Evaluation of a New 1H/31P Dual-Tuned Birdcage Coil for 31P Spectroscopy

We introduce a new dual-tuned Hydrogen/Phosphorus (¹H/³¹P) birdcage coil, referred to as split birdcage coil, and evaluate its performance using both simulations and magnetic resonance (MR) experiments on a 3 T MR scanner. The proposed coil simplifies the practical matters of tuning and matching, which makes the coil easily reproducible. Simulations were run with the Finite Difference in Time Domain (FDTD) method to evaluate the sensitivity and homogeneity of the magnetic field generated by the proposed ¹H coils. Following simulations, MR experiments were conducted using both a phantom and human thigh to compare the proposed design with a currently available commercial dual-tuned flexible surface coil, referred to as flex surface coil, for signal to noise ratio (SNR) as well as homogeneity for the ³¹P coil. At regions deep within the human thigh, the split birdcage coil was able to acquire spectroscopic signal with a higher average SNR than the flex surface coil. For all regions except those close to the flex surface coil, the split birdcage coil matched or exceeded the performance of the flex surface coil.

Electrical stimulation-evoked resistance exercise therapy improves arterial health after chronic spinal cord injury

STUDY DESIGN

Repeated measures training intervention.

OBJECTIVES

To evaluate the effects of neuromuscular electrical stimulation (NMES)-induced resistance exercise therapy on lower extremity arterial health in individuals with chronic, complete spinal cord injury (SCI). We define "arterial health" using three surrogate markers: (a) resting diameter, (b) flow-mediated dilation (FMD), and (c) arterial range.

SETTING

Department of Kinesiology, University of Georgia, USA.

METHODS

We assessed five 36+/-5-year-old male individuals with chronic, complete SCI before, during, and after 18 weeks of training. The quadriceps femoris muscle group of both legs were trained twice a week with 4 x 10 repetitions of unilateral, dynamic knee extensions. The health of the posterior tibial artery was assessed using a B-mode ultrasound unit equipped with a high-resolution video capture device. Proximal occlusion was used to evoke ischemia for 5 min and then for 10 min. FMD was calculated using the peak diameter change (above rest) following 5 min occlusion. Arterial range was calculated using minimum (during occlusion) and maximum diameters (post 10 min occlusion). Hierarchical linear modeling accounted for the nested (repeated measures) experimental design.

RESULTS

FMD improved from 0.08+/-0.11 mm (2.7%) to 0.18+/-0.15 mm (6.6%) (P=0.004), and arterial range improved from 0.36+/-0.28 to 0.94+/-0.40 mm (P=0.001), after 18 weeks of training. Resting diameter did not significantly change.

CONCLUSIONS

Home-based, self-administered NMES resistance exercise therapy consisting of 80 contractions/week improved FMD and arterial range. This provides evidence that resistance exercise therapy can improve arterial health after SCI, which may reduce the risk of future cardiovascular disease.

Blood flow response in individuals with incomplete spinal cord injuries

STUDY DESIGN

Cross sectional comparison, control group.

OBJECTIVE

To determine if incomplete spinal cord injured patients (SCI) have an abnormal blood flow response to cuff ischemia compared to able-bodied individuals (AB).

SETTING

Academic institution.

METHODS

Blood flow in five chronic incomplete SCI patients (C4-C5) and 17 able bodied individuals was measured in the common femoral artery using quantitative Doppler ultrasound (GE LogiQ 400CL) at rest and after distal thigh cuff occlusion of 2, 4 and 10 min to investigate whether blood flow or vascular control were different in SCI's and AB.

RESULTS

Blood flow and the diameter of the common femoral artery at rest were similar in incomplete SCI and AB. Peak flow after 10 min of cuff ischemia (the highest found) was also comparable between incomplete SCI and AB. The half-time for recovery of blood flow to baseline after 2, 4 or 10 min of ischemia was 50% longer for incomplete SCI compared to the AB (P = 0.023). In addition, peak blood flow after 2 and 4 min of ischemia relative to the maximum, 10 min value (2/10 and 4/10 ratios) was lower in incomplete SCI compared to AB (0.65 +/- 0.06 vs 0.76 +/- 0.15, P = 0.029 and 0.75 +/- 0.10 vs 0.89 +/- 0.11, P = 0.014, respectively).

CONCLUSION

This study demonstrated that incomplete spinal cord injured patients have impaired vascular control seen as a slower return to resting flow after cuff ischemia and reduced sensitivity to ischemia relative to maximum flow. However, incomplete SCI patients did not demonstrate impaired flow capacity as seen in complete SCI patients suggesting that smaller cardiovascular abnormalities are seen with incomplete versus complete SCI injury. Impaired vascular control may serve to limit exercise capacity and may contribute to increased cardiovascular disease. Impaired circulation could contribute to impaired muscle function and poor cardiovascular health in incomplete SCI's, although these findings need to be replicated in a study with more subjects.

Near-infrared spectroscopy: what can it tell us about oxygen saturation in skeletal muscle?

Near-infrared spectroscopy (NIRS) measures hemoglobin saturation in small vessels. A number of interesting studies have used this method. However, difficulties with signal quantification and studies in which NIRS oxygen saturation did not behave as expected raise concerns. NIRS remains promising for studies of skeletal muscle, but a better understanding of the method is needed.

Impaired oxygen delivery to muscle in chronic fatigue syndrome

The purpose of this study was to determine if chronic fatigue syndrome (CFS) is associated with reduced oxygen delivery to muscles. Patients with CFS according to CDC (Center for Disease Control) criteria (n=20) were compared with normal sedentary subjects (n=12). Muscle oxygen delivery was measured as the rate of post-exercise and post-ischaemia oxygen-haem resaturation. Oxygen-haem resaturation was measured in the medial gastrocnemius muscle using continuous-wavelength near-IR spectroscopy. Phosphocreatine resynthesis was measured simultaneously using (31)P magnetic resonance spectroscopy. The time constant of oxygen delivery was significantly reduced in CFS patients after exercise (46.5+/-16 s; mean+/-S.D.) compared with that in controls (29.4+/-6.9 s). The time constant of oxygen delivery was also reduced (20.0+/-12 s) compared with controls (12.0+/-2.8 s) after cuff ischaemia. Oxidative metabolism was also reduced by 20% in CFS patients, and a significant correlation was found between oxidative metabolism and recovery of oxygen delivery. In conclusion, oxygen delivery was reduced in CFS patients compared with that in sedentary controls. This result is consistent with previous studies showing abnormal autonomic control of blood flow. Reduced oxidative delivery in CFS patients could be specifically related to CFS, or could be a non-specific effect of reduced activity levels in these patients. While these results suggest that reduced oxygen delivery could result in reduced oxidative metabolism and muscle fatigue, further studies will be needed to address this issue.

