The shortening of rabbit muscles during rigor mortis; its relation to the breakdown of adenosine triphosphate and creatine phosphate and to muscular contraction

Declined flesh quality resulting from niacin deficiency is associated with elevated glycolysis and impaired mitochondrial homeostasis in grass carp (Ctenopharyngodon idella)

Energy metabolism exerts profound impacts on flesh quality. Niacin can be transformed into nicotinamide adenine dinucleotide (NAD), which is indispensable to energy metabolism. To investigate whether niacin deficiency could affect energy metabolism and flesh quality, six diets with graded levels of 0.49, 9.30, 21.30, 33.30, 45.30 and 57.30 mg/kg niacin were fed to grass carp (Ctenopharyngodon idella) for 63 days. The results showed that niacin deficiency declined flesh quality by changing amino acid and fatty acid profiles, decreasing shear force, increasing cooking loss and accelerating pH decline. The accelerated pH decline might be associated with enhanced glycolysis as evident by increased hexokinase (HK), pyruvate kinase (PK) and lactic dehydrogenase (LDH) activities, and mitochondrial dysfunction as evident by destroyed mitochondrial morphology, impaired respiratory chain complex I and antioxidant ability. Based on PWG and cooking loss, the niacin requirements for sub-adult grass carp were 31.95 mg/kg and 29.66 mg/kg diet, respectively.

Connecting Heat Tolerance and Tenderness in Bos indicus Influenced Cattle

Simple Summary

Bos indicus (also known as zebu or humped) cattle are heat tolerant and parasite resistant, which is advantageous in hot, humid climates. However, Bos indicus cattle are also known for excitable temperaments, slower growth, and variation in meat quality characteristics. The relationships between thermotolerance, temperament, and meat production traits are poorly understood. Due to its contribution to body weight, muscle may play an important role in determining the thermoregulatory capacity of Bos indicus cattle. Ultimately, defining relationships between muscle metabolism and heat tolerance are necessary in order to enhance tenderness, without sacrificing heat tolerance of Bos indicus breeds.

Abstract

Bos indicus cattle are widely utilized in tropical and subtropical climates. Their heat tolerance and parasite resistance are integral for beef production in these regions; however, a reputation for excitable temperaments, slower growth, and variation in tenderness has limited their use in commercial beef production. This suggests that there is antagonism between heat tolerance and meat production traits. Meat quality characteristics are determined by the properties of skeletal muscle as well as conditions during slaughter and processing. Thus, it is possible that adaptations related to heat tolerance in the living animal affect tenderness and other meat quality attributes. Since muscle represents a large proportion of body mass, relatively small changes at the cellular level could impact overall heat production of the animal. Specifically, protein degradation and mitochondria function are aspects of organ and cellular metabolism that may help limit heat production and also have a connection to tenderness. Protein degradation postmortem is critical to structural changes that enhance tenderness whereas mitochondria may influence tenderness through their roles in energy metabolism, calcium regulation, cell death signaling, and oxidative stress. This review explores potential relationships between cellular metabolism in vivo and beef quality development in Bos indicus and Bos indicus influenced cattle.

A genome-wide case-control association study of dark cutting in beef cattle

The genetic architecture of dark cutting was investigated with a case-control genome-wide association study on two groups of beef cattle analyzed separately and together (combined group). Groups I (n = 64) and II (n = 150) were genotyped using the 70K GeneSeek Genomic Profiler for Beef Cattle-HD and the 50K Illumina BovineSNP50v2 BeadChip, respectively. Dark cutting was analyzed as a binary trait (case versus control) using logistic regression in an additive model implemented in PLINK version 1.9. Significant loci were not identified when correcting for multiple testing (false discovery rate), suggesting that the trait is not controlled by genes with big effects, or the sample size was not large enough to detect these major genes. Regions harbouring single-nucleotide polymorphisms (SNPs) with a raw p < 0.01 using 1 MB window were analyzed for gene function using the ingenuity pathway analysis. For groups I, II, and the combined group, 449, 301, and 191 SNPs were identified, respectively. Genes identified were involved in pyruvic acid modification and release, 2-deoxyglucose clearance and disposal, sucrose recognition, energy production, and metabolism of carbohydrate. Although detected SNP associations require validation in a large population, results suggested the possibility for marker-assisted or genomic selection of beef cattle to reduce dark cutting.

