Fiber Type Distribution Research Articles

Diffusion Tensor Imaging of Skeletal Muscle - Correlation of Fractional Anisotropy to Muscle Power

Recent DTI studies demonstrated the possibility of fiber geometry visualization in skeletal muscle. We tested for an association between muscle power and standard DTI parameters, e. g. fractional anisotropy. Maximal muscle power (Lmax) of the soleus muscle was determined in 11 healthy subjects. Subsequently DTI was performed and standard parameters (fractional anisotropy - FA, mean diffusivity - MD, parallel diffusivity - PD, radial diffusivity - RD) were extracted in an ROI of the soleus muscle. We found a signficant association of Lmax with FA (neg. correlation: r = -0.85, p = 0.0015) and RD (pos. correlation r = 0.80, p = 0.047). There was no signficant association of MD or PD. Maximum muscle power is an indirect measure of fiber type distribution. The correlation between muscle power and DTI parameters can be explained by differences in fiber diameter and differences in the intracellular microstructure of type-1 and type-2 fibers. DTI should be evaluated as a tool for non-invasive quantification of fiber type distribution in skeletal muscle.

Distinct roles for Ste20-like kinase SLK in muscle function and regeneration

BackgroundCell growth and terminal differentiation are controlled by complex signaling systems that regulate the tissue-specific expression of genes controlling cell fate and morphogenesis. We have previously reported that the Ste20-like kinase SLK is expressed in muscle tissue and is required for cell motility. However, the specific function of SLK in muscle tissue is still poorly understood.MethodsTo gain further insights into the role of SLK in differentiated muscles, we expressed a kinase-inactive SLK from the human skeletal muscle actin promoter. Transgenic muscles were surveyed for potential defects. Standard histological procedures and cardiotoxin-induced regeneration assays we used to investigate the role of SLK in myogenesis and muscle repair.ResultsHigh levels of kinase-inactive SLK in muscle tissue produced an overall decrease in SLK activity in muscle tissue, resulting in altered muscle organization, reduced litter sizes, and reduced breeding capacity. The transgenic mice did not show any differences in fiber-type distribution but displayed enhanced regeneration capacity in vivo and more robust differentiation in vitro.ConclusionsOur results show that SLK activity is required for optimal muscle development in the embryo and muscle physiology in the adult. However, reduced kinase activity during muscle repair enhances regeneration and differentiation. Together, these results suggest complex and distinct roles for SLK in muscle development and function.

The influence of training status, age, and muscle fiber type on cycling efficiency and endurance performance

The purpose of this study was to assess the influence of age, training status, and muscle fiber-type distribution on cycling efficiency. Forty men were recruited into one of four groups: young and old trained cyclists, and young and old untrained individuals. All participants completed an incremental ramp test to measure their peak O2 uptake, maximal heart rate, and maximal minute power output; a submaximal test of cycling gross efficiency (GE) at a series of absolute and relative work rates; and, in trained participants only, a 1-h cycling time trial. Finally, all participants underwent a muscle biopsy of their right vastus lateralis muscle. At relative work rates, a general linear model found significant main effects of age and training status on GE (P < 0.01). The percentage of type I muscle fibers was higher in the trained groups (P < 0.01), with no difference between age groups. There was no relationship between fiber type and cycling efficiency at any work rate or cadence combination. Stepwise multiple regression indicated that muscle fiber type did not influence cycling performance (P > 0.05). Power output in the 1-h performance trial was predicted by average O2 uptake and GE, with standardized β-coefficients of 0.94 and 0.34, respectively, although some mathematical coupling is evident. These data demonstrate that muscle fiber type does not affect cycling efficiency and was not influenced by the aging process. Cycling efficiency and the percentage of type I muscle fibers were influenced by training status, but only GE at 120 revolutions/min was seen to predict cycling performance.