Identification of peripheral vascular disease in elderly subjects using optical spectroscopy

The purpose of this study was to determine the usefulness of near-infrared spectroscopy (NIRS) measurements to identify peripheral vascular disease (PVD). Usefulness was determined by the frequency of a successful test, as well as comparison with standard clinical assessments. Study subjects (N = 117, mean age = 67.8 +/- 8.1 yrs) responded to a free screening for PVD. NIRS was used to measure the relative O2 saturation of hemoglobin in the soleus muscle. The time to 1/2 recovery of O2 saturation (O2T1/2) was measured after 1 minute of repeated plantar flexions using a Cybex Eagle seated calf machine. O2T1/2 was used as many subjects had recovery curves that did not have an exponential line shape. The test was done on both legs and the worst leg was used for analysis. For comparative purposes, a clinical history and physical examination were performed by a physician or nurse practitioner, which included questions on intermittent claudication, examination of peripheral pulses, and questions to identify cardiovascular risk factors. NIRS signals were obtained on 105 of 117 subjects (89% success rate). Subjects with body mass index (BMI) values above 32 appeared to have NIRS O2T1/2 values that were less reliable than subjects with BMI values < or = 32 (77% success rate). The O2T1/2 was longer in subjects with claudication and reduced pulses than in subjects without these conditions. Sensitivity comparing O2T1/2 to claudication and reduced pulse varied from 51-76% and specificity from 65-80%, depending on the cutoff value for O2T1/2 that was used (normal value plus 1 or 2 SD). A longer O2T1/2 was significantly associated with incidence of diabetes, smoking, hypercholesterol, and coronary bypass surgery. In summary, successful NIRS O2T1/2 measurements were made in 77% of the subjects, with failure primarily occurring in obese subjects. NIRS O2T1/2 measurements showed reasonable although not strong agreements with clinical assessment of PVD, and with some risk factors for cardiovascular disease.

Noninvasive measurement of phosphocreatine recovery kinetics in single human muscles

The rate at which phosphocreatine (PCr) is resynthesized after exercise is related to muscle oxidative capacity (Vmax). With the use of a one-dimensional image-guided, localized nuclear magnetic resonance spectroscopy technique, PCr kinetics were monitored in the medial gastrocnemius of eight healthy subjects after voluntary, short duration, maximal rate exercise. Localized spectra were obtained every 6 s with <5% contamination from nonselected regions. Maximal rate exercise elicited near-maximal to maximal muscle activation, as indicated by the high-PCr hydrolysis rate (2.26 +/- 0.07 mM/s) and extensive PCr depletion. At the end of 9 s of maximal rate exercise, PCr was depleted by 61.4 +/- 2.4% and intracellular pH was 7.04 +/- 0.03. After 9 s of maximal rate exercise, PCr recovered with a rate constant (kPCr) of 1.87 +/- 0.15 min(-1) and a Vmax of 67.2 +/- 6.0 mM/min. Independent of prior activity, aerobic ATP synthesis rates reached 48.6 +/- 4.9 mM/min within 9 s. Extending maximal rate exercise to 30 s resulted in 92.0 +/- 1.2% PCr depletion and an intracellular pH of 6.45 +/- 0.07. The intracellular acidosis separated the direct relationship between kPCr and muscle Vmax but did not affect the initial PCr resynthesis rate.

Reduced oxidative muscle metabolism in chronic fatigue syndrome

The purpose of this study was to determine if chronic fatigue syndrome (CSF) is characterized by abnormalities in oxidative muscle metabolism. Patients with CFS according to Centers for Disease Control (CDC) criteria (n = 22) were compared to normal sedentary subjects (n = 15). CFS patients were also tested before and 2 days after a maximal treadmill test. Muscle oxidative capacity was measured as the maximal rate of postexercise phosphocreatine (PCr) resynthesis using the ADP model (Vmax) in the calf muscles using 31P magnetic resonance spectroscopy. Vmax was significantly reduced in CFS patients (39.6 +/- 2.8 mmol/L/min, mean +/- SE) compared to controls (53.8 +/- 2.8 mmol/L/min). Two days postexercise there was no change in resting inorganic phosphate (Pi)/PCr or Vmax in the CFS patients (n = 14). In conclusion, oxidative metabolism is reduced in CFS patients compared to sedentary controls. In addition, a single bout of strenuous exercise did not cause a further reduction in oxidative metabolism, or alter resting Pi/PCr ratios.

Reduced oxidative muscle metabolism in chronic fatigue syndrome

The purpose of this study was to determine if chronic fatigue syndrome (CSF) is characterized by abnormalities in oxidative muscle metabolism. Patients with CFS according to Centers for Disease Control (CDC) criteria (n = 22) were compared to normal sedentary subjects (n = 15). CFS patients were also tested before and 2 days after a maximal treadmill test. Muscle oxidative capacity was measured as the maximal rate of postexercise phosphocreatine (PCr) resynthesis using the ADP model (Vmax) in the calf muscles using 31P magnetic resonance spectroscopy. Vmax was significantly reduced in CFS patients (39.6 +/- 2.8 mmol/L/min, mean +/- SE) compared to controls (53.8 +/- 2.8 mmol/L/min). Two days postexercise there was no change in resting inorganic phosphate (Pi)/PCr or Vmax in the CFS patients (n = 14). In conclusion, oxidative metabolism is reduced in CFS patients compared to sedentary controls. In addition, a single bout of strenuous exercise did not cause a further reduction in oxidative metabolism, or alter resting Pi/PCr ratios.