Review of the methods used for calculating physiological cross-sectional area (PCSA) for ecological questions

This review examines literature that used physiological cross-sectional area (PCSA) as a representative measure of an individual muscle's maximal isometric force production. PCSA is used to understand the muscle architecture and how a trade-off between muscle force and muscle contractile velocity reflect adaptations of the musculoskeletal system as a reflection of functional demands. Over the decades, methods have been developed to measure muscle volume, fascicle lengths, and pennation angle to calculate PCSA. The advantages and limitations of these methods (especially the inclusion/elimination of pennation angle) are discussed frequently; however, these method descriptions are scattered throughout the literature. Here, we reviewed and summarised the different approaches to collecting and recording muscle architectural properties to subsequently calculate PCSA. By critically discussing the advantages and limitations of each methodology, we aim to provide readers with an overview of repeatable methods to assess muscle architecture. This review may serve as a guide to facilitate readers searching for the appropriate techniques to calculate PCSA and measure muscle architecture to be applied in ecomorphology research. RESEARCH HIGHLIGHTS: Discuss the theories behind PCSA in a synthesised review to inform researchers about PCSA methodology.

Effect of Amniotic Injection of N-Carbamylglutamate on Meat Quality of Broilers

Simple Summary

N-carbamylglutamate (NCG) is a key activator of endogenous arginine production, which plays a vital role in meat quality and antioxidant performance. To the best of our knowledge, there is no study about NCG enhancing the meat quality of broilers. The present research was aimed at exploring the effects of amniotic injection of NCG on meat quality of pectoral muscle in broilers. The data showed that the in ovo feeding (IOF) of NCG enhanced the arginine content, improved the nutritional properties, enhanced the antioxidant capacity, and improved the meat quality in the pectoral muscle of broilers. In summary, amniotic injection of NCG on day 17.5 of incubation could be an effective and novel approach to improving the meat quality of broilers.

Abstract

The current study was performed to determine the influence of amniotic injection of N-carbamylglutamate (NCG) on meat quality of pectoral muscle in broilers. A total of 792 alive broiler embryos at 17 d of incubation were assigned to three treatments randomly (non-injected control, saline-injected control, or NCG-injected treatment). The two injection treatments were an injection with 0.1 mL 0.85% aseptic saline alone or containing 2 mg NCG per egg at 17.5 d of incubation. After hatching, 72 healthy male chicks were selected from each treatment and housed in six pens for a 42 day feeding study. Pectoral muscles from six 42-day-old broilers were collected from each treatment group and were dissected for meat quality assays. The results showed that arginine contents in pectoral muscle in either free or hydrolytic form in the NCG group were higher than those in the non-injection control group (p < 0.05). In comparison to the non-injection or saline-injection control groups, NCG injection resulted in a lower lactic acid content in pectoral muscle (p < 0.05). Muscular antioxidant capacity in the NCG group was higher, as evidenced by the higher activity of catalase and glutathione peroxidase and lower content of malondialdehyde (p < 0.05). In addition, the group of in ovo administration of NCG had decreased drip loss and increased crude fat content in pectoral muscle in comparison to those of either control group (p < 0.05) and had enhanced crude protein content compared to that of the saline-injection control group (p < 0.05). Briefly, these results indicate that amniotic administration of NCG in the late incubation phase increased the arginine content, improved the nutritional properties, enhanced the antioxidant capacity, and improved the meat quality in the pectoral muscle of broilers. Amniotic injection of NCG may serve as a novel approach to improving the meat quality of broilers.

Capillary Network Morphometry of Pig Soleus Muscle Significantly Changes in 24 Hours After Death

Capillary network characteristics are invaluable for diagnostics of muscle diseases. Biopsy material is limited in size and mostly not accessible for intensive research. Therefore, especially in human tissue, studies are performed on autopsy material. To approach the problem whether it is reliable to deduce hypotheses from autopsy material to explain physiological and pathological processes, we studied capillarity in pig soleus muscle 1 and 24 hr after death. Capillaries and muscle fibers were immunofluorescently marked, and images were acquired with a confocal microscope. Characteristics of the capillary network were estimated by image analysis methods using several plugins of the Ellipse program. Twenty-four hours after death, the measured characteristics of the capillary network differ by up to 50% when compared with samples excised 1 hr after death. Muscle fiber diameter, the measured capillary length, and tortuosity were reduced, and capillary network became more anisotropic. The main postmortem change that affects capillaries is evidently geometric deformation of muscle tissue. In conclusion, when comparing results from biopsy samples with those from autopsy samples, the effect of postmortem changes on the measured parameters must be carefully considered.