Aging-associated exacerbation in fatty degeneration and infiltration after rotator cuff tear

Rotator cuff tears are one of the most common musculoskeletal complaints and a substantial source of morbidity in elderly patients. Chronic cuff tears are associated with muscle atrophy and an infiltration of fat to the area, a condition known as "fatty degeneration." To improve the treatment of cuff tears in elderly patients, a greater understanding of the changes in the contractile properties of muscle fibers and the molecular regulation of fatty degeneration is essential. Using a full-thickness, massive supraspinatus and infraspinatus tear model in elderly rats, we measured fiber contractility and determined changes in fiber type distribution that develop 30 days after tear. We also measured the expression of messenger RNA and micro-RNA transcripts involved in muscle atrophy, lipid accumulation, and matrix synthesis. We hypothesized that a decrease in specific force of muscle fibers, an accumulation of type IIb fibers, and an upregulation in atrophic, fibrogenic, and inflammatory gene expression would occur in torn cuff muscles. Thirty days after the tear, we observed a reduction in muscle fiber force and an induction of RNA molecules that regulate atrophy, fibrosis, lipid accumulation, inflammation, and macrophage recruitment. A marked accumulation of advanced glycation end products and a significant accretion of macrophages in areas of fat accumulation were observed. The extent of degenerative changes in old rats was greater than that observed in adults. In addition, we identified that the ectopic fat accumulation that occurs in chronic cuff tears does not occur by activation of canonical intramyocellular lipid storage and synthesis pathways.

A shift in energy metabolism anticipates the onset of sarcopenia in rhesus monkeys

Age-associated skeletal muscle mass loss curtails quality of life and may contribute to defects in metabolic homeostasis in older persons. The onset of sarcopenia occurs in middle age in rhesus macaques although the trigger has yet to be identified. Here, we show that a shift in metabolism occurs in advance of the onset of sarcopenia in rhesus vastus lateralis. Multiphoton laser-scanning microscopy detects a shift in the kinetics of photon emission from autofluorescent metabolic cofactors NADH and FAD. Lifetime of both fluorophores is shortened at mid-age, and this is observed in both free and bound constituent pools. Levels of FAD and free NADH are increased and the NAD/NADH redox ratio is lower. Concomitant with this, expression of fiber-type myosin isoforms is altered resulting in a shift in fiber-type distribution, activity of cytochrome c oxidase involved in mitochondrial oxidative phosphorylation is significantly lower, and the subcellular organization of mitochondria in oxidative fibers is compromised. A regulatory switch involving the transcriptional coactivator PGC-1α directs metabolic fuel utilization and governs the expression of structural proteins. Age did not significantly impact total levels of PGC-1α; however, its subcellular localization was disrupted, suggesting that PGC-1α activities may be compromised. Consistent with this, intracellular lipid storage is altered and there is shift to larger lipid droplet size that likely reflects a decline in lipid turnover or a loss in efficiency of lipid metabolism. We suggest that changes in energy metabolism contribute directly to skeletal muscle aging in rhesus monkeys.

Marked phenotypic differences of endurance performance and exercise-induced oxygen consumption between AMPK and LKB1 deficiency in mouse skeletal muscle: changes occurring in the diaphragm

LKB1 phosphorylates members of the AMP-activated protein kinase (AMPK) family. LKB1 and AMPK in the skeletal muscle are believed to regulate not only fuel oxidation during exercise but also exercise capacity. LKB1 was also required to prevent diaphragm fatigue, which was shown to affect exercise performance. Using mice expressing dominant negative (DN) mutants of LKB1 and AMPK, specifically in the skeletal muscle but not in the heart, we investigated the roles of LKB1 and AMPK activity in exercise performance and the effects of these kinases on the characteristics of respiratory and locomotive muscles. In the diaphragm and gastrocnemius, both AMPK-DN and LKB1-DN mice showed complete loss of AMPKα2 activity, and LKB1-DN mice showed a reduction in LKB1 activity. Exercise capacity was significantly reduced in LKB1-DN mice, with a marked reduction in oxygen consumption and carbon dioxide production during exercise. The diaphragm from LKB1-DN mice showed an increase in myosin heavy chain IIB and glycolytic enzyme expression. Normal respiratory chain function and CPT I activity were shown in the isolated mitochondria from LKB1-DN locomotive muscle, and the expression of genes related to fiber type, mitochondria function, glucose and lipid metabolism, and capillarization in locomotive muscle was not different between LKB1-DN and AMPK-DN mice. We concluded that LKB1 in the skeletal muscle contributes significantly to exercise capacity and oxygen uptake during exercise. LKB1 mediated the change of fiber-type distribution in the diaphragm independently of AMPK and might be responsible for the phenotypes we observed.

