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Kim H, Heckman CJ. A dynamic calcium-force relationship model for sag behavior in fast skeletal muscle. PLoS Comput Biol 2023; 19:e1011178. [PMID: 37289805 DOI: 10.1371/journal.pcbi.1011178] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/30/2022] [Accepted: 05/12/2023] [Indexed: 06/10/2023] Open
Abstract
In vitro studies using isolated or skinned muscle fibers suggest that the sigmoidal relationship between the intracellular calcium concentration and force production may depend upon muscle type and activity. The goal of this study was to investigate whether and how the calcium-force relationship changes during force production under physiological conditions of muscle excitation and length in fast skeletal muscles. A computational framework was developed to identify the dynamic variation in the calcium-force relationship during force generation over a full physiological range of stimulation frequencies and muscle lengths in cat gastrocnemius muscles. In contrast to the situation in slow muscles such as the soleus, the calcium concentration for the half-maximal force needed to drift rightward to reproduce the progressive force decline, or sag behavior, observed during unfused isometric contractions at the intermediate length under low-frequency stimulation (i.e., 20 Hz). The slope at the calcium concentration for the half-maximal force was required to drift upward for force enhancement during unfused isometric contractions at the intermediate length under high-frequency stimulation (i.e., 40 Hz). The slope variation in the calcium-force relationship played a crucial role in shaping sag behavior across different muscle lengths. The muscle model with dynamic variations in the calcium-force relationship also accounted for the length-force and velocity-force properties measured under full excitation. These results imply that the calcium sensitivity and cooperativity of force-inducing crossbridge formation between actin and myosin filaments may be operationally altered in accordance with the mode of neural excitation and muscle movement in intact fast muscles.
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Affiliation(s)
- Hojeong Kim
- Division of Biotechnology, Institute of Convergence Research, DGIST, Daegu, Republic of Korea
- Department of Interdisciplinary Studies, DGIST, Daegu, Republic of Korea
| | - Charles J Heckman
- Department of Neuroscience, Feinberg School of Medicine, Northwestern University, Chicago, Illinois, United States of America
- Physical Therapy and Human Movement Sciences, Feinberg School of Medicine, Northwestern University, Chicago, Illinois, United States of America
- Physical Medicine and Rehabilitation, Feinberg School of Medicine, Northwestern University, Chicago, Illinois, United States of America
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Andersen OE, Kristensen AM, Nielsen OB, Overgaard K. Force potentiation during eccentric contractions in rat skeletal muscle. J Appl Physiol (1985) 2023; 134:777-785. [PMID: 36759160 DOI: 10.1152/japplphysiol.00676.2022] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/09/2022] [Revised: 02/01/2023] [Accepted: 02/06/2023] [Indexed: 02/11/2023] Open
Abstract
Postactivation potentiation refers to an acute enhancement of contractile properties following muscle activity. Previously, the effects of prior muscle activation on eccentric force at tetanic activation frequencies have only been sparsely reported. This paper aimed to study acute activity-induced effects on eccentric force of slow and fast-twitch muscles and characterize them in relation to postactivation potentiation. We elicited eccentric contractions in isolated rat extensor digitorum longus and soleus muscles by actively lengthening muscles at a constant velocity. We assessed contractile properties by measuring force over shortly interspaced, identical eccentric, and isometric contractions. We then analyzed stretch force, isometric peak force, rate of force development, and relaxation times. Finally, we compared the time courses for the development and cessation of changes in stretch force to known features of postactivation potentiation. In extensor digitorum longus, muscles stretch force consistently increased in a contraction-to-contraction manner by up to 49% [95% confidence interval (CI): 35-64%] whereas isometric peak force simultaneously showed minor declines (8%, 95% CI: 5-10%). The development and cessation of eccentric force potentiation coincided with the development of twitch potentiation and increases in rate of force development. In soleus muscles we found no consistent eccentric potentiation. Characterization of the increase in eccentric force revealed that force only increased in the very beginning of an active stretch. Eccentric force at tetanic activation frequencies potentiates substantially in extensor digitorum longus muscles over consecutive contractions with a time course coinciding with postactivation potentiation. Such eccentric potentiation may be important in sport performance.NEW & NOTEWORTHY Force during eccentric contractions can increase to a magnitude that may have profound consequences for our understanding of skeletal muscle locomotion. This increase in eccentric force occurs over consecutive, shortly interspaced, tetanic contractions in rat extensor digitorum longus muscles-not in rat soleus muscles-and coincides with well-known traits of postactivation potentiation. Eccentric force potentiation may significantly enhance muscle performance in activities involving stretch-shortening cycles.