The effect of a submaximal exercise orientation on cardiopulmonary cycle ergometer stress test results in older adults

BACKGROUND

Orientation sessions are sometimes used to habituate subjects before exercise stress testing. The extent of habituation in older subjects has not been clearly defined. Additionally, the use of repetitive maximal stress testing as an orientation method may not be necessarily applicable in the aged.

METHODS

To determine if the employment of a submaximal orientation session would effect cardiopulmonary cycle ergometer exercise results, 266 older adults (68.6 +/- 5.0 years) male (n = 100) and female (n = 166) subjects participated in this study. One hundred thirty-one subjects received an orientation before stress testing. One hundred thirty-five did not.

RESULTS

Analysis of resting values revealed no significant differences. Separate gender analysis was performed at submaximal workloads. Men were examined at 0, 60, and 105 Watts; women at 0, 45, and 75 Watts. Oriented subjects displayed significantly lower heart rates for both males and females at all submaximal workloads. Oxygen uptake was significantly lower for oriented women at 45 Watts (P < or = .05) and men at 60 Watts (P < or = .05). Oriented males displayed significantly lower systolic blood pressure at 0 Watts (P < or = .05), 60 Watts (P < or = .01), and 105 Watts (P < or = .05). The oriented group reached ventilatory threshold (VeT) at a higher workload (P < or = .001), lower heart rate (P < or = .001), and higher VO2 uptake (P < or = .05). Nonoriented subjects obtained a significantly higher maximal heart rate than oriented subjects (147 +/- 15.7 beats per minute (bpm) vs. 140 +/- 17.1 bpm, P < or = .01). Separate gender analysis revealed a significant difference (P < or = .01) in maximal heart rate in males (oriented = 137.4 +/- 18.8 bpm vs. nonoriented = 147.7 +/- 15.7 bpm). Although nonoriented women achieved a higher maximal heart rate, the difference was not significant (146.9 +/- 15.8 bpm vs. 142.6 +/- 16.0 bpm).

CONCLUSION

These results indicate that both male and female older subjects display significant modification in physiologic performance from habituation after a single submaximal exercise orientation session. This effect was greater at submaximal than maximal workloads, and appears to be greater than that previously reported in younger subjects.

Use of exercise for treatment of chronic fatigue syndrome

Chronic fatigue syndrome (CFS) is a condition that results in moderate to severe disability, the primary feature of which is fatigue of unknown origin. There is a lot of interest in classifying, characterising and treating patients with CFS. Currently, the two major theories of a medical cause of CFS are viral infection and immune dysregulation. Patients report critical reductions in levels of physical activity, and many experience 'relapses' of severe symptoms following even moderate levels of exertion. Despite this, most studies report CFS patients to have normal muscle strength and either normal or slightly reduced muscle endurance. Histological and metabolic studies report mixed results: CFS patients have either no impairment or mild impairment of mitochondria and oxidative metabolism compared with sedentary controls. Current treatments for CFS are symptom-based, with psychological, pharmacological and rehabilitation treatments providing some relief but no cure. Immunological and nutritional treatments have been tried but have not provided reproducible benefits. Exercise training programmes are thought to be beneficial (if 'relapses' can be avoided), although few controlled studies have been performed. CFS is a long-lasting disorder that can slowly improve with time, but often does not. Further studies are needed to better understand the multiple factors that can cause chronic fatigue illness, as well as the effect that exercise training has on the symptoms of CFS.

The application of blood flow measurements to the study of aging muscle

Blood flow to skeletal muscle is a potentially important factor in the reduction of muscle function associated with aging (sarcopenia). The main influence of reduced blood flow capacity on muscle function is in limiting oxidative metabolism. Direct measures of blood flow include: intravital-microscopy, plethysmography, radioactive microspheres, 133Xenon washout, thermodilution, and Doppler ultrasound. Indirect measurement of blood flow includes arm-to-ankle pressure index and the rate of phosphocreatine recovery after exercise. Several new methodologies have been developed to evaluate muscle blood flow, including color-Doppler imaging, magnetic resonance imaging/angiography (MRI/MRA), and near-infrared spectroscopy (NIRS). As adaptations of traditional techniques, these methods promise more precise information under less invasive conditions. MRI is an expensive and technically challenging method able to measure vessel location, blood flow, and wall diameter in blood vessels throughout the cardiac cycle. Color-Doppler provides excellent temporal resolution blood flow throughout the cardiac cycle, along with some anatomical information. NIRS is an inexpensive and portable technology that can measure changes in oxygen saturation and provide information on tissue oxygen delivery in studies of frailer and more difficult-to-study subjects. Muscle blood flow is not thought to limit oxidative metabolism under normal conditions in young individuals. However, it is not clear what happens to muscle blood flow in healthy older individuals. Reduced capillary density, less maximal blood flow, and a slower hyperemic flow response have been reported in some, but not all, studies. Further studies with the newer methodologies are needed to re-examine age-related changes in muscle blood flow.(ABSTRACT TRUNCATED AT 250 WORDS)

Injuries during the one repetition maximum assessment in the elderly

PURPOSE

A decrease in strength, and its associated loss of functional ability is common among the elderly. Although resistance training can reverse this decline, associated injuries with frequently used strength assessments may present a greater risk.

METHODS

To evaluate the injuries associated with maximal strength evaluations, 83 relatively healthy elderly subjects (40 men and 43 women, 65.8 +/- 6.2 years) with and without prior weight training experience performed 1 repetition maximum testing (1 RM) involving 5 different exercises: chest press, leg extension, abdominal curl, arm curl, and seated calf raise. Subjects were separated into three groups depending on weight training experience, Group 1 had no weight training experience (n = 32), Group 2 had < 6 months of training (n = 24), and Group 3 had > 6 months of training (n = 27). Injury assessment was made 30 minutes, 2 days, and 7 days posttesting.

RESULTS

Two Group 1 subjects sustained an injury (2.4% of total subjects, 8% of Group 1). Eighty-one subjects safely completed the 1 RM assessment without injury (97.6% of total). Forty-eight of the 83 subjects complained of muscle soreness after testing (58% of total). This complaint alone was not sufficient to be categorized as an injury.

CONCLUSIONS

These results indicate that 1 RM testing is an acceptable tool in strength evaluations in the elderly. Additional precautions may be needed in inexperienced, elderly individuals to prevent injury.

