Contractile properties of adjacent segments of single human muscle fibers.

A comparison of the contractile properties of adjacent segments of single human muscle fibers may help to explain the interaction among nuclear domains within the myofiber. Biopsy samples were obtained from the vastus lateralis muscle of 20 healthy untrained women (age 18-79 years). Single fibers (n = 38) were dissected and cut into halves (segments A and B). Segment diameter and depth were measured using an image analysis system. Maximal force (Po) was recorded during activation with calcium (pCa 4.5). Maximal unloaded shortening velocity (Vo) was calculated using the slack test. Myosin heavy chain (MyHC) expression was determined using sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS-PAGE). A significant difference ( approximately 7%) in Po was seen between adjacent segments expressing type I MyHC that could not be attributed to differences in fiber size. Significant differences were observed in Vo even after adjusting for fiber type. A positive correlation was seen in Po (concordance coefficient Rho_C = 0.803) and Vo (Rho_C = 0.690) between segments, but concordance was less than perfect in both cases. Possible explanations for nonuniformity of contractile properties include random variations in physiological systems or variability of protein expression among nuclear domains.

Age- and gender-related differences in maximum shortening velocity of skeletal muscle fibers.

OBJECTIVE: To determine age- and gender-related differences in maximum unloaded shortening velocity (Vo) of Type I and IIA single muscle fibers. Muscle fibers must have a broad range of contractile velocities to generate the full range of power required for varied activities. DESIGN: Percutaneous needle biopsies of the vastus lateralis were obtained from 31 healthy subjects (n = 7 young men [YM], n = 7 young women [YW], n = 12 older men [OM], n = 12 older women [OW]). The slack test was used to determine Vo of individual fibers; 916 muscle fibers were chemically skinned. Fiber type was determined by myosin heavy chain isoform identification. RESULTS: Among men, Vo (fiber lengths/sec) was reduced with age in Type IIA fibers (OM vs. YM: 1.78 vs. 2.14; P < 0.05) but unchanged in Type I fibers. Among women, Vo was reduced with age in Type I fibers (OW vs. YW: 0.70 vs. 0.75; P < 0.05) but not IIA. OW had a lower Vo than did OM in both fiber types (Type I: OW = 0.70, OM = 0.77; Type IIA: OW = 1.51, OM = 1.78; P < 0.05). YW did not differ from YM. CONCLUSIONS: Both age and gender affect Vo. Age- and gender-related differences in Vo may partially explain the impairments in muscle function that occur with aging and the greater impairment in muscle function observed in OW compared with that observed in OM.

Longitudinal muscle strength changes in older adults: influence of muscle mass, physical activity, and health.

The longitudinal changes in isokinetic strength of knee and elbow extensors and flexors, muscle mass, physical activity, and health were examined in 120 subjects initially 46 to 78 years old. Sixty-eight women and 52 men were reexamined after 9.7 +/- 1.1 years. The rates of decline in isokinetic strength averaged 14% per decade for knee extensors and 16% per decade for knee flexors in men and women. Women demonstrated slower rates of decline in elbow extensors and flexors (2% per decade) than men (12% per decade). Older subjects demonstrated a greater rate of decline in strength. In men, longitudinal rates of decline of leg muscle strength were approximately 60% greater than estimates from a cross-sectional analysis in the same population. The change in leg strength was directly related to the change in muscle mass in both men and women, and it was inversely related to the change in medication use in men. Physical activity declined yet was not directly associated with strength changes. Although muscle mass changes influenced the magnitude of the strength changes over time, strength declines in spite of muscle mass maintenance or even gain emphasize the need to explore the contribution of other cellular, neural, or metabolic mediators of strength changes.

Skeletal muscle fiber quality in older men and women.

Whole muscle strength and cross-sectional area (WMCSA), and contractile properties of chemically skinned segments from single fibers of the quadriceps were studied in 7 young men (YM, 36.5 +/- 3. 0 yr), 12 older men (OM, 74.4 +/- 5.9 yr), and 12 older women (OW, 72.1 +/- 4.3 yr). WMCSA was smaller in OM compared with YM (56.1 +/- 10.1 vs. 79.7 +/- 13.1 cm(2); P = 0.031) and in OW (44.9 +/- 7.5; P < 0.003) compared with OM. Age-related, but not sex-related, differences in strength were eliminated after adjusting for WMCSA. Maximal force was measured in 552 type I and 230 type IIA fibers. Fibers from YM (type I = 725 +/- 221; type IIA = 792 +/- 271 microN) were stronger (P < 0.001) than fibers from OM (I = 505 +/- 179; IIA = 577 +/- 262 microN) even after correcting for size. Type IIA fibers were stronger (P < 0.005) than type I fibers in YM and OM but not in OW (I = 472 +/- 154; IIA = 422 +/- 97 microN). Sex-related differences in type I and IIA fibers were dependent on fiber size. In conclusion, differences in WMCSA explain age-related differences in strength. An intrinsic defect in contractile proteins could explain weakness in single fibers from OM. Sex-related differences exist at the whole muscle and single fiber levels.