PREDICTION OF MUSCLE FORCES USING STATIC OPTIMIZATION FOR DIFFERENT CONTRACTILE CONDITIONS

In this study, we introduced a novel cost function for the prediction of individual muscle forces for a one degree-of-freedom musculoskeletal system. Unlike previous models, the new approach incorporates the instantaneous contractile conditions represented by the force-length and force-velocity relationships and accounts for physiological properties such as fiber type distribution and physiological cross-sectional area (PCSA) in the cost function. Using this cost function, it is possible to predict experimentally observed features of force-sharing among synergistic muscles that cannot be predicted using the classical approaches. Specifically, the new approach allows for predictions of force-sharing loops of agonistic muscles in one degree-of-freedom systems and for simultaneous increases in force in one muscle and decreases in a corresponding agonist. We concluded that the incorporation of the contractile conditions in the weighting of cost functions provides a natural way to incorporate observed force-sharing features in synergistic muscles that have eluded satisfactory description.

Fiber type characterization in skeletal muscle by diffusion tensor imaging

Fiber type distribution within a skeletal muscle, i.e. the quantification of the relative amount of type 1 (slow-twitching) and type 2 (fast-twitching) muscle fibers, is of great interest for the monitoring of the effects of training or the treatment of muscle diseases. The purpose of this study was to determine the feasibility of diffusion tensor imaging (DTI) as a tool for noninvasive fiber type quantification in human skeletal muscle. The right calves of 12 healthy volunteers were examined using DTI at 1.5 T. Standard DTI parameters, including fractional anisotropy (FA), and mean, radial and parallel diffusivity (MD, RD and PD, respectively), were determined in the soleus muscle. Fiber type proportion and mean fiber diameter within the soleus muscle were quantified from tissue specimens obtained via a fine needle biopsy. Linear regression analysis tested for associations between DTI and biopsy results. FA values were correlated significantly with fiber type proportion, such that higher FA values indicated a higher proportion of type 1 fibers (R(2) = 0.5, p = 0.01). This was based on lower diffusivity perpendicular to the main axis of the fiber in subjects with a higher type 1 fiber proportion (RD: R(2) = 0.52, p = 0.008). MD was also correlated with the proportion of type 1 fibers (R(2) = 0.37, p = 0.037), whereas PD showed no significant correlation. DTI is a promising method for the noninvasive estimation of fiber type proportion in skeletal muscle. This technique may be used to monitor training effects or may be further developed as a biomarker in certain muscle diseases.

Masseter function and skeletal malocclusion

The aim of this work is to review the relationship between the function of the masseter muscle and the occurrence of malocclusions. An analysis was made of the masseter muscle samples from subjects who underwent mandibular osteotomies. The size and proportion of type-II fibers (fast) decreases as facial height increases. Patients with mandibular asymmetry have more type-II fibers on the side of their deviation. The insulin-like growth factor and myostatin are expressed differently depending on the sex and fiber diameter. These differences in the distribution of fiber types and gene expression of this growth factor may be involved in long-term postoperative stability and require additional investigations. Muscle strength and bone length are two genetically determined factors in facial growth. Myosin 1H (MYOH1) is associated with prognathia in Caucasians. As future objectives, we propose to characterize genetic variations using "Genome Wide Association Studies" data and their relationships with malocclusions.