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Affiliation(s)
- Ole Emil Andersen
- Department of Public Health, Aarhus University, Aarhus, Denmark
- Steno Diabetes Center Aarhus, Aarhus University, Aarhus, Denmark
| | | | - Ole B Nielsen
- Department of Public Health, Aarhus University, Aarhus, Denmark
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Optimization and comparison of two methods for spike train estimation in an unfused tetanic contraction of low threshold motor units. J Electromyogr Kinesiol 2022; 67:102714. [PMID: 36209700 DOI: 10.1016/j.jelekin.2022.102714] [Citation(s) in RCA: 3] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/05/2022] [Revised: 09/02/2022] [Accepted: 09/28/2022] [Indexed: 12/14/2022] Open
Abstract
BACKGROUND Recent findings have shown that imaging voluntarily activated motor units (MUs) by decomposing ultrasound-based displacement images provides estimates of unfused tetanic signals evoked by spinal motoneurons' neural discharges (spikes). Two methods have been suggested to estimate its spike trains: band-pass filter (BPM) and Haar wavelet transform (HWM). However, the methods' optimal parameters and which method performs the best are unknown. This study will answer these questions. METHOD HWM and BPM were optimized using simulations. Their performance was evaluated based on simulations and 21 experimental datasets, considering their rate of agreement, spike offset, and spike offset variability to the simulated or experimental spikes. RESULTS A range of parameter sets that resulted in the highest possible agreement with simulated spikes was provided. Both methods highly agreed with simulated and experimental spikes, but HWM was a better spike estimation method than BPM because it had a higher agreement, less bias, and less variation (p < 0.001). CONCLUSIONS The optimized HWM will be an important contributor to further developing the identification and analysis of MUs using imaging, providing indirect access to the neural drive of the spinal cord to the muscle by the unfused tetanic signals.
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Smith IC, Onasch F, Kryściak K, Celichowski J, Herzog W. Contractile history affects sag and boost properties of unfused tetanic contractions in human quadriceps muscles. Eur J Appl Physiol 2020; 121:645-658. [PMID: 33221935 DOI: 10.1007/s00421-020-04561-9] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/07/2020] [Accepted: 11/11/2020] [Indexed: 10/22/2022]
Abstract
PURPOSE A period of extra-efficient force production ("boost") followed by a decline in force ("sag") is often observed at the onset of unfused tetanic contractions. We tested the hypothesis that in human muscle boost and sag are diminished in repeated contractions separated by short rest periods and are re-established or enhanced following long rest periods. METHODS Two sets of 3 unfused tetanic contractions were evoked in the right quadriceps muscle group of 29 participants via percutaneous stimulation of the femoral nerve. Contractions consisted of 20 pulses evoked at inter-pulse intervals of 1.25 × twitch time to peak torque. Contractions were evoked 5 s apart and sets were evoked 5 min apart. RESULTS The ratio of the angular impulse of pulses 1-10 to the angular impulse of pulses 11-20 was used as the boost indicator. By this metric, boost was higher (P < 0.05) in the first relative to the second and third contractions within a set, but did not differ between sets (Set 1: 1.31 ± 0.15, 1.18 ± 0.12, 1.14 ± 0.12 vs Set 2: 1.34 ± 0.17, 1.17 ± 0.13, 1.14 ± 0.13). Sag (the percent decline in torque within each contraction) was also higher (P < 0.05) in the first relative to the second and third contractions within a set, but did not differ between sets (Set 1: 40.8 ± 7.5%, 35.4 ± 6.8%, 33.2 ± 7.8% vs Set 2: 42.1 ± 8.0%, 35.5 ± 6.8%, 33.9 ± 7.2%). Participants' sex and resistance training background did not influence boost or sag. CONCLUSION Boost and sag are sensitive to contractile history in whole human quadriceps. Optimizing boost may have application in strength and power sports.