Muscle fatigue and muscle injury.

The purpose of this review is to acquaint the reader with the neurobiology of muscle fatigue. Muscle fatigue is a complex, multifactorial process. The authors have covered the chain of events bringing about skeletal muscle contraction and the manner in which fatigue may affect each step. Advances in technology continue to increase understanding of central fatigue. Many excellent studies of peripheral fatigue have been designed to delineate the mechanisms that influence the excitation-contraction coupling, energy supply, and force generation processes. Although much of this work has considered mechanisms in isolation, different mechanisms may be responsible under different conditions. Fatigue is a common complaint among patients with a variety of neuromuscular and metabolic diseases. Armed with an enhanced knowledge of the mechanisms of muscle fatigue, one can more fully recognize the signs and symptoms of metabolic disorders and neuromuscular diseases and use diagnostic testing. The clinician should anticipate the role of muscle fatigue in injury and focus on injury prevention strategies, especially during the restorative phase of rehabilitation. As a clinician-scientist concerned with optimizing patients' and athletes' performance, one must design ways to identify, measure, and treat muscle fatigue. Beyond illustrating what is currently known about muscle fatigue, the authors hope this review inspires the reader to solve problems of great clinical importance to patients and athletes alike.

Aging of skeletal muscle: a 12-yr longitudinal study.

The present study examines age-related changes in skeletal muscle size and function after 12 yr. Twelve healthy sedentary men were studied in 1985-86 (T1) and nine (initial mean age 65.4 +/- 4.2 yr) were reevaluated in 1997-98 (T2). Isokinetic muscle strength of the knee and elbow extensors and flexors showed losses (P < 0.05) ranging from 20 to 30% at slow and fast angular velocities. Computerized tomography (n = 7) showed reductions (P < 0.05) in the cross-sectional area (CSA) of the thigh (12.5%), all thigh muscles (14.7%), quadriceps femoris muscle (16.1%), and flexor muscles (14. 9%). Analysis of covariance showed that strength at T1 and changes in CSA were independent predictors of strength at T2. Muscle biopsies taken from vastus lateralis muscles (n = 6) showed a reduction in percentage of type I fibers (T1 = 60% vs. T2 = 42%) with no change in mean area in either fiber type. The capillary-to-fiber ratio was significantly lower at T2 (1.39 vs. 1. 08; P = 0.043). Our observations suggest that a quantitative loss in muscle CSA is a major contributor to the decrease in muscle strength seen with advancing age and, together with muscle strength at T1, accounts for 90% of the variability in strength at T2.

Contractile properties of single muscle fibers in myotonic dystrophy.

Muscle fiber contractile dysfunction in myotonic dystrophy (MD) is poorly understood. We biopsied the tibialis anterior of two symptomatic and three asymptomatic subjects (aged 21-31 years) with the MD mutation. Biopsies were freeze dried. A total of 103 single muscle fibers were activated with Ca(++), allowing maximal force measurements and specific force (SF) estimates. The slack test was performed to calculate maximum unloaded shortening velocity (V(o)). The myosin heavy chain composition of each fiber was determined using sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS-PAGE). Type I and IIA fibers of all subjects had reduced SF when compared with healthy control subjects (P < 0.001). In addition, the type I fibers of symptomatic subjects generated less SF than those of asymptomatic subjects (P < 0.001). Type I fibers from asymptomatic and symptomatic subjects did not differ in V(o), but V(o) was lower than in control subjects (P < 0.001). There was no significant difference in V(o) of type IIA fibers from symptomatic, asymptomatic, and control subjects. These results indicate that the MD mutation leads to a diminished force-generating capacity of the myofilaments in both symptomatic and asymptomatic individuals. The results further suggest that reduction in force-generating capacity at the cellular level develops prior to clinical weakness.

Drink composition, voluntary drinking, and fluid balance in exercising, trained, heat-acclimatized boys.