Characterization of Longissimus thoracis, Semitendinosus and Masseter muscles and relationships with technological quality in pigs. 1. Microscopic analysis of muscles

Three porcine muscles (Longissimus thoracis, Semitendinosus, Masseter), known to have large differences in biochemical and histological traits, were fully characterized and the link between muscle structure and quality evaluated. The oxidative Masseter had more pigment, higher content of metmyoglobin, haem iron, protein and collagen, and was redder with higher fibre numbers, fibre circularity, pH and water holding capacity than the glycolytic Longissimus. Fibre type distribution showed predominance of type IIB in Longissimus and Semitendinosus white, type I in Semitendinosus red and IIA in Masseter. Type I fibres were larger than type IIB and IIA in Semitendinosus and Masseter, respectively, but not in the Longissimus, indicating that fibre size is muscle dependent. Muscle redness was positively correlated with type I fibre traits, haem iron and metmyoglobin, and negatively associated with type II fibre characteristics, non-haem iron and oxymyoglobin. Expressible juice had positive correlation with fibre size and negative with fibre number and connective tissue.

AQP4-Dependent Water Transport Plays a Functional Role in Exercise-Induced Skeletal Muscle Adaptations

In this study we assess the functional role of Aquaporin-4 (AQP4) in the skeletal muscle by analyzing whether physical activity modulates AQP4 expression and whether the absence of AQP4 has an effect on osmotic behavior, muscle contractile properties, and physical activity. To this purpose, rats and mice were trained on the treadmill for 10 (D10) and 30 (D30) days and tested with exercise to exhaustion, and muscles were used for immunoblotting, RT-PCR, and fiber-type distribution analysis. Taking advantage of the AQP4 KO murine model, functional analysis of AQP4 was performed on dissected muscle fibers and sarcolemma vesicles. Moreover, WT and AQP4 KO mice were subjected to both voluntary and forced activity. Rat fast-twitch muscles showed a twofold increase in AQP4 protein in D10 and D30 rats compared to sedentary rats. Such increase positively correlated with the animal performance, since highest level of AQP4 protein was found in high runner rats. Interestingly, no shift in muscle fiber composition nor an increase in AQP4-positive fibers was found. Furthermore, no changes in AQP4 mRNA after exercise were detected, suggesting that post-translational events are likely to be responsible for AQP4 modulation. Experiments performed on AQP4 KO mice revealed a strong impairment in osmotic responses as well as in forced and voluntary activities compared to WT mice, even though force development amplitude and contractile properties were unvaried. Our findings definitively demonstrate the physiological role of AQP4 in supporting muscle contractile activity and metabolic changes that occur in fast-twitch skeletal muscle during prolonged exercise.

Fiber type composition of the architecturally distinct regions of human supraspinatus muscle: a cadaveric study.

The human supraspinatus muscle is clinically important as it is frequently injured in older adults and the elderly. We have previously shown that the supraspinatus has a complex architecture with two distinct regions each consisting of three parts. Further we have found dynamic changes in architectural parameters such as fiber bundle length markedly vary between these regions. Fiber types of the supraspinatus have not been thoroughly investigated throughout its volume and are of interest to clinicians treating supraspinatus pathologies. In this study we investigated the distribution of fiber types within the distinct regions and parts of supraspinatus. Samples of supraspinatus were excised from six distinct parts of each muscle from five formalin embalmed specimens (one male, four female; mean age 77±11.1 years) free of tendon pathology. Samples were frozen in liquid nitrogen and then cryosectioned. Serial sections were labeled using immunohistochemical techniques and antibodies against fast or slow myosin heavy chain isoforms. The mean percentage of Type I (slow) fibers ranged from 56.73% to 63.97%. Results demonstrated significant variations in fiber type distribution. The middle part of the anterior region has a significantly greater percentage of Type I fibers compared to that of the posterior. The superficial part of the anterior region has a greater percentage of Type II (fast) fibers compared to the middle and deep parts. Findings aid in highlighting the distinct functions of the anterior and posterior regions, and prompt the need to re-evaluate assessment and treatment techniques established on a limited understanding of the fiber type distribution.