Effects of dietary energy sources on early postmortem muscle metabolism of finishing pigs

OBJECTIVE

This study investigated the effects of different dietary energy sources on early postmortem muscle metabolism of finishing pigs.

METHODS

Seventy-two barrow (Duroc×Landrace×Yorkshire, DLY) pigs (65.0±2.0 kg) were allotted to three iso-energetic and iso-nitrogenous diets: A (44.1% starch, 5.9% crude fat, and 12.6% neutral detergent fibre [NDF]), B (37.6% starch, 9.5% crude fat, and 15.4% NDF) or C (30.9% starch, 14.3% crude fat, and 17.8% NDF). After the duration of 28-day feeding experiment, 24 pigs (eight per treatment) were slaughtered and the M. longissimus lumborum (LL) samples at 45 min postmortem were collected.

RESULTS

Compared with diet A, diet C resulted in greater adenosine triphosphate and decreased phosphocreatine (PCr) concentrations, greater activity of creatine kinase and reduced percentage bound activities of hexokinase (HK), and pyruvate kinase (PK) in LL muscles (p<0.05). Moreover, diet C decreased the phosphor-AKT level and increased the hydroxy-hypoxia-inducible factor-1α (HIF-1α) level, as well as decreased the bound protein expressions of HK II, PKM2, and lactate dehydrogenase A (p<0.05).

CONCLUSION

Diet C with the lowest level of starch and the highest levels of fat and NDF could enhance the PCr utilization and attenuate glycolysis early postmortem in LL muscle of finishing pigs.

Effects of dietary starch types on early postmortem muscle energy metabolism in finishing pigs

This study aimed to investigate the effects of different dietary starch types on early postmortem muscle energy metabolism in finishing pigs. Ninety barrows (68.0±2.0kg) were randomly allotted to three experimental diets with five replicates of six pigs, containing pure waxy maize starch (WMS), nonwaxy maize starch (NMS), and pea starch (PS) (amylose/amylopectin were 0.07, 0.19 and 0.28 respectively). Compared with the WMS diet, pigs fed the PS diet exhibited greater creatine kinase activity, higher adenosine triphosphate and adenosine diphosphate contents, lower phosphocreatine (PCr), adenosine monophosphate and glycogen contents, and lower glycolytic potential (P<0.05). Moreover, the PS diet led to reduced percentage of bound hexokinase activity, decreased level of phosphorylated AKT (P<0.05) and increased level of hypoxia-inducible factor-1α (P<0.05). In conclusion, diet with high amylose content might promote PCr degradation and inhibit the rate of glycolysis, followed by attenuation of early postmortem glycolysis in finishing pigs.

Muscle structure, sarcomere length and influences on meat quality: A review

The basic contractile unit of muscle, the sarcomere, will contract as the muscle goes into rigor post-mortem. Depending on the conditions, such as the rate of pH decline, the cooling rate and the mechanical restraints on the muscles, this longitudinal shortening will result in various post-mortem sarcomere lengths as well as lateral differences in the distances between the myosin and actin filaments. This shortening is underlying the phenomena described as rigor contraction, thaw rigor, cold shortening and heat shortening. The shortening in combination with the molecular architecture of the sarcomere as defined by the myosin filaments and their S-1 and S-2 units, the interaction with the actin filaments, and the boundaries formed by the Z-disks will subsequently influence basic meat quality traits including tenderness and water-holding capacity. Biochemical reactions from proteolysis and glycogen metabolism interrelate with the sarcomere length in a complex manner. The sarcomere length is also influencing the eating quality of cooked meat and the water-holding in meat products.

Association of blood glucose, blood lactate, serum cortisol levels, muscle metabolites, muscle fiber type composition, and pork quality traits

The objective of this study was to investigate the relationship of blood glucose levels with blood lactate, serum cortisol levels, postmortem muscle glycogen and lactate content, muscle fiber type composition, and pork quality traits. Compared to pigs with lower blood glucose levels, pigs with higher blood glucose levels showed higher blood lactate and serum cortisol levels at exsanguination, and they had lower residual glycogen and higher lactate content in the muscle at 45min postmortem. In addition, pigs with higher blood glucose levels had higher type IIB and lower type I area composition and finally exhibited lower muscle pH, paler color, and excessive loss of fluid on surface. These results imply that measuring blood glucose levels at exsanguination can be useful to indicate early glycolytic rates during postmortem and thus may be of value in the identification of pork with undesirable quality traits.