This study examined the effects of beverage composition on the voluntary drinking pattern, body fluid balance, and thermoregulation of heat-acclimatized trained boys exercising intermittently in outdoor conditions (wet bulb globe temperature 30.4 +/- 1.0 degreesC). Twelve boys (age 13.4 +/- 0.4 yr) performed two 3-h sessions, each consisting of four 20-min cycling bouts at 60% maximal aerobic power alternating with 25-min rest. One of two beverages was assigned: unflavored water (W) or flavored water plus 6% carbohydrate and 18 mmol/l Na (CNa). Drinking was ad libitum. Total intake was higher (P < 0.05) during CNa (1,943 +/- 190 g) compared with W (1,470 +/- 143 g). Euhydration was maintained with CNa (+0.18% body wt), but a mild dehydration resulted with W (-0.94% body wt; P < 0.05). Sweat loss, much higher than previously published for children of similar age, was similar between conditions (CNa = 1,644.7 +/- 117.5; W = 1,750.2 +/- 152.7 g). The increase in rectal temperature (CNa = 0.86 +/- 0.3; W = 0.76 +/- 0.1 degreesC), heart rate, and all perceptual variables did not differ between conditions. In conclusion, a flavored carbohydrate-electrolyte drink prevents voluntary dehydration in trained heat-acclimatized boys exercising in a tropical climate despite their large sweat losses. Because hydration changes were minor, the thermoregulatory strain observed was similar between conditions.

Dietary and performance assessment of elite soccer players during a period of intense training.

This study examined the nutritional and performance status of elite soccer players during intense training. Eight male players (age 17+/-2 years) of the Puerto Rican Olympic Team recorded daily activities and food intake over 12 days. Daily energy expenditure was 3,833+/-571 (SD) kcal, and energy intake was 3,952+/-1,071 kcal, of which 53.2+/-6.2% (8.3 g x kg BW(-1)) was from carbohydrates (CHO), 32.4+/-4.0% from fat, and 14.4+/-2.3% from protein. With the exception of calcium, all micronutrients examined were in accordance with dietary guidelines. Body fat was 7.6+/-1.1% of body weight. Time to completion of three runs of the soccer-specific test was 37.65+/-0.62 s, and peak torques of the knee flexors and extensors at 60 degrees x s(-1) were 139+/-6 and 225+/-9 N x m, respectively. Players' absolute amounts of CHO seemed to be above the minimum recommended intake to maximize glycogen storage, but calcium intakes were below recommended. Their body fat was unremarkable, and they had a comparatively good capacity to endure repeated bouts of intense soccer-specific exercise and to exert force with their knee extensors and flexors.

Muscle strength and hormonal levels in adolescents: gender related differences.

The purpose of the present investigation was to study muscle strength in adolescents and its relationship to serum levels of testosterone and growth hormone in both genders. Thirty active adolescents (15 boys; age range 11 -12 y/o) participated in the first study. Isokinetic muscle strength of the dominant knee extensors (KE) was determined at 0, 12, 20, 30, 120, 180 and 240 deg/sec using a Cybex 340 dynamometer. The assessment of pubertal status was accomplished using the criteria of Tanner. Serum levels of total testosterone (T) and growth hormone (GH) were determined using radioimmunoassay techniques. Boys had higher (p< 0.001) T levels but no differences in muscle strength were detected between genders. Fifty-seven additional subjects representing three age groups (11-12 y/o, n=18; 13-14, n=21; 17-18, n=18) participated in the second study. A significant increase in peak torque (absolute and corrected for body weight) with age was observed in both genders. There were no significant gender differences in strength for the two youngest age groups, but boys were stronger than girls in the oldest age group (group 3). Testosterone and GH levels increased with age in boys but not in girls. Gender related differences in T were found in groups 2 and 3. A positive correlation (r=0,64 boys; r=0.46 girls) between testosterone levels and absolute muscle strength was seen in both genders. Our results suggest that increases in anabolic hormones precede muscle strength gains in adolescent males. In addition, gender related differences in muscle strength during adolescents cannot be explained solely on the basis of difference in body size or T levels.

Firing rate of the lower motoneuron and contractile properties of its muscle fibers after upper motoneuron lesion in man.