Asetil L-karnitin ve doksorubisinin rat çizgili kası üzerine etkilerinin araştırılması

Aim. Doxorubicin is a widely used anticancer agent which can cause an unpredictable cardiac toxicity increases with its cumulative dose. Previous studies demonstrated that side effects of doxorubicin can be reduced by L-carnitine. However in only a few studies, toxic effects of doxorubicin on skeletal muscles was investigated widely. The objective of present study was to investigate the effects of doxorubicin and Acetyl L-carnitine on skeletal muscles in rats. Method. The study was performed on adult male Wistar-Albino rats which are 200-300 gr in weight. Rats were divided to four groups as control, doxorubicine, ALCAR and ALCAR + doxorubicine. The first group (n=7) was the control. The second and forth groups (n=6/n=6) received intraperitoneal doxorubicine (cumulative dose 15 mg/kg). Third and forth groups (n=7/n=6) received intraperitoneal ALCAR (300 mg/kg/day) and it was continued 10 days. All animals in each group were sacrificed on tenth day of the study and muscle biopsy samples were taken from quadriceps muscles. All samples were frozen and 8-micron sections were cut using cryostat. Slides were stained with routine hematoxylin eosin, enzyme histochemically combined COX and SDH, histochemically with modified Gomori’s trichrome, PAS and oil red O. Fast myosin antibody was used for discriminating myofiber type. Results. No significant differences were found in muscle fiber type distribution, lipid and glucogen accumulation, or mitocondrial function in Doxorubicine or ALCAR groups compared with control group. Conclusion. This study indicated that treatment with doxorubicine or ALCAR cannot produce significant injury to non-cardiac striated muscle tissue.

Anatomic and neuromuscular characterisation of the equine cricothyroid muscle

As part of investigation into laryngeal stability and reanimation using functional electrical stimulation, the cricothyroid muscle might be utilised to increase laryngeal cross-sectional area in horses with recurrent laryngeal neuropathy. For optimal electrode placement and muscle recruitment, the neuroanatomy and excitability of the equine cricothyroid muscle needs to be defined. To describe the anatomy, innervation and function of the equine cricothyroid muscle and its contribution to laryngeal diameter. Seventeen equine larynges were collected at necropsy and 12 were grossly dissected. Seven larynges (five grade 1, two grade 4) were prepared for immunohistochemistry following gross dissection and 5 larynges were prepared for special staining: acetylcholinesterase staining of motor endplates (n = 3) and Sihler's staining (n = 2). Three larynges were stimulated following in situ cadaver dissection and 2 larynges were removed and stimulated ex vivo. Three neuromuscular compartments, each innervated by a primary nerve branch of the external branch of the cranial laryngeal nerve, were identified in all larynges. Stimulation of each neuromuscular compartment resulted in ventral displacement of the thyroid cartilage with respect to the cricoid cartilage, thereby increasing dorsoventral height of the rima glottis. The equine cricothyroid muscle has 3 distinct neuromuscular compartments with discrete innervation, fibre type distribution and muscle fibre sizes. All neuromuscular compartments tense the vocal cords by increasing dorsoventral height of the rima glottis through ventral displacement of the thyroid cartilage with respect to the cricoid cartilage. Simultaneous functional electrical stimulation of the cricothyroid and dorsal cricoarytenoid muscles may enhance laryngeal cross-sectional area in horses with recurrent laryngeal neuropathy.

Human Aging and Expression of Proteins Interacting with the Ryanodine Receptor in Skeletal Muscle