Physical determinants of independence in mature women

The purpose of this study was to determine the relationship in mature women between muscle strength and whole body oxidative capacity and the ability to perform activities of daily living (ADL). Sixty-one women (mean age 69 years) without major disease or disability were recruited from either a community exercise center or a personal care facility. Physiological measurements consisted of peak oxygen consumption on a cycle ergometer (VO2peak) and one repetition maximum strength of nine muscle groups (1-RM). Ability to perform ADL was measured with a balance and gait test, "Bag Carrying Test", and ADL questionnaires. Significant correlations were found with VO2peak and calf muscle strength and ability to perform ADL, with weaker or no correlations for other muscle groups. For some relationships, it was possible to identify the minimum level of physiological functioning associated with successful performance of independence tasks. In summary, physiological capacities, particularly VO2peak and strength of the calf muscles, predicted ability to perform activities needed for functional independence in healthy mature women.

Simultaneous in vivo measurements of HbO2 saturation and PCr kinetics after exercise in normal humans

Simultaneous measurements of phosphocreatine (PCr) and oxyhemoglobin (HbO2) saturation were made during recovery from exercise in calf muscles of five male subjects. PCr was measured using magnetic resonance spectroscopy in a 2.0-T 78-cm-bore magnet with a 9-cm-diam surface coil. Relative HbO2 saturation was measured as the difference in absorption of 750- and 850-nm light with use of near-infrared spectroscopy. The light source and detectors were 3 cm apart. Exercise consisted of isokinetic plantar flexion in a supine position. Two 5-min submaximal protocols were performed with PCr depletion to 60% of resting values and with pH values of > 7.0. Then two 1-min protocols of rapid plantar flexion were performed to deplete PCr values to 5-20% of resting values with pH values of < 6.8. Areas of PCr peaks (every 8 s) and HbO2 saturation (every 1 s) were fit to a monoexponential function, and a time constant was calculated. The PCr time constant was larger after maximal exercise (68.3 +/- 10.5 s) than after submaximal exercise (36.0 +/- 6.5 s), which is consistent with the effects of low pH on PCr recovery. HbO2 resaturation approximated submaximal PCr recovery and was not different between maximal (29.4 +/- 5.5 s) and submaximal (27.6 +/- 6.0 s) exercise. We conclude that magnetic resonance spectroscopy measurements of PCr recovery and near-infrared spectroscopy measurements of recovery of HbO2 saturation provide similar information as long as muscle pH remains near 7.0.

Exercise-induced changes in oxygen saturation in the calf muscles of elderly subjects with peripheral vascular disease

This study noninvasively measured hemoglobin oxygen saturation in young (n = 6, 28 +/- 6 yrs), old healthy (n = 20, 68 +/- 7 yrs), and old subjects with mild peripheral vascular disease (PVD) (n = 8, 72 +/- 4 yrs). Hemoglobin oxygen saturation was measured as the difference in absorption at 760 and 850 nm light using near-infrared spectroscopy (NIRS). The lateral soleus muscle was studied after mild plantar flexion exercise. The time constant of recovery of oxygen saturation (HBO2Tc) was 21.5 +/- 8.6 s in young, 26.9 +/- 13.5 s in old healthy subjects. PVD subjects had HbO2Tc values of 104.4 +/- 41.1 s in their "bad" leg and 57.6 +/- 42.2 s in their "good" leg. The correlation between HbO2Tc and ankle-arm pressures was significant (r2 = .63, p < .001). Six younger normal, five older normal, and seven PVD subjects also performed a progressive walking test. Normal subjects completed the test without difficulty, and older normal subjects showed progressive deoxygenation. PVD subjects complained of calf pain (4 out of 5 were able to finish) and showed near-maximal deoxygenation throughout the test. In summary, NIRS measurements show promise as a method of evaluating the kinetics of oxygen saturation, particularly in patients with mild peripheral vascular disease.

Further impairment of muscle phosphate kinetics by lengthening exercise in DMD/BMD carriers. An in vivo 31P-NMR spectroscopy study

We used phosphorus magnetic resonance spectroscopy (31P-MRS) to study the effect of exercise-induced muscle injury in the calf muscle of 7 DMD/BMD carriers and 6 non-carrier females. All subjects performed 50-80 lengthening contractions with the right calf muscles. 48 h after lengthening exercise non-carriers showed increased sensitivity to pressure in their gastrocnemius accompanied by increased T2 relaxation times and by elevated Pi/PCr ratios at rest. DMD/BMD carriers did not show any effect of lengthening exercise on these measurements. In-magnet exercise revealed in all carriers a reduced initial rate of Pi recovery and an increased time to fully recovery the resting value of intracellular pH. Lengthening exercise further decreased the initial rate of Pi recovery. Non-carriers did not show any variation attributable to lengthening exercise either during in-magnet work or during recovery from exercise. We found that lengthening exercise contractions causes: (1) less muscle injury in carriers compared to non-carriers, (2) even slower rate of Pi recovery, but (3) no effect on Pi recovery in non-carriers. The use of lengthening exercise and measurements of Pi recovery may be a useful method to evaluate the disease process in DMD/BMD.

Relationships between in vivo and in vitro measurements of metabolism in young and old human calf muscles

This study compared in vivo measurements of muscle metabolism in humans with magnetic resonance spectroscopy (MRS) and in vitro analysis of biopsies. Healthy subjects [4 young males, 28.2 +/- 6.8 (SD) yr, and 6 older subjects (5 males, 1 female), 66 +/- 6.0 yr] performed a maximal cycle ergometer test, and MRS measurements of the calf muscles and needle biopsies of the lateral gastrocnemius were performed. Biopsies were analyzed for fiber type and citrate synthase (CS) activity. MRS measurements of inorganic phosphate (Pi), phosphocreatine (PCr), ATP, and pH were made using a 1.8-T 78-cm clear-bore magnet-and-spectrometer system. Two or three 5-min bouts of plantar flexion were performed against variable resistance to deplete PCr levels to 50% of resting values (mean end pH 6.99). PCr values during recovery were fit to an exponential curve, and the rate constant (PCrrate) was calculated. PCrrate was used as an index of oxidative metabolism. Older subjects had lower peak O2 uptake (VO2 peak) values (19.2 +/- 5.6 vs. 49.5 +/- 8.1 ml O2.min-1 x kg-1), CS activities (16 +/- 2.8 vs. 25 +/- 2.6 mmol.kg wet wt-1 x min-1), and PCrrate values (25.3 +/- 8. vs. 37.5 +/- 5.3 mmol PCr.kg wet wt-1.min-1) than young subjects. PCrrate correlated with CS activity, and both PCrrate and CS activity correlated with VO2 peak (P < 0.05). No correlations were found between percent fiber type and PCrrate, CS activity, and VO2 peak. These results support studies that showed decreases in muscle metabolism with age in healthy humans and show a good correlation between in vivo and in vitro measurements of oxidative metabolism.