We studied motor unit (MU) firing rate and contractile properties and myosin isoform composition of single muscle fibers after upper motoneuron lesion. Single-MUs and surface electromyogram (EMG) were recorded during voluntary contractions and locomotion in the paretic (P) and nonparetic (NP) tibialis anterior (TA) of 15 hemiparetics. P TA low-threshold MUs fired within the lower end of their normal range. High-threshold MUs fired below their normal range or were not recruited. Surface EMG was abnormally low and high in the P TA and NP TA, respectively. On muscle cross sections stained with histochemical methods, type I fibers represented 99.4%, 74.3% and 66.6% of NP, P, and control TA, respectively. P TA fibers expressing type I myosin heavy chain (MyHC) were smaller, weaker, and slower. In conclusion, low MU firing rate and activity in the P TA was associated with slower type I MyHC fibers, while increased activity in NP TA resulted in homogenous expression of type I MyHC.

Contractile studies of single human skeletal muscle fibers: a comparison of different muscles, permeabilization procedures, and storage techniques.

The study of single muscle fibers has improved our understanding of muscle physiology and pathology. To compare three techniques for fiber preparation and storage, biopsies were obtained from the tibialis anterior and vastus lateralis muscles of a hemiparetic patient and a control subject. Single fibers were prepared with: (1) chemical skinning (CS) and storage at -20 degrees C; (2) chemical skinning followed by sucrose (SU) incubation and storage at -80 degrees C; or (3) freeze-drying (FD) and -80 degrees C storage. Cross-sectional area (CSA), resting, maximal (P0), and specific tension (P0/CSA), and maximum shortening velocity (V0) were determined in 189 cells. CSA was similar in all groups. Resting tension was higher and P0 and P0/CSA lower after FD. In general, V0 was the same in all groups. Our data suggest that CS and SU preserve the properties of single muscle fibers better than FD. SU may allow longer storage of fibers.

The reproducibility of the Bruce protocol exercise test for the determination of aerobic capacity in older women.

The reproducibility of the Bruce exercise test protocol for the determination of maximal aerobic activity was evaluated in sedentary older women. Seventeen women between the ages of 51 and 68 yr performed five maximal graded exercise tests to volitional fatigue on a treadmill. VO2max (mL.kg-1.min-1) values averaged 27.5 +/- 1.1; 28.3 +/- 1.3; 28.4 +/- 1.3; 29.6 +/- 1.5; and 28.2 +/- 1.4 for trials 1-5, respectively, and were not significantly different. Criteria for a plateau in VO2 at the point of exhaustion were met in 21 out of 85 tests (25%). The mean coefficient of variation VO2max for the subjects for the 5 tests was 6.5% (range, 2.0-14%). Pearson's correlation coefficients for the study variables were significant, indicating good agreement between repeated tests (r2: between 0.70 to 0.89). Although there were no significant differences among the mean VO2max values in the 5 trials, 11 subjects had a 1.0 ml.kg-1.min-1 or greater increase in the VO2max from test 1 to test 2, and only 6 subjects had no change or a decrease in VO2max. The mean difference between T2 and T3 was lower (T1 vs T2: 0.8 mL.kg-1.min-1, T2 vs T3: 0.1 mL.kg-1.min-1) indicating slightly better agreement between the second and third set. Estimates of the between and within subject variance revealed a low within subject variance (4.2 (mL.kg-1.min-1)2) compared to the between subject variance (22.1 (mL.kg-1.min-1)2). This study demonstrates that a commonly used exercise testing protocol generates highly reproducible measurements of VO2max in women between 51 and 68 yr. The mean differences between tests and the high level of agreement between repeated tests suggests that a single measurement of VO2max can be performed to assess functional aerobic capacity in this population.

Effects of aging on shortening velocity and myosin isoform composition in single human skeletal muscle cells.

Maximum unloaded shortening velocity (V0) and maximum force normalized to cross-sectional fiber area (specific tension) were determined in 400 single quadriceps muscle cells of young (n = 4, 25-31 yr) and old (n = 4, 73-81 yr) men. Two of the old men were physically very active, and the subjects were divided into young control, old control, and old physically active groups. The expressions of types I, IIa, and IIb myosin heavy chains (MHC) and essential and regulatory myosin light chains were determined by 6 and 12% sodium dodecyl sulfate-polyacrylamide gel electrophoresis. V0 was highly dependent on the MHC isoform composition, and a gradient from slow to fast was observed in the sequence I, I-IIa, IIa, IIab, and IIb MHC cells. The V0 values and specific tensions of types I and IIa MHC fibers in the young control group were significantly higher than those in the old control and old physically active groups. Thus the present results provide evidence of qualitative changes in contractile properties of human skeletal muscle in old age, which probably play an important role in the age-related impairment of skeletal muscle function.