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Affiliation(s)
- Ian C Smith
- Human Performance Laboratory, Faculty of Kinesiology, University of Calgary, 2500 University Drive NW Calgary, Alberta, T2N 1N4, Canada.
| | - Franziska Onasch
- Human Performance Laboratory, Faculty of Kinesiology, University of Calgary, 2500 University Drive NW Calgary, Alberta, T2N 1N4, Canada
| | - Katarzyna Kryściak
- Department of Neurobiology, Poznan University of Physical Education, 27/39 Królowej Jadwigi Street, 61-871, Poznań, Poland
| | - Jan Celichowski
- Department of Neurobiology, Poznan University of Physical Education, 27/39 Królowej Jadwigi Street, 61-871, Poznań, Poland
| | - Walter Herzog
- Human Performance Laboratory, Faculty of Kinesiology, University of Calgary, 2500 University Drive NW Calgary, Alberta, T2N 1N4, Canada.,Biomechanics Laboratory, School of Sports, Federal University of Santa Catarina, Florianopolis, SC, Brazil
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Kryściak K, Smith IC, Drzymała-Celichowska H, Celichowski J. Initial force production before sag is enhanced by prior contraction followed by a 3-minute rest period in fast motor units of the rat medial gastrocnemius. J Electromyogr Kinesiol 2020; 53:102429. [PMID: 32505088 DOI: 10.1016/j.jelekin.2020.102429] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/10/2020] [Revised: 04/06/2020] [Accepted: 05/15/2020] [Indexed: 11/28/2022] Open
Abstract
Unfused tetanic contractions evoked in fast motor units exhibit extra-efficient force production at the onset of contraction, an effect called "boost". Boost is diminished in subsequent contractions if there is a short rest period between contractions, but can be re-established with a longer period of rest. We tested the hypothesis that contractile activity and rest could enhance boost-related metrics. Two sets of 3 unfused tetani were evoked 3 min apart in fast fatigable (FF) and fast fatigue-resistant (FR) motor units of the rat medial gastrocnemius. The greatest changes occurred in the first unfused tetanic contractions. Relative to the first contraction in the first set, the first contraction in the second set exhibited higher peak force during boost in a subset of motor units (76% of FF and 48% of FR). Enhanced force during boost was influenced by interaction of slowing of twitch contraction time (up to 20% and 25%, for FF and FR motor units, respectively), half-relaxation time (up to 37% and 49% for FF and FR motor units, respectively), and potentiation of the first twitch (up to 13% and 5% for FF and FR motor units, respectively). Examination of twitches evoked between sets suggested opportunity for greater enhancement of boost with shorter intervening rest periods. The phenomenon of enhanced boost following motor unit activity may interest sports scientists.