Regulation of post-mortem glycolysis in ruminant muscle

As a tissue, muscle has the unique ability to switch its metabolic source of ATP, the energy currency underpinning muscle function. During oxygen debt, such as that occurring immediately following the death of animals, anaerobic metabolism is initiated in an attempt to restore homeostasis within the muscle. The cascade of biochemical events that are initiated is paramount in the context of meat quality. This review revisits this reasonably well-known subject but takes a new perspective by drawing on the understanding outside the traditional discipline of meat science. Our understanding of the intrinsic regulators of glycolytic flux has improved but knowledge gaps remain. Further efforts to understand how the glycolytic enzyme kinetics are influenced by both pre- and post-slaughter factors will be beneficial in the ongoing quest to maximise fresh meat quality.

The concentrations of free magnesium and free calcium ions both increase in skeletal muscle fibres entering Rigor mortis

Metabolic inhibitors have been used to induce rigor in mammalian muscle fibres previously injected with the Ca(2+)- and Mg(2+)-binding dye arsenazo III. The spectral changes which ensue, after the onset of rigor, indicate that a large increase in intracellular free Ca(2+) occurs (to more than 10(-4)m) but that this is preceded and accompanied by a substantial increase in free Mg(2+) concentration (to 2 mm or more) presumably as a result of the decline in the ATP concentration within the cell. Fibres in rigor have been treated with the divalent cation ionophore A23187 to enable Ca(2+) and Mg(2+) to be extracted selectively from the cell by extracelluar chelating agents. Optical measurements performed during this extraction, confirm that both Ca(2+)- and Mg(2+)-binding to arsenazo III contribute to the absorption change which is recorded, whilst the treatment of relaxed fibres with the ionophore together with EDTA, produces an absorption change which, in combination with other measurements, indicates that in the sarcoplasm of the resting relaxed cell, approximately one third of the dye is complexed with Mg(2+). This indicates that the cytoplasmic free Mg(2+) concentration, prior to rigor onset, is about 0·5 mm.

Muscle-fibre types in porcine longissimus muscle of different genotypes and their association with the status of energy metabolism

To study the difference in muscle-fibre types in porcine muscle among different genotypes and its association with energy metabolism, composition of myosin heavy chain (MyHC) mRNA and energy metabolism indices were determined in the longissimus muscle (LM). Pig breeds included Jinhua (JHP), Zhongbai (ZBP), Duroc × Zhongbai cross (DZP) and Duroc × Yorkshire × Landrace cross (DYL). JHP pigs were found to have the highest proportions of MyHC I, IIa and IIx mRNA (P < 0.05), creatine kinase (CK) activity (P < 0.05) and the lowest glycolytic potential (GP) compared with the other genotypes. The proportions of MyHC I and IIa mRNA increased in the order of DYL < DZP < ZBP < JHP, whereas the trend was opposite for MyHC IIb mRNA. The proportions of MyHC I, IIa and IIx mRNA were positively correlated with CK activity and the turnover ratio of creatine phosphate (CP) (P < 0.01), and negatively correlated with GP, glucose-6-phosphate (G-6-P) and lactate (LA) contents (P < 0.01), with the trends being opposite for MyHC IIb mRNA. The results indicate that muscle-fibre type in porcine LM is influenced by the genetic background of pigs. For example, JHP pigs had more of Types I, IIa and IIx fibres than did other genotypes. Proportions of Types I, IIa and IIx fibres were positively correlated with CK reaction (ATP-CP) capacity and negatively correlated with GP. These data provide some evidence for exploring the effective mechanism of muscle-fibre type on pork quality.

Sarcomere length of torn rotator cuff muscle

BACKGROUND

Sarcomere length is one of the factors related to the contractile ability of muscle. There is no report about sarcomere length of torn rotator cuff muscles. The purpose of this study was to clarify sarcomere length of torn rotator cuff muscles.