Muscle metabolism in track athletes, using 31P magnetic resonance spectroscopy

We tested whether preferred running event in track athletes would correlate with the initial rate of phosphocreatine (PCr) resynthesis following submaximal exercise. PCr recovery was measured in the calf muscles of 16 male track athletes and 7 male control subjects following 5 min of repeated plantar flexion against resistance. Pi, PCr, and pH were measured using phosphorus magnetic resonance spectroscopy (31P MRS) with an 8-cm surface coil in a 1.8-T magnet. During exercise, work levels were gradually increased to deplete PCr to 50-60% of the initial value. No drop in pH was seen in any of the subjects during this exercise. The areas of the PCr peaks following exercise were fit to monoexponential curves. Two or three tests were performed on each subject and the results averaged. Athletes were divided into three groups based on their primary event: sprinters running 400 m or less, middle-distance athletes running 400-1500 m, and long-distance athletes running farther than 1500 m. The maximal rates of PCr resynthesis (mmol.min-1.kg-1 muscle weight) were 64.8 +/- 8.6, for long-distance runners; 41.4 +/- 11, for middle-distance runners; 32.0 +/- 7.0, for sprinters; and 38.6 +/- 10, for controls (mean +/- SE). The faster PCr recovery rates seen in long-distance runners compared with sprinters indicate greater oxidative capacity, which is consistent with the known differences between athletes in these events.

Contribution of skeletal muscle atrophy to exercise intolerance and altered muscle metabolism in heart failure

BACKGROUND

The purpose of this study was to investigate the prevalence of skeletal muscle atrophy and its relation to exercise intolerance and abnormal muscle metabolism in patients with heart failure (HF).

METHODS AND RESULTS

Peak VO2, percent ideal body weight (% IBW), 24-hour urine creatinine (Cr), and anthropometrics were measured in 62 ambulatory patients with HF. 31P magnetic resonance spectroscopy (MRS) and imaging (MRI) of the calf were performed in 15 patients with HF and 10 control subjects. Inorganic phosphorus (Pi), phosphocreatine (PCr), and intracellular pH were measured at rest and during exercise. Calf muscle volume was determined from the sum of the integrated area of muscle in 1-cm-thick contiguous axial images from the patella to the calcaneus. A reduced skeletal muscle mass was noted in 68% of patients, as evidenced by a decrease in Cr-to-height ratio of less than 7.4 mg/cm and/or upper arm circumference of less than 5% of normal. Calf muscle volume (MRI) was also reduced in the patients with HF (controls, 675 +/- 84 cm3/m2; HF, 567 +/- 112 cm3/m2; p less than 0.05). Fat stores were largely preserved with triceps skinfold of less than 5% of normal and/or IBW of less than 80% in only 8% of patients. Modest linear correlations were observed between peak VO2 and both calf muscle volume per meter squared (r = 0.48) and midarm muscle area (r = 0.36) (both p less than 0.05). 31P metabolic abnormalities during exercise were observed in the patients with HF, which is consistent with intrinsic oxidative abnormalities. The metabolic changes were weakly correlated with muscle volume (r = -0.42, p less than 0.05).

CONCLUSIONS

These findings indicate that patients with chronic HF frequently develop significant skeletal muscle atrophy and metabolic abnormalities. Atrophy contributes modestly to both the reduced exercise capacity and altered muscle metabolism.

Muscle metabolism in older subjects using 31P magnetic resonance spectroscopy

We used phosphorus magnetic resonance spectroscopy to study the calf muscles of elderly normal (mean +/- SD) (80.0 +/- 5.12 years), elderly impaired (80.7 +/- 0.58 years), old normal (66.8 +/- 1.92 years), and young normal people (24.6 +/- 4.72 years). Relative levels of inorganic phosphate (Pi), phosphocreatine (PCr), and adenosine triphosphate were measured with a 1.9-tesla, 30-cm bore magnet at rest and following plantra flexon exercise. No differences were found at rest or during recovery from exercise in the elderly normal subjects with respect to gender or the presence of stable medical problems treated with medication. At rest there was an age-related decrease in the ratio of PCr/Pi. After exercise, the time constant of PCr recovery increased with age. A mild 7-week exercise regimen consisting of plantar flexion had no effect on time constant of PCr recovery in the elderly subjects. Four elderly impaired subjects had lower PCr/Pi ratios at rest and slower time constant of PCr recovery after exercise than normal elderly subjects. We conclude that gender and the presence of stable medical problems had no effect on muscle metabolism in the elderly and that the elderly recovered slower than young controls. This slower recovery was not corrected with a mild exercise program.

Biochemical adaptations to training: implications for resisting muscle fatigue

Skeletal muscle activity is invariably associated with a decline in force-generating capacity (fatigue). The build-up of metabolic by-products such as intracellular H+ and inorganic phosphate (Pi) has been shown to be one of the potential mechanisms of muscle fatigue. The use of phosphorus magnetic resonance spectroscopy is a repeatable and useful tool to study the effect of pH and Pi on force development. When maximal exercise is preceded by submaximal exercise to reduce the starting muscle pH and increase Pi, the degree of muscle fatigue correlates more strongly with H2PO4- than pH or Pi alone. However, other studies in humans have found that H2PO4- does not always correlate well with fatigue. The use of ramp exercise protocols allow repeatable and sensitive measurement of changes in muscle metabolism in response to endurance training. Chronic electrical stimulation in dogs and endurance training in humans results in reduced pH and Pi changes at the same exercise intensities. This means that the effect of pH and Pi in depressing force development is reduced, which could partially explain the increased fatigue resistance seen following endurance training.