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Affiliation(s)
- Katarzyna Kryściak
- Department of Neurobiology, Poznan University of Physical Education, Poland.
| | - Ian C Smith
- Faculty of Kinesiology, University of Calgary, Canada
| | - Hanna Drzymała-Celichowska
- Department of Neurobiology, Poznan University of Physical Education, Poland; Division of Biochemistry, Poznan University of Physical Education, Poland
| | - Jan Celichowski
- Department of Neurobiology, Poznan University of Physical Education, Poland
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Smith IC, Adam H, Herzog W. A brief contraction has complex effects on summation of twitch pairs in human adductor pollicis. Exp Physiol 2020; 105:676-689. [PMID: 32052487 DOI: 10.1113/ep088401] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/17/2019] [Accepted: 02/10/2020] [Indexed: 11/08/2022]
Abstract
NEW FINDINGS What is the central question of this study? How do contraction-induced reductions in twitch duration, without changes in twitch force, affect summation of twitch pairs into higher force contractions in skeletal muscle? What is the main finding and its importance? Abbreviating twitch duration with a brief contraction resulted in enhanced summation of fully fused twitch pairs, but impaired summation in partially fused twitch pairs even after accounting for the differences in relaxation of the first twitch. An inherent mechanism which enhances relaxation without sacrificing force generation in forceful contractions would benefit cyclic muscle activities, such as locomotion. ABSTRACT During electrically evoked contractions of skeletal muscle, the interplay between twitch duration and the time between electrical stimuli (inter-pulse interval, IPI) determines how effectively twitch forces summate into high force contractions. A brief muscle contraction can impair summation by abbreviating twitch duration, though it is not clear if these impairments occur at all physiologically relevant IPI. This study was designed to test how a brief contraction affects summation of nominally isometric twitch pairs with IPIs lasting 10-5000 ms. Left adductor pollicis muscles of human participants (n = 9) were electrically activated using stimulus pairs applied both before (Pre) and after (Post) a 10 Hz, 1.0 s contraction. Force-time records were mathematically separated into Pulse 1 (single twitch) and Pulse 2 (summated twitch) components. The ratio of Pulse 2 peak force to Pulse 1 peak force was used as our measure of summation effectiveness. Consistent with the observed decline of Pulse 1 duration at Post relative to Pre (4.7 ± 0.6%; P < 0.001; duration was defined as the time from stimulation to the time required for active force to decline by 50%), summation effectiveness was higher at Pre than at Post at IPIs of 100-333 ms. Summation effectiveness was not different between Pre and Post at IPIs of 50-83 ms or 500-5000 ms. Intriguingly, summation effectiveness was higher at Post than at Pre at IPIs of 10-25 ms. In summary, a brief contraction has complex effects on the relationship between inter-pulse interval and summation effectiveness. Future experiments are needed to reveal the mechanisms behind this novel observation.
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Affiliation(s)
- Ian C Smith
- Human Performance Lab, Faculty of Kinesiology, University of Calgary 2500 University Drive NW, Calgary, Alberta, Canada, T2N 1N4
| | - Helen Adam
- Human Performance Lab, Faculty of Kinesiology, University of Calgary 2500 University Drive NW, Calgary, Alberta, Canada, T2N 1N4
| | - Walter Herzog
- Human Performance Lab, Faculty of Kinesiology, University of Calgary 2500 University Drive NW, Calgary, Alberta, Canada, T2N 1N4
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Smith IC, Vandenboom R, Tupling AR. Caffeine attenuates contraction-induced diminutions of the intracellular calcium transient in mouse lumbrical muscle ex vivo. Can J Physiol Pharmacol 2019; 97:429-435. [PMID: 30661369 DOI: 10.1139/cjpp-2018-0658] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/31/2023]
Abstract
The amount of calcium released from the sarcoplasmic reticulum in skeletal muscle rapidly declines during repeated twitch contractions. In this study, we test the hypothesis that caffeine can mitigate these contraction-induced declines in calcium release. Lumbrical muscles were isolated from male C57BL/6 mice and loaded with the calcium-sensitive indicator, AM-furaptra. Muscles were then stimulated at 8 Hz for 2.0 s in the presence or absence of 0.5 mM caffeine, at either 30 °C or 37 °C. The amplitude and area of the furaptra-based intracellular calcium transients and force produced during twitch contractions were calculated. For each of these measures, the values for twitch 16 relative to twitch 1 were higher in the presence of caffeine than in the absence of caffeine at both temperatures. We conclude that caffeine can attenuate contraction-induced diminutions of calcium release during repeated twitch contractions, thereby contributing to the inotropic effects of caffeine.