MATERIALS AND METHODS

Twenty-eight embalmed cadaver shoulders (14 shoulders with intact rotator cuff and 14 shoulders with full-thickness rotator cuff tears: an isolated tear of the supraspinatus in 3, a combined tear of the supraspinatus and infraspinatus in 4, and a combined tear of the supraspinatus, infraspinatus and subscapularis in 7) were used in this study. Muscle fiber length was measured using a digital caliper. Sarcomere length was measured by laser diffraction method.

RESULTS

Muscle fiber lengths of the supraspinatus and infraspinatus (33.0 +/- 6.5 mm and 61.5 +/- 14.0 mm, respectively) in the cuff tear group were significantly shorter than those in the intact cuff group (56.9 +/- 10.1 mm and 74.2 +/- 10.0 mm: P < .001 and P = .010). The sarcomere lengths of these muscles were 3.00 +/- 0.44 microm and 3.12 +/- 0.45 microm in the intact cuff group and 2.90 +/- 0.34 microm and 3.01 +/- 0.34 microm in the cuff tear group. The sarcomere lengths showed no significant difference (P = 0.46 and P = .37).

CONCLUSION

The sarcomere lengths of the supraspinatus and infraspinatus with torn tendons were not significantly different from those with intact tendons, although the muscle fiber lengths were significantly shorter with torn tendons.

Comparing human skeletal muscle architectural parameters of cadavers with in vivo ultrasonographic measurements

The purpose of this study was to document and compare the architectural parameters (fibre bundle length, angle of pennation) of human skeletal muscle in cadaveric specimens and live subjects. The medial (MG) and lateral (LG) gastrocnemius, and posterior (PS) and anterior (AS) soleus were examined bilaterally in 5 cadavers (mean age 72.6, range 65-83 y) and 9 live subjects (mean age 76.3, range 70-92 y). Data were obtained from direct measurement of cadaveric specimens and from ultrasonographic scans of the live subjects. In cadaveric muscle, fibre bundles were isolated; their length was measured in millimetres and pennation angles were recorded in degrees. In live muscle, similar measurements were taken from ultrasonographic scans of relaxed and contracted muscle. For the scans of relaxed muscle, subjects were positioned prone with the foot at a 90 degrees angle to the leg, and for scans of contracted muscle, subjects were asked to sustain full plantarflexion during the scanning process. Fibre bundle length and angle of pennation were compared at matched locations in both groups. It was found that the relationship between cadaveric and in vivo values for fibre length and angle of pennation varied between muscle parts. The cadaveric architectural parameters did not tend to lie consistently towards either extreme of relaxation or contraction. Rather, within MG, PS and AS, cadaveric fibre bundle lengths lay between those for relaxed and contracted in vivo muscle. Similarly both the anterior and posterior cadaveric fibre angles of pennation lay between the in vivo values within LG and PS. In summary, architectural characteristics of cadaveric muscle differ from both relaxed and contracted in vivo muscle. Therefore, when developing models of skeletal muscle based on cadaveric studies, the architectural differences between live and cadaveric tissue should be taken into consideration.

Active state of muscle in iodoacetate rigor

Frog sartorius muscles, equilibrated to 2 x 10^-4M iodoacetic acid-Ringer's solution and activated by a series of twitches or a long tetanus, perform a rigor response consisting in general of a contractile change which plateaus and is then automatically reversed. Isotonic rigor shortening obeys a force-velocity relation which, with certain differences in value of the constants, accords with Hill's equation for this relation. Changes in rigidity during either isotonic or isometric rigor response show that the capacity of the rigor muscle to bear a load increases more abruptly than the corresponding onset of the ordinarily recorded response, briefly plateaus, and then decays. A quick release of about 1 mm. applied at any instant of isometric rigor output causes the tension to drop instantaneously to zero and then redevelop, the rate of redevelopment varying as does the intensity of the load-bearing capacity. These results demonstrate that rigor mechanical responses result from interaction of a passive, undamped series elastic component, and a contractile component with active state properties like those of normal contraction. Adenosinetriphosphate is known to break down in association with development of the rigor active state. This is discussed in relation to the apparent absence of ATP splitting in normal activation of the contractile component.