Noninvasive measurements of activity-induced changes in muscle metabolism

Two noninvasive measurement techniques were used to monitor activity-induced changes in skeletal muscle in humans. Phosphorus magnetic resonance spectroscopy (P-MRS) was used to measure changes in energy metabolism by measuring the ratio of inorganic phosphate to phosphocreatine (Pi/PCr) during steady level work in the wrist flexor muscles in a 30 cm bore, 1.9 Telsa magnet, and the rate of PCr recovery from exercise in the calf muscles in a 76 cm bore, 1.8 Tesla magnet. Near red spectroscopy (NRS) was used to measure changes in oxygen saturation of hemoglobin and myoglobin during and after exercise. Fourteen days of wrist flexion exercise resulted in significant improvement in muscle metabolism as measured by MRS. This improvement disappeared after 35 days of inactivity. Indications of muscle stress during training such as muscle soreness and decreased maximum strength were associated with increases in resting Pi/PCr. A similar training protocol using plantar flexion exercise resulted in an improved rate of PCr resynthesis, which returned to control values 42 days after training stopped. NRS measurements of the wrist flexor muscles during a ramp exercise protocol demonstrated a decrease in the oxygen saturation of hemoglobin-myoglobin from 60% at rest to 15% at the highest work levels. The half time of recovery of oxygen saturation was faster than that of PCr in both young and old subjects, supporting the hypothesis that oxygen delivery is not rate limiting in submaximal exercise in healthy individuals.

Metabolic effects of training in humans: a 31P-MRS study

The purpose of this study was to determine the feasibility of measuring with 31P nuclear magnetic resonance the effects of an endurance training program on the high-energy phosphate metabolism of exercising human skeletal muscle. The system used included a 1.9-T 30-cm-bore Oxford Systems superconducting magnet, a PhosphoEnergetics spectrometer, and a modified Cybex isokinetic ergometer. Seven healthy human volunteers exercised their wrist flexor muscles 20 min/day 5 days/wk for 8 wk. Testing before and after the training period consisted of a performance test to measure muscle functional capacity and a ramp test to measure the work-energy cost relationship of the exercising muscles. The results indicate that the subjects had a significant increase in their work output on the 10-min performance test after training. They also exhibited an increase in the work-energy cost relationship on the ramp test as indicated by a decrease in peak Pi-to-phosphocreatine ratio and an increase in pH at the same relative power output after training. These results indicate that 1) the training program was sufficient to elicit a training effect and 2) this effect was observed with 31P nuclear magnetic resonance as an increased potential for oxidative metabolism, particularly at the high exercise levels.

Effects of ovariectomy on energy metabolism in exercising rat muscle studied by 31P-NMR

The effects of ovariectomy on metabolism of high-energy phosphate compounds during and after exercise were studied in hindleg muscles of 14 rats. Sciatic nerve stimulation was used to establish different work loads, and the changes in inorganic phosphate-to-phosphocreatine ratios (Pi/PCr) were recorded by 31P nuclear magnetic resonance (NMR) in vivo. Four weeks after ovariectomy, there was evidence of significantly higher Pi/PCr during work at stimulation rates greater than 0.5 Hz. The slope for the stimulation rate-to-Pi/PCr relationship decreased from 1.98 +/- 0.15 to 1.36 +/- 0.2 Hz/Pi/PCr after ovariectomy. The normalized tension output of these muscles, tested separately using identical stimulation protocols, was not changed with ovariectomy. Thus the relationship between work (tension-time integral) and bioenergetic cost (Pi/PCr) suggested reduced maximal enzyme activity (Vmax) by 9-17% as a result of lack of ovarian sex hormones, but no change in Michaelis-Menten constant (Km) was found. Postexercise recovery was also significantly slower (3.27 +/- 0.54 PCr/Pi units per minute compared with 4.04 +/- 1.08 in controls). It is suggested that reduced levels of ovarian sex hormones decrease oxidative phosphorylation. Cytochrome oxidase activity was reduced in these muscles by 40%, but other mitochondrial enzyme systems may be affected as well. The possible significance of these data is the implication of a reduced capacity for menopausal women or amenorrheic female athletes to perform prolonged intensive exercise.

Wrist flexor muscles of elite rowers measured with magnetic resonance spectroscopy

We used phosphorus magnetic resonance spectroscopy to study skeletal muscle metabolism of trained vs. untrained humans. The forearms of highly trained rowers (n = 10) and age-matched controls were placed in a 1.9-T magnet so that the wrist flexor muscles were placed over a 4-cm transmitting and receiving coil. The subjects performed work by depressing a handle attached to an isokinetic ergometer. Relative concentrations of Pi, phosphocreatine (PCr), and ATP were measured along with intracellular pH. Measurements were made at rest, during steady-state and ramp exercise protocols, and during recovery from exercise. At rest the rowers had Pi/PCr that were not different from control. During steady-state exercise rowers (n = 4) had lower Pi/PCr at the same relative power levels, and the slope of the power vs. Pi-PCr curve was significantly greater than for controls. Rowers (n = 4) also had faster rates of PCr recovery after exercise than controls (T1/2 of 24 +/- 2.0 s for rowers and 47 +/- 8.4 s for controls) when power level was adjusted so that all subjects recovered from the same level of Pi/PCr. During a ramp exercise protocol, the initial slope of the power vs. Pi-PCr curve was greater in three of six rowers compared with controls and at the highest power level rowers had lower Pi/PCr values with less drop in pH. At the end of the ramp test, the same degree of muscle fatigue was associated with much lower levels of H2PO-4 (5.7 +/- 0.70 mM) in rowers compared with controls (13.0 +/- 1.8 mM).(ABSTRACT TRUNCATED AT 250 WORDS)

Relationship of muscular fatigue to pH and diprotonated Pi in humans: a 31P-NMR study

Seventeen normal subjects performed maximal wrist flexion exercise with continuous monitoring of forearm muscle pH and H2PO4-, measured with 31P nuclear magnetic resonance, and muscle fatigue, expressed as a percentage of decline in maximal developed force. Four minutes of exercise (flexion duration = 1 s) reduced maximal developed force from 100 to 74 +/- 9% and pH from 6.99 +/- 0.04 to 6.17 +/- 0.33 and increased H2PO4- to 927 +/- 401% of resting levels. In all subjects, linear relationships were noted between developed force and pH (r = 0.90 +/- 0.08) and between developed force and H2PO4- (r = -0.89 +/- 0.08). Doubling the contraction duration to 2 s produced more rapid changes in developed force, pH, and H2PO4- but no change in the relationship of force to pH and H2PO4-. Two minutes of submaximal exercise before maximal exercise significantly reduced pH and increased H2PO4-. During subsequent maximal exercise, the relationship between developed force and H2PO4- remained unchanged. In contrast, the relationship between developed force and pH was shifted leftward; muscle pH remained lower throughout maximal exercise, and developed force remained comparable to that noted during control exercise. These observations suggest that muscle fatigue during intense short-term exercise is primarily caused by an increase in intramuscular H2PO4- rather than by a decrease in intramuscular pH.