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Affiliation(s)
- Ian C Smith
- a Human Performance Lab, Faculty of Kinesiology, University of Calgary, Calgary, AB T2N 1N4, Canada.,b Department of Kinesiology, University of Waterloo, Waterloo, ON N2L 3G1, Canada
| | - Rene Vandenboom
- c Department of Kinesiology, Brock University, St. Catharines, ON L2S 3A1, Canada
| | - A Russell Tupling
- b Department of Kinesiology, University of Waterloo, Waterloo, ON N2L 3G1, Canada
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Kryściak K, Celichowski J, Krutki P, Raikova R, Drzymała-Celichowska H. Factors contributing to sag in unfused tetanic contractions of fast motor units in rat medial gastrocnemius. J Electromyogr Kinesiol 2018; 44:70-77. [PMID: 30529806 DOI: 10.1016/j.jelekin.2018.11.011] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/10/2018] [Revised: 11/20/2018] [Accepted: 11/28/2018] [Indexed: 11/15/2022] Open
Abstract
The sag phenomenon can be observed in fast motor units (MUs) as a transitional decline in force during unfused tetanic contractions; however, its mechanisms are poorly understood. The study aimed to identify in the rat muscle factors that contribute to sag in two types of fast MUs: fast fatigable (FF) and fast resistant to fatigue (FR). First, we performed mathematical decomposition of sagging tetanic contractions of FF and FR MUs into twitch-like responses to consecutive stimuli. This process indicated an increase in the amplitudes of a few initial responses (up to the 2nd-3rd for FF and up to the 2nd-7th for FR MUs), followed by a decrease in the amplitudes of later responses. In comparison to the first twitch, the relative increase in force amplitudes of the several subsequent decomposed responses was smaller, and their contraction and relaxation times were shorter for FF than for FR units, which corresponded to observed differences in their sag profiles. Additionally, after occlusion of the blood circulation, sag disappeared, but it reappeared after restoration of the blood supply. This indicates that the presence of sag depends on the proper circulation in the muscle.
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Affiliation(s)
- K Kryściak
- Department of Neurobiology, Poznan University of Physical Education, Poland.
| | - J Celichowski
- Department of Neurobiology, Poznan University of Physical Education, Poland
| | - P Krutki
- Department of Neurobiology, Poznan University of Physical Education, Poland
| | - R Raikova
- Institute of Biophysics and Biomedical Engineering, Bulgarian Academy of Sciences, Sofia, Bulgaria
| | - H Drzymała-Celichowska
- Department of Neurobiology, Poznan University of Physical Education, Poland; Division of Biochemistry, Poznan University of Physical Education, Poland
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Morris SR, Gittings W, Vandenboom R. Epinephrine augments posttetanic potentiation in mouse skeletal muscle with and without myosin phosphorylation. Physiol Rep 2018; 6:e13690. [PMID: 29718592 PMCID: PMC5931225 DOI: 10.14814/phy2.13690] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/29/2017] [Revised: 03/19/2018] [Accepted: 03/20/2018] [Indexed: 11/24/2022] Open
Abstract
Sympathetic tone may influence force potentiation, that is, the stimulation-induced increase in skeletal muscle mechanical function associated with myosin phosphorylation, although the mechanism for this effect remains unknown. The purpose of this study was to examine the influence of epinephrine on concentric twitch force potentiation of wild-type and skeletal myosin light-chain kinase devoid mouse muscle (skMLCK-/- ). To this end, concentric twitch force was assessed before and after a potentiating stimulus (PS) to determine the peak and the duration of potentiation in the absence (-EPI) and presence (+EPI) of 1 μmol/L epinephrine in both genotypes. Twitch force of wild-type and skMLCK-/- muscles was increased by up to 31 and 35% and 18 and 23% in the -EPI and EPI conditions, respectively (all data n = 8, P < 0.05). In wild-type muscles, the PS increased RLC phosphorylation from 0.14 ± 0.05 (rest) to 0.66 ± 0.08 mol phos mol RLC; by 480 sec RLC phosphorylation had returned to baseline (all data n = 4 each time point, P < 0.05). Neither resting nor peak levels of RLC phosphorylation were altered by +EPI, although the duration of RLC phosphorylation was prolonged. In skMLCK-/- muscles, RLC phosphorylation was not elevated above constituent levels by stimulation in either the -EPI or +EPI condition. Thus, given the similarity in potentiation responses between genotypes our data suggest that the influence of epinephrine on potentiation was independent of skMLCK catalyzed phosphorylation of the RLC, although the clinical significance of this pathway for skeletal muscle function remains to be identified.