Sarcopenia is characterised by reduced muscle mass and reduced force that is only partially attributable to muscle atrophy. Fiber type distribution changes with aging (fast twitch decreases, slow twitch increases). Excitation-contraction coupling (ECC) may be impaired by an uncoupling of the dihydropyridine receptor (DHPR) and the ryanodine receptor (RyR1), possibly due to decreased expression of the DHPR α1s subunit (Delbono et al., J Membr Biol. 1995;148:211-22). The DHPR β1a subunit, which may contribute to ECC (Rebbeck et al., Biophys J. 2011;100:922-30), increases (Taylor et al., Aging cell 2009;8:584-94). The 12KDa FK506 binding protein (FKBP12) stabilizes RyR1 and its dissociation may contribute to decreased ECC (Andersson et al., Cell Metab.2011; 14:196-207). Previous studies have used animal models; here we investigate the expression of these proteins in aging human muscle.Human muscle samples were obtained from 42 male and female donors (age 40-90) undergoing knee (vastus medialis) and hip (gluteus minimus or gluteus medius) replacements. Results of fiber type distribution in muscle homogenates from female subjects shows fast twitch fibers decreasing and slow twitch increasing with age. Preliminary data shows that levels of FKBP12 relative to RyR1, determined in microsomal vesicles using western blot and densitometry decrease with age. Expression levels of the DHPR α1s and β1a subunits relative to that of actin determined in muscle homogenates using western blot and densitometry are being analysed. We find that β1a subunit is mostly in the homogenate and not associated with membrane fractions. The preliminary data indicates that the levels of several of the proteins that associate with RyR1 and modulate its activity change with age and could affect the release of Ca2+ during ECC and muscle function in human sarcopenia.

Noninvasive Monitoring of Training Induced Muscle Adaptation with -MRS: Fibre Type Shifts Correlate with Metabolic Changes

Purpose. To evaluate training induced metabolic changes noninvasively with 31P magnetic resonance spectroscopy (31P-MRS) for measuring muscle fibre type adaptation. Methods. Eleven volunteers underwent a 24-week training, consisting of speed-strength, endurance, and detraining (each 8 weeks). Prior to and following each training period, needle biopsies and 31P-MRS of the resting gastrocnemius muscle were performed. Fibre type distribution was analyzed histologically and tested for correlation with the ratios of high energy phosphates ([PCr]/[Pi], [PCr]/[βATP] and [PCr + Pi]/[βATP]). The correlation between the changes of the 31P-MRS parameters during training and the resulting changes in fibre composition were also analysed. Results. We observed an increased type-II-fibre proportion after speed-strength and detraining. After endurance training the percentage of fast-twitch fibres was reduced. The progression of the [PCr]/[Pi]-ratio was similar to that of the fast-twitch fibres during the training. We found a correlation between the type-II-fibre proportion and [PCr]/[Pi] (r = 0.70, P < 0.01) or [PCr]/[βATP] (r = 0.69, P < 0.01); the correlations between its changes (delta) and the fibre-shift were significant as well (delta[PCr]/[Pi] r = 0.66, delta[PCr]/[βATP] r = 0.55, P < 0.01). Conclusion. Shifts in fibre type composition and high energy phosphate metabolite content covary in human gastrocnemius muscle. Therefore 31P-MRS might be a feasible method for noninvasive monitoring of exercise-induced fibre type transformation.

Myosin Isoforms and Contractile Properties of Single Fibers of Human Latissimus Dorsi Muscle

The aim of our study was to investigate fiber type distribution and contractile characteristics of Latissimus Dorsi muscle (LDM). Samples were collected from 18 young healthy subjects (9 males and 9 females) through percutaneous fine needle muscle biopsy. The results showed a predominance of fast myosin heavy chain isoforms (MyHC) with 42% of MyHC 2A and 25% of MyHC 2X, while MyHC 1 represented only 33%. The unbalance toward fast isoforms was even greater in males (71%) than in females (64%). Fiber type distribution partially reflected MyHC isoform distribution with 28% type 1/slow fibers and 5% hybrid 1/2A fibers, while fast fibers were divided into 30% type 2A, 31% type A/X, 4% type X, and 2% type 1/2X. Type 1/slow fibers were not only less abundant but also smaller in cross-sectional area than fast fibers. During maximal isometric contraction, type 1/slow fibers developed force and tension significantly lower than the two major groups of fast fibers. In conclusion, the predominance of fast fibers and their greater size and strength compared to slow fibers reveal that LDM is a muscle specialized mainly in phasic and powerful activity. Importantly, such specialization is more pronounced in males than in females.

Muscle Fiber Types, Characteristics and Meat Quality

Muscle fiber is the basic unit of muscle tissue, this paper summarized the types of muscle fiber of animals, the influence factors of muscle fiber type distribution and the muscle fiber type conversion in the process of growth constantly. Discuss the important effect of muscle fiber type on meat quality.