Effects of removal of weight-bearing function on contractility and myosin isoform composition in single human skeletal muscle cells.

The purpose of this study was to investigate the effects of a 6-week period without weight bearing, achieved by bed rest, on the contractile behaviour, myosin isoform expression and myofibrillar protein content of single human muscle fibres. Percutaneous biopsied specimens of the quadriceps muscle were taken from three healthy male volunteers before and at the end of the experimental period. Maximum force normalised to cross-sectional area (specific tension), maximum velocity of unloaded shortening (V0), and myosin heavy chain (MyHC) and light chain (MyLC) isoform composition were measured in single membrane-permeabilised muscle cells obtained from these specimens. At the end of the experimental period, specific tension was reduced (P < 0.001) by 40% and there was a parallel decline in myofibrillar protein content per muscle cell volume. V0 did not change significantly in response to bed rest when data from all muscle cells were pooled. In two of the subjects, however, V0 decreased (P < 0.01-0.001) in muscle cells expressing the beta/slow (type I) MyHC isoform, but there was no change in fibres expressing type IIA or a combination of type IIA and IIB MyHCs. The slowing in type I MyHC fibres was associated with a change in the isoform composition of the regulatory MyLC.

Effects of hyperhydration on total body water, temperature regulation and performance of elite young soccer players in a warm climate.

The purpose of this study was to test the effect of increased fluid intake on temperature regulation and performance in elite soccer players. Eight players of the Puerto Rico National Team were studied under field conditions at a Wet Bulb Globe Temperature (WBGT) heat stress index of 25.3 +/- 0.5 degrees C. They were young (17.0 +/- 0.6 yr) and well aerobically conditioned, as shown by their maximum aerobic power of 69.2 +/- 0.7 ml.kg(-1).min(-1). Players were randomly allocated to a week of voluntary hydration (VH) (fluid intake = 2.7 +/- 0.2.day(-1)) and a week of hyperhydration (HH) (fluid intake = 4.6 +/- 0.2.day(-1)) (p <0.05) prior to a soccer match. Their total body water (TBW) increased in HH compared to VH (p<0.05), despite a significant increase in urine output (p < 0.01). The soccer match was played at 82 +/- 7% (VH) and 83 +/- 6% (HH) of maximum heart rate. Sweat losses and core temperature increases during the match were similar in both hydration conditions. When the environmental conditions were taken into consideration, the increase in core temperature during the match rose, as a function of the heat stress index, only in VH (p < 0.05). Plasma volume was slightly reduced in both hydration conditions (ns). Performance assessment at the end of the soccer match revealed that average time to complete 7 repetitions of a soccer specific test was significantly increased (p < 0.05) after the match in both hydration conditions but showed no difference between them. Peak torque and fatigability of knee flexor and extensor muscle groups at 240 deg.sec(-1), measured on a Cybex 340 dynamometer were similar in both hydration conditions and not affected by the soccer match. The data suggest that additional water intake in these heat-acclimated players increased body water reserves and improved temperature regulation during a soccer match with no significant effect on the decrement in soccer specific performance observed at the end of a soccer match.

Muscle strength and body composition: associations with bone density in older subjects.

This study investigated the relationship between isokinetic muscle strength and bone density (SPA and DPA) in men and women aged 45-77 yr. Regression models were adjusted for age, weight, smoking status, and calcium supplementation. Elbow extensors (but not flexors) peak torque was correlated with radial density in men (partial r = 0.26, P < 0.05) and women (partial r = 0.24, P < 0.05). Knee flexor (but not extensor) peak torque in women was significantly correlated with spine density (partial r = 0.28, P < 0.05), and muscle mass was significantly correlated with Ward's triangle density (partial r = 0.35, P < 0.05). No associations between knee flexor or extensor muscle strength and spine or femur bone density were observed in men. Fat-free mass (FFM, hydrodensitometry) was associated with all bone density sites in males and females (partial r = 0.30-0.55; P < 0.05). These results demonstrate that 1) significant associations observed between elbow extensor strength and radial bone density in men and women may reflect loading along the longitudinal axis of the radius associated with elbow extensor activity; and 2) significant associations exist between knee flexor muscle strength and lumbar density in women only. Additionally, the associations between FFM and bone density do not necessarily reflect associations between isokinetic muscle strength and bone density.