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Affiliation(s)
| | - William Gittings
- Department of KinesiologyBrock UniversitySt. CatharinesOntarioCanada
| | - Rene Vandenboom
- Department of KinesiologyBrock UniversitySt. CatharinesOntarioCanada
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Abstract
PURPOSE We examined how muscle length and time between stimuli (inter-pulse interval, IPI) influence declines in force (sag) seen during unfused tetani in the human adductor pollicis muscle. METHODS A series of 16-pulse contractions were evoked with IPIs between 1 × and 5 × the twitch time to peak tension (TPT) at large (long muscle length) and small (short muscle length) thumb adduction angles. Unfused tetani were mathematically deconstructed into a series of overlapping twitch contractions to examine why sag exhibits length- and IPI-dependencies. RESULTS Across all IPIs tested, sag was 62% greater at short than long muscle length, and sag increased as IPI was increased at both muscle lengths. Force attributable to the second stimulus increased as IPI was decreased. Twitch force declined from maximal values across all IPI tested, with the greatest reductions seen at short muscle length and long IPI. At IPI below 2 × TPT, the twitch with highest force occurred earlier than the peak force of the corresponding unfused tetani. Contraction-induced declines in twitch duration (TPT + half relaxation time) were only observed at IPI longer than 1.75 × TPT, and were unaffected by muscle length. CONCLUSIONS Sag is an intrinsic feature of healthy human adductor pollicis muscle. The length-dependence of sag is related to greater diminution of twitch force at short relative to long muscle length. The dependence of sag on IPI is related to IPI-dependent changes in twitch duration and twitch force, and the timing of peak twitch force relative to the peak force of the associated unfused tetanus.
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Affiliation(s)
- Ian C Smith
- Human Performance Lab, Faculty of Kinesiology, University of Calgary, 2500 University Drive NW, Calgary, AB, T2N 1N4, Canada.
| | - Jahaan Ali
- Human Performance Lab, Faculty of Kinesiology, University of Calgary, 2500 University Drive NW, Calgary, AB, T2N 1N4, Canada
| | - Geoffrey A Power
- Human Health and Nutritional Sciences, College of Biological Sciences, University of Guelph, 50 Stone Rd E, Guelph, ON, N1G 2W1, Canada
| | - Walter Herzog
- Human Performance Lab, Faculty of Kinesiology, University of Calgary, 2500 University Drive NW, Calgary, AB, T2N 1N4, Canada
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Smith IC, Vandenboom R, Tupling AR. Contraction-induced enhancement of relaxation during high force contractions of mouse lumbrical muscle at 37°C. J Exp Biol 2017; 220:2870-2873. [PMID: 28576821 DOI: 10.1242/jeb.158998] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/06/2017] [Accepted: 05/28/2017] [Indexed: 11/20/2022]
Abstract
Repeated stimulation of unfatigued rodent fast-twitch skeletal muscle accelerates the kinetics of tension relaxation through an unknown mechanism. This effect varies with muscle type and stimulation parameters, and has been observed at physiological temperatures for submaximal but not maximal contractions. The purpose of this study was to compare relaxation kinetics of C57BL/6 mouse lumbrical muscles ex vivo from maximal isometric force (500 Hz for 20 ms) when evoked before (pre) and after (post) an intervening tetanic contraction at 37°C. During post contractions, we noted significant increases in the rate of tension decline during both the slow linear phase and the fast exponential phase of relaxation, as well as a reduced duration of the slow phase of relaxation compared with pre contractions (all P<0.05). This is the first demonstration of enhanced slow and fast relaxation phases from maximal isometric tension induced by prior stimulation in intact muscle at a physiological temperature.