Effects of dietary zinc level and ractopamine HCl on pork chop tenderness and shelf-life characteristics (2013)

A total of 320 finishing pigs (PIC 327 × 1050; initially 216 lb) were utilized to determine the effects of adding Zn to diets containing ractopamine HCl (RAC) on muscle fiber type distribution, fresh chop color, and cooked meat characteristics. Dietary treatments were fed for approximately 35 d and consisted of: a corn-soybean meal–based negative control (CON); a positive control diet with 10 ppm of RAC (RAC+); and the RAC+ diet plus 75, 150, or 225 ppm added Zn from either ZnO or Availa-Zn. Loins from 80 barrow and 80 gilt carcasses were evaluated. No Zn source effect or Zn source × level interactions we observed during the study (P > 0.10). Pigs fed the RAC+ had increased (P < 0.02) percentage type IIX and a tendency for increased percentage type IIB muscle fibers. Increasing added Zn decreased (linear, P = 0.01) percentage type IIA and tended to increase (P = 0.09) IIX muscle fibers. On d 1, 2, 3, 4, and 5 of display, pork chops from pigs fed the RAC+ treatment had greater (P < 0.03) L* values (lighter) compared with the CON. On d 0 and 3 of display, increasing added Zn tended to decrease (quadratic, P = 0.10) L* values and decreased (quadratic, P < 0.03) L* values on d 1, 2, 4, and 5. Pigs fed RAC+ had decreased (P < 0.05) a* values (less red) on d 1 and 4 of display and tended to have decreased (P < 0.10) a* values on d 0 and 2 compared with CON pork chops. RAC+ decreased (P < 0.001) metmyoglobin reducing ability (MRA) of pork chops on d 5. Chops from pigs fed added Zn had increased (quadratic, P < 0.03) MRA on d 3 and 5 of the display period. There was a trend for increased (linear, P = 0.07) cooking loss as added Zn increased in RAC diets. In conclusion, RAC+ diets produced chops that were lighter and less red but maintained a higher percentage of surface oxymyoglobin throughout a 5-d simulated retail display. RAC+ reduced MRA at the end of the display period, but supplementing Zn to RAC diets restored MRA to near CON treatment levels at the end of the display period.; Swine Day, Manhattan, KS, November 21, 2013

Increasing Exercise Intensity Reduces Heterogeneity of Glucose Uptake in Human Skeletal Muscles

Proper muscle activation is a key feature of survival in different tasks in daily life as well as sports performance, but can be impaired in elderly and in diseases. Therefore it is also clinically important to better understand the phenomenon that can be elucidated in humans non-invasively by positron emission tomography (PET) with measurements of spatial heterogeneity of glucose uptake within and among muscles during exercise. We studied six healthy young men during 35 minutes of cycling at relative intensities of 30% (low), 55% (moderate), and 75% (high) of maximal oxygen consumption on three separate days. Glucose uptake in the quadriceps femoris muscle group (QF), the main force producing muscle group in recreational cycling, and its four individual muscles, was directly measured using PET and 18F-fluoro-deoxy-glucose. Within-muscle heterogeneity was determined by calculating the coefficient of variance (CV) of glucose uptake in PET image voxels within the muscle of interest, and among-muscles heterogeneity of glucose uptake in QF was expressed as CV of the mean glucose uptake values of its separate muscles. With increasing intensity, within-muscle heterogeneity decreased in the entire QF as well as within its all four individual parts. Among-muscles glucose uptake heterogeneity also decreased with increasing intensity. However, mean glucose uptake was consistently lower and heterogeneity higher in rectus femoris muscle that is known to consist of the highest percentage of fast twitch type II fibers, compared to the other three QF muscles. In conclusion, these results show that in addition to increased contribution of distinct muscle parts, with increases in exercise intensity there is also an enhanced recruitment of muscle fibers within all of the four heads of QF, despite established differences in muscle-part specific fiber type distributions. Glucose uptake heterogeneity may serve as a useful non-invasive tool to elucidate muscle activation in aging and diseased populations.