Application of 31P magnetic resonance spectroscopy to the study of athletic performance

Magnetic resonance spectroscopy is a non-invasive and repeatable method of studying muscle metabolism. Magnetic resonance spectroscopy uses specific radiofrequency pulses in a strong magnetic field to determine the relative concentrations of chemical compounds in the sample. 31P Magnetic resonance spectroscopy provides indirect measures of phosphate compounds such as adenosine triphosphate (ATP), phosphocreatine and inorganic phosphate. Muscle intracellular pH can also be determined. Exercise tests can be performed in the magnet such that the metabolic response to steady-state exercise can be measured. The ratio of inorganic phosphate to phosphocreatine reflects the relative metabolic rate of mitochondrial respiration (V) and the extrapolated maximum capacity of oxidative metabolism (Vm). Normal humans vary considerably in their metabolic response to exercise. These differences are reflected in their Vms and the degree of acidosis during exercise. Active muscles in endurance trained athletes have higher Vms and faster recovery rates than normal controls. Preliminary studies have been done to assess muscle glycolytic capacity by measuring the degree of acidosis during ischaemic exercise. Exercise-induced muscle injury can be detected as an increased inorganic phosphate to phosphocreatine ratio in resting muscle. The increase in the inorganic phosphate to phosphocreatine ratio with injury reaches a peak 1 to 2 days after the injury and lasts for up to a week. Similar increases in the inorganic phosphate to phosphocreatine ratio occur in patients with destructive neuromuscular diseases. Thus changes in the resting inorganic phosphate to phosphocreatine ratio may be used to detect the degree of muscle injury following exercise. Levels of H2PO4- in muscle are thought to be important in causing muscle fatigue during exercise. As 31P magnetic resonance spectroscopy can measure H2PO4-, magnetic resonance spectroscopy has become a useful technique in the study of the metabolic causes of muscle fatigue. It may also be possible to identify the relative populations of fast twitch and slow twitch fibres in a skeletal muscle using pH changes measured with 31P magnetic resonance spectroscopy. Magnetic resonance spectroscopy using other nuclei, such as 1H, 13C and 23Na, have the potential to provide information on other metabolic changes which occur with exercise. Magnetic resonance spectroscopy has shown promise as a technique to monitor the effects of training, including overtraining, in specific muscle groups in athletes.

Detection of muscle injury in humans with 31-P magnetic resonance spectroscopy

Strenuous exercise can result in muscle injury that may persist for 2 weeks. Our purpose was to determine if muscle injury can be detected with 31-P magnetic resonance spectroscopy. Normal subjects performed repeated lengthening contractions with either arms or legs designed to result in mild muscle injury. One hour after the arm exercise, there was a significant increase in the inorganic phosphate to phosphocreatine ratio (Pi/PCr), with the maximum increase in Pi/PCr occurring 1 day postexercise (0.12 +/- 0.01 to 0.21 +/- 0.05). Pi/PCr remained elevated for 3-10 days. Similar results were seen following the leg exercise protocol. ATP/(Pi + PCr) decreased in all the arm exercised subjects. Exercise protocols that did not contain lengthening contractions did not result in changes of Pi/PCr or ATP/(Pi + PCr). Patients with various neuromuscular diseases with evidence of muscle damage (elevated CK, muscle soreness, and histopathological findings) also showed increased Pi/PCr at rest. We conclude that elevated Pi/PCr at rest can reflect nonspecific muscle damage in normal and diseased subjects.

Exercise-induced injury to skeletal muscle

Strenuous or unaccustomed exercise can cause injury to skeletal muscle. This paper reviews our understanding of the mechanisms of exercise-induced injury. Measurements of exercise-induced injury have included muscle soreness, increased serum levels of intracellular enzymes, increased lysosomal enzyme activities, structural changes in muscle fibers, and prolonged decreases in force development that cannot be attributed to fatigue. Injury can be induced by exercise of small muscle groups, which suggests that it involves processes localized in skeletal muscles. Exercise of relatively short duration can result in injury, which indicates that long durations of exercise and associated metabolic changes are not necessary for injury to occur. Exercise that involves lengthening contractions results in greater evidence of muscle injury than exercise involving isometric or shortening contractions. Lengthening contractions are associated with higher levels of force and lower metabolic costs per muscle fiber than isometric or shortening contractions. These results suggest that changes in muscle metabolism are not responsible for exercise-induced injury to skeletal muscle. Exercise-induced injury is more likely the result of mechanical disruption of muscle fibers.

Characteristics of lengthening contractions associated with injury to skeletal muscle fibers

Lengthening (eccentric) contractions result in injury to skeletal muscle fibers. Two hypotheses were tested through lengthening contractions of an in situ muscle preparation: the extent of injury increases with increases in the duration; and the extent of injury increases with increases in the peak force. Mice were anesthetized, and distal tendons of the extensor digitorum longus muscles were attached to a servomotor. Muscles were stimulated at 150 Hz and lengthened 20% of fiber length (Lf). Lengthening contractions were performed at 0.2, 0.5, or 1.0 Lf/s with durations of 0.5-15 min. Peak force during lengthening contractions at 1.0 Lf/s was decreased by inducing fatigue with isometric contractions, stimulating at 70-100 Hz, or 3) lengthening 10% of Lf. Injury was assessed 3 days after lengthening contractions by histological appearance and maximum force (Po) development. Injury increased with duration up to 5 min. After 5 min, fatigue appeared to prevent further injury. Results for 0.2 and 0.5 Lf/s were similar to those for 1.0 Lf/s but with less injury. A high correlation was observed between histological appearance of injury and the decrease in Po. The extent of injury was related to the peak force developed during the lengthening contractions.