Iodoacetic acid-induced rigor in ileal longitudinal smooth muscle of guinea-pig

1. Ileal tensions to iodoacetic acid (IAA) develop when tissue ATP concentration falls below approximately 60-55% of the control. 2. As the IAA concentration is increased (0.1-10 mM), the ATP concentrations decrease rapidly, and both the time of the onset and the duration of the contraction shorten. 3. In the presence of lactate, IAA failed to decrease the tissue ATP concentration and did not develop tension. 4. The contraction to IAA developed in the presence of Ca2+ antagonist, D-600 or in Ca2(+)-free solution, however, onset time was prolonged. 5. These results suggest that the contraction to IAA is referred to as 'rigor' because it increases with decreasing tissue ATP concentration in ileum.

Early post-mortem metabolism and muscle shortening in the Pectoralis major muscle of broiler chickens

Three experiments were conducted to examine the effects of sodium pentobarbital (SP), iodoacetate (IO), tubocurarine (TC), and surgical denervation (DN) on early rigor development in broiler breast muscle. In Experiment 1, birds were either anesthetized or not with SP before receiving an injection of IO or TC or maintained as noninjected controls. Experiment 2 was identical except that a treatment of denervation of the breast muscle was added. Experiment 3 was conducted to contrast birds at 1 day (DN1) and 3 days (DN3) denervation prior to slaughter to nonoperated controls. Measurements of muscle lactate, ATP, R value (ratio of inosine to adenine nucleotides), pH, sarcomere lengths, and shear were used to evaluate treatment effects. Results for Experiment 1 showed no significant differences among treatment and control groups for ATP and lactate contents, R values, or sarcomere lengths; however, significantly lower pH and higher shear values were observed for control birds. In Experiment 2, no significant differences were observed among the treatment groups for ATP, R values, or sarcomere lengths. However, lactate and shear values were significantly lower, and pH higher, for the DN and SP treated birds. Experiment 3 resulted in lower lactate and higher pH values for the DN3 treatment in comparison with both DN1 and control groups. Results of these studies indicate that the use of SP and DN can be used to alter the early profiles of rigor development.

Microstructure and biochemistry of avian muscle and its relevance to meat processing industries

This review is an attempt to summarize information concerning the gross and microscopic anatomy of avian muscle. In addition, specific proteins found in muscle tissue such as the troponins, tropomyosin, alpha- and beta-actinin, desmin, vimentin, myomesin, and creatine kinase are described. The role of protein synthesis and degradation, leading to net protein accumulation in growing animals, is addressed. It is thought that learning to decrease the rate of protein degradation would have immense economic significance. Satellite cells are mentioned as some of these cells fuse with muscle cells to become true muscle nuclei. Understanding the mechanism by which the nuclei of satellite cells differentiate into muscle nuclei could lead to a practical means of increasing protein synthesis within muscle tissue. Finally, postmortem biochemistry is discussed to show how the pH of muscle tissue interacts with environmental temperature to affect the final tenderness of the product.

Contraction of rabbit skinned skeletal muscle fibers at low levels of magnesium adenosine triphosphate

The contractile properties of skinned single fibers from rabbit psoas muscle were investigated under conditions of low MgATP and no Ca2+ (i.e., less than 10(-8) M). At 1 microM MgATP, fibers shortened at a maximum velocity of 660 +/- 420 A/half sarcomere/s (n = 9), compared with 34,000 A/half sarcomere/s measured during maximum Ca2+-activation at 1 mM MgATP (Moss, R. L., 1982. J. Muscle Res. Cell. Motil ., 3:295-311). The observed dependence of Vmax on pMgATP between 7.0 and 5.3 was similar to that of actomyosin ATPase measured previously by Weber, A., R. Herz , and I. Reiss (1969, Biochemistry, 8:2266-2270). Isometric tension was found to vary with pMgATP in a manner much like that reported by Reuben , J. P., P. W. Brandt, M. Berman , and H. Grundfest (J. Gen. Physiol. 1971. 57:385-407). A simple cross-bridge model was developed to simulate contractile behaviour at both high and low levels of MgATP. It was found that the pMgATP dependence of Vmax and ATPase could be successfully modeled if the rate of detachment of the cross-bridge was made proportional to the concentration of MgATP. In the model, the similar dependence of Vmax and ATPase on pMgATP was derived from the fact that in this range of pMgATP every pass of a cross-bridge by an actin site resulted in an attachment-detachment cycle, and every such cycle caused hydrolysis of one molecule of ATP.