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Affiliation(s)
- Ian C Smith
- Human Performance Lab, University of Calgary, 2500 University Drive NW, Calgary, Alberta, Canada, T2N 1N4 .,Department of Kinesiology, University of Waterloo, 200 University Avenue West, Waterloo, Ontario, Canada, N2L 3G1
| | - Rene Vandenboom
- Department of Kinesiology, Faculty of Applied Health Sciences, Brock University, St. Catharines, Ontario, Canada, L2S 3A1
| | - A Russell Tupling
- Department of Kinesiology, University of Waterloo, 200 University Avenue West, Waterloo, Ontario, Canada, N2L 3G1
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Smith IC, Bellissimo C, Herzog W, Tupling AR. Can inorganic phosphate explain sag during unfused tetanic contractions of skeletal muscle? Physiol Rep 2016; 4:4/22/e13043. [PMID: 27884960 PMCID: PMC5358005 DOI: 10.14814/phy2.13043] [Citation(s) in RCA: 11] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/25/2016] [Accepted: 10/31/2016] [Indexed: 12/31/2022] Open
Abstract
We test the hypothesis that cytosolic inorganic phosphate (Pi) can account for the contraction‐induced reductions in twitch duration which impair summation and cause force to decline (sag) during unfused tetanic contractions of fast‐twitch muscle. A five‐state model of crossbridge cycling was used to simulate twitch and unfused tetanic contractions. As Pi concentration ([Pi]) was increased from 0 to 30 mmol·L−1, twitch duration decreased, with progressive reductions in sensitivity to Pi as [Pi] was increased. When unfused tetani were simulated with rising [Pi], sag was most pronounced when initial [Pi] was low, and when the magnitude of [Pi] increase was large. Fast‐twitch extensor digitorum longus (EDL) muscles (sag‐prone, typically low basal [Pi]) and slow‐twitch soleus muscles (sag‐resistant, typically high basal [Pi]) were isolated from 14 female C57BL/6 mice. Muscles were sequentially incubated in solutions containing either glucose or pyruvate to create typical and low Pi environments, respectively. Twitch duration was greater (P < 0.05) in pyruvate than glucose in both muscles. Stimuli applied at intervals approximately three times the time to peak twitch tension resulted in sag of 35.0 ± 3.7% in glucose and 50.5 ± 1.4% in pyruvate in the EDL (pyruvate > glucose; P < 0.05), and 3.9 ± 0.3% in glucose and 37.8 ± 2.7% in pyruvate in the soleus (pyruvate > glucose; P < 0.05). The influence of Pi on crossbridge cycling provides a tenable mechanism for sag. Moreover, the low basal [Pi] in fast‐twitch relative to slow‐twitch muscle has promise as an explanation for the fiber‐type dependency of sag.
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Affiliation(s)
- Ian C Smith
- Human Performance Lab, University of Calgary, Calgary, Alberta, Canada
| | | | - Walter Herzog
- Human Performance Lab, University of Calgary, Calgary, Alberta, Canada
| | - A Russell Tupling
- Department of Kinesiology, University of Waterloo, Waterloo, Ontario, Canada
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