Injury to skeletal muscle fibers of mice following lengthening contractions

We tested the hypothesis that lengthening contractions result in greater injury to skeletal muscle fibers than isometric or shortening contractions. Mice were anesthetized with pentobarbital sodium and secured to a platform maintained at 37 degrees C. The distal tendon of the extensor digitorum longus muscle was attached to a servomotor. A protocol consisting of isometric, shortening, or lengthening contractions was performed. After the contraction protocol the distal tendon was reattached, incisions were closed, and the mice were allowed to recover. The muscles were removed after 1-30 days, and maximum isometric force (Po) was measured in vitro at 37 degrees C. Three days after isometric and shortening contractions and sham operations, histological appearance was not different from control and Po was 80% of the control value. Three days after lengthening contractions, histological sections showed that 37 +/- 4% of muscle fibers degenerated and Po was 22 +/- 3% of the control value. Muscle regeneration, first seen at 4 days, was nearly complete by 30 days, when Po was 84 +/- 3% of the control value. We conclude that, with the protocol used, lengthening, but not isometric or shortening contractions, caused significant injury to muscle fibers.

Characteristics of cat skeletal muscles grafted with intact nerves or with anastomosed nerves

Grafting of 3-g extensor digitorum longus (EDL) muscles of cats may be made with (i) severence of the nerve with spontaneous reinnervation, termed standard grafts (ii) severence of the nerve with reinnervation facilitated by anastomosis of the nerve, termed nerve-anastomosed grafts; and (iii) preservation of the nerve, termed nerve-intact grafts. In previous studies, standard grafts developed a maximum isometric tetanic tension (P0) that was 22% of the value for control EDL muscles. We hypothesized that the low values of P0 resulted from incomplete reinnervation of muscle fibers. To test this hypothesis, EDL muscles were grafted in cats with nerves intact and with nerves anastomosed. In standard grafts differences were observed in both structure and function at 120 compared with 240 days after grafting. Characteristics of the nerve-intact and nerve-anastomosed grafts did not change significantly between 120 and 240 days and the data were pooled for comparisons with control EDL muscles. Nerve-anastomosed and nerve-intact grafts developed P0 values that were 34 and 64% of the control values, respectively. Nerve-intact grafts had a mass and fiber cross-sectional area not different from control EDL muscles. Compared with control values, all grafts had fewer fibers, more connective tissue, lower absolute and normalized P0, reduced capillary density, and increased fatigability. The greater P0 of nerve-intact compared with standard and nerve-anastomosed grafts supported our hypothesis that the degree of reinnervation is a factor that limits graft development. The presence of a necrotic core and the low tension development of even the nerve-intact grafts suggested that revascularization is a significant limitation as well.

Length-tension relationship of mammalian diaphragm muscles

We tested the hypothesis that the length-tension relationship of diaphragm muscle is different from that of other skeletal muscle. Isometric contractile properties of small bundles of diaphragm muscle from rats, cats, rhesus monkeys, dogs, and pigs were measured in vitro at 37 degrees C. For diaphragm muscles from all species, twitch contraction and relaxation times and histochemical myofibrillar ATPase indicated a mixture of fast and slow muscle fiber types. By use of tetanic stimulations of 400-ms duration, isometric tension was recorded from bundles of diaphragm muscle at lengths between 50 and 130% of the muscle fiber length at which active tension was maximal (Lo). At lengths below 60-70% of Lo, tetanic tension did not reach a plateau within 400 ms. With longer stimulation durations, active tension could be recorded between 30 and 40% of Lo. The length-tension curves for bundles of fibers from the diaphragm muscle were not different among species and were consistent with results reported for intact limb muscles.

Contractile properties of bundles of fiber segments from skeletal muscles

Our purpose was to determine whether contractile properties of bundles of skeletal muscle fiber segments were significantly different from those of bundles of intact fibers. In frog muscles, the only difference between the contractile properties of fiber segments and intact fibers was a lower maximum velocity of shortening (Vo) for the fiber segments. In mammalian muscles, the contraction time (TPT), relaxation time (RT1/2), and maximum tetanus tension (Po) of bundles of fiber segments were not different from those of intact fibers, but the rate of tension development (dP/dt), twitch-to-tetanus ratio (Pt/Po) and Vo were lower. The lower dP/dt and Pt/Po resulted from increased compliance due to damaged sarcomeres near cut ends. Within 4-9 mm of a cut end, membrane potentials were less than control values, and sarcomeres lengthened during a fixed-end contraction. after the length of fiber segments was corrected for the exact portion that was not shortening, the Vo of fiber segments was not different from that of intact fibers. We conclude that valid estimates of contractile properties can be obtained from bundles of skeletal muscle fiber segments.

Contractile properties of the muscles of mastication of rhesus monkeys (Macaca mulatta) following increase in muscle length

The hypothesis was tested that increasing the resting length of the masseter and temporalis muscles by a bite-opening appliance with or without detachment and re-attachment of the masseter would not affect the contractile properties of these muscles. Appliances opened the bite of 10 adult female monkeys 20 mm. Five received the appliance alone (Group A); five received the appliance and in addition the masseter was detached and re-attached (Group ADR). Comparisons were made 48 weeks later. Small bundles of fibres were excised from the masseter and temporalis muscles of experimental animals and from 8 control animals. Isometric and isotonic contractile properties were measured in vitro and fibre classification and fibre areas were determined histochemically. No significant differences were observed within either masseter or temporalis muscles between animals in Groups A and ADR. In both groups, the bundles of fibres from the masseter had prolonged contraction and relaxation times compared to control masseter muscles but no difference was observed in the percentage of Type II fibres. As detachment and re-attachment had no significant effect on morphological or physiological characteristics, other than those due to lengthening, this procedure may be useful in decreasing the passive tension induced when orthognathic surgery increases muscle length. The significant prolongation of the contractile response of the masseter is similar to the adaptation induced by long-term stimulation at low frequency.