knee extensor muscles. Ten healthy male subjects (age: 23.6 +/- 4.2 years; weight: 73.8 +/- 7.7 kg; height: 1.79 +/- 0.05 m) executed maximal voluntary contractions that were measured before a fatigue test (pre-exercise), immediately after (after-exercise) and after 1 h of recovery (after-recovery). The fatigue test consisted of 60 maximal (100%) or submaximal (40%) dynamic concentric or eccentric knee extensions at an angular velocity of 60 degrees /s. The isometric torque produced by low- (20 Hz) and high- (100 Hz) frequency stimulation was also measured at these times and the 20:100 Hz ratio was calculated to assess LFF. One-way ANOVA for repeated measures followed by the Newman-Keuls post hoc test was used to determine significant (P < 0.05) differences. LFF was evident after-recovery in all trials except following submaximal eccentric contractions. LFF was not evident after-exercise, regardless of exercise intensity or contraction type. Our results suggest that low-frequency fatigue was evident after submaximal concentric but not submaximal eccentric contractions and was more pronounced after 1-h of recovery.

Potentiation of isometric and isotonic contractions during high-frequency stimulation

Activity dependent potentiation is an enhanced contractile response resulting from previous contractile activity. It has been proposed that even a maximal effort contraction may be enhanced by prior activity if there is an increase in the peak rate of force development. This should increase the peak active force during a very brief maximal effort contraction. The purpose of these experiments was to evaluate potentiation during brief sequential contractions with high-frequency stimulation. For this experiment, the rat medial gastrocnemius muscle was isolated in situ. Sequential stimulation (two contractions per second for 4 s) with 200, 300, or 400 Hz doublets, triplets, and quadruplets was applied. A small degree of force potentiation was observed in isometric contractions at the reference length (RL), but the activity dependent potentiation of isometric contractions was greater at short muscle length. For example, peak rate of force development for 200 Hz doublets increased significantly from the first to the eighth contraction (from 0.30+/-0.02 to 0.34+/-0.02 N.s(-1) at RL and from 0.18+/-0.02 to 0.28+/-0.01 N.s(-1) at RL-3 mm). During isotonic contractions, there were significant increases in peak shortening from the first to the eighth contraction. With 200 Hz doublet stimulation, shortening increased from 0.85+/-0.14 to 1.14+/-0.17 mm, and this corresponded with an increase in peak velocity (from 116+/-18 to 136+/-19 mm.s(-1)). Remarkably, even 400 Hz quadruplets showed a significant increase in shortening during repeated contractions (2 s(-1)). These observations indicate the possibility that activity dependent potentiation can result in significant improvement in both isometric and dynamic contractions, even when activated at very high frequency.

Impact of length during repetitive contractions on fatigue in rat skeletal muscle

The magnitude of fatigue resulting from repeated contractions at a short length has been reported to be less than that which occurs with contractions at a long length. However, there have been what appear to be contradictory reports; the rate of fatigue is greater at a short length. The purpose of this study was to evaluate the impact of length on the magnitude and the rate of fatigue resulting from a series of repetitive stimulations. Experiments were done with anesthetized rats and the medial gastrocnemius muscle was stimulated via the sciatic nerve. Submaximal force-length relationships were obtained prior to and 45 min after repeated contractions (50 Hz, 300 ms) at short or long length. Stimulation was applied at 1 Hz or 0.5 Hz for 5 min at the long length or 1 Hz at the short length (difference = 3.6mm). This approach permitted evaluation of the impact of rate of muscle activation as well as length on subsequent contractile response. Repetitive stimulation at a short length resulted in more potentiation and a greater (relative) rate of fatigue but after 5 min the depression of relative active force was similar between the series at long and short length. The submaximal force-length relationship obtained after 45 min of recovery revealed that depression of force was greater after 1 Hz contractions at the long length. These results are consistent with both sides of the apparent contradiction in the literature; rate of fatigue is greater at a short length and magnitude of fatigue is greater at a long length.

Dantrolene, like fatigue, has a length-dependent effect on submaximal force-length relationships of rat gastrocnemius muscle

AIM

Fatigue is length-dependent; relative active force depression is greater when measured at short lengths than at long lengths. Several unsatisfactory mechanisms have been proposed to explain this length dependence of fatigue, including: damaged myofilaments, stretch of 'in-series' structures, impaired t-tubule conduction and reduced intensity of activation. Dantrolene targets the ryanodine receptors, inhibiting stimulation-induced release of Ca2+. The purpose of this study was to determine if the force depression caused by dantrolene treatment also has a length dependence.

METHODS

Submaximal (single-pulse, double-pulse and 50 Hz stimulation) active force-length relationships were obtained from the medial gastrocnemius muscle of anaesthetized rats, before and after intravenous injection with dantrolene dissolved in propylene glycol.

RESULTS

Dantrolene treatment was sufficient to reduce twitch amplitude by 25%. Similar to the consequences of repetitive stimulation, dantrolene treatment caused the same decrease in absolute active force across a broad range of test lengths, for twitch, double-pulse and 50 Hz contractions. Considering that active force is smaller at short lengths than at long lengths, this similar absolute force decrease represents a greater relative decrease at short lengths. Clearly, there is a length-dependent impact of attenuated Ca2+ release by dantrolene on relative active force.

CONCLUSION

This study demonstrates that there is a length dependence of force depression associated with decreased Ca2+ release due to dantrolene treatment; therefore, if fatigue is due to decreased Ca2+ release, then additional length-dependent mechanisms are not required to explain the reported length dependence of force depression.