Christopher P. Elder

Intramuscular fat and glucose tolerance after spinal cord injury – a cross-sectional study

Study design: Survey.Objective: Determine intramuscular fat (IMF) in affected skeletal muscle after complete spinal cord injury using a novel analysis method and determine the correlation of IMF to plasma glucose or plasma insulin during an oral glucose tolerance test.Setting: General community of Athens, GA, USA.Methods: A total of 12 nonexercise-trained complete spinal cord injured (SCI) persons (10 males and two females 40±12 years old (mean±SD), range 26–71 years, and 8±5 years post SCI) and nine nonexercise-trained nondisabled (ND) controls 29±9 years old, range 23–51 years, matched for height, weight, and BMI, had T1 magnetic resonance images of their thighs taken and underwent an oral glucose tolerance test (OGTT) after giving consent.Results: Average skeletal muscle cross-sectional area (CSA) (mean±SD) was 58.6±21.6 cm2 in spinal cord subjects and 94.1±32.5 cm2 in ND subjects. Average IMF CSA was 14.5±6.0 cm2 in spinal cord subjects and 4.7±2.5 cm2 in nondisabled subjects, resulting in an almost four-fold difference in IMF percentage of 17.3±4.4% in spinal cord subjects and 4.6±2.6% in nondisabled subjects. The 60, 90 and 120 min plasma glucose or plasma insulin were higher in the SCI group. IMF (absolute and %) was related to the 90 or 120 min plasma glucose or plasma insulin (r 2=0.71–0.40).Conclusions: IMF is a good predictor of plasma glucose during an OGTT and may be a contributing factor to the onset of impaired glucose tolerance and type II diabetes, especially in SCI. In addition, reports of skeletal muscle CSA should be corrected for IMF.

Effects of Electrical Stimulation Parameters on Fatigue in Skeletal Muscle

STUDY DESIGN Experimental laboratory study. OBJECTIVES The primary purpose was to investigate the independent effects of current amplitude, pulse duration, and current frequency on muscle fatigue during neuromuscular electrical stimulation (NMES). A second purpose was to determine if the ratio of the evoked torque to the activated area could explain muscle fatigue. BACKGROUND Parameters of NMES have been shown to differently affect the evoked torque and the activated area. The efficacy of NMES is limited by the rapid onset of muscle fatigue. METHODS AND MEASURES Seven healthy participants underwent 4 NMES protocols that were randomly applied to the knee extensor muscle group. The NMES protocols were as follows: standard protocol (Std), defined as 100-Hz, 450-micros pulses and amplitude set to evoke 75% of maximal voluntary isometric torque (MVIT); short pulse duration protocol (SP), defined as 100-Hz, 150-micros pulses and amplitude set to evoke 75% of MVIT; low-frequency protocol (LF), defined as 25-Hz, 450-micros pulses and amplitude set to evoke 75% of MVIT; and low-amplitude protocol (LA), defined as 100-Hz, 450-micros pulses and amplitude set to evoke 45% of MVIT. The peak torque was measured at the start and at the end of the 4 protocols, and percent fatigue was calculated. The outcomes of the 4 NMES protocols on the initial peak torque and activated cross-sectional area were recalculated from a companion study to measure torque per active area. RESULTS Decreasing frequency from 100 to 25 Hz decreased fatigue from 76% to 39%. Decreasing the amplitude and pulse duration resulted in no change of muscle fatigue. Torque per active area accounted for 57% of the variability in percent fatigue between Std and LF protocols. CONCLUSIONS Altering the amplitude of the current and pulse duration does not appear to influence the percent fatigue in NMES. Lowering the stimulation frequency results in less fatigue, by possibly reducing the evoked torque relative to the activated muscle area.

Electrically stimulated resistance training in SCI individuals increases muscle fatigue resistance but not femoral artery size or blood flow

Study design:Longitudinal.Objectives:The purpose of this study was to evaluate the effect of lower extremity resistance training on quadriceps fatigability, femoral artery diameter, and femoral artery blood flow.Setting:Academic Institution.Methods:Five male chronic spinal cord injury (SCI) individuals (American Spinal Injury Association (ASIA): A complete; C5–T10; 36±5 years old) completed 18 weeks of home-based neuromuscular electrical stimulation (NMES) resistance training. Subjects trained the quadriceps muscle group twice a week with four sets of 10 dynamic knee extensions against resistance while in a seated position. All measurements were made before training and after 8, 12, and 18 weeks of training. Ultrasound was used to measure femoral artery diameter and blood flow. Blood flow was measured before and after 5 and 10 min of distal cuff occlusion, and during a 4-min isometric electrical stimulation fatigue protocol.Results:Training resulted in significant increases in weight lifted and muscle mass, as well as a 60% reduction in muscle fatigue (P=0.001). However, femoral arterial diameter did not increase. The range was 0.44±0.03 to 0.46±0.05 cm over the four time points (P=0.70). Resting, reactive hyperemic, and exercise blood flow did not appear to change with training.Conclusion:NMES resistance training improved muscle size and fatigue despite an absence of response in the supplying vasculature. These results suggest that the decreases in arterial caliber and blood flow seen with SCI are not tightly linked to muscle mass and fatigue resistance. In addition, muscle fatigue in SCI patients can be improved without increases in arterial diameter or blood flow capacity.Sponsorship:Grants HL65179, HD39676, and HD39676S2.

Combined diffusion and strain tensor MRI reveals a heterogeneous, planar pattern of strain development during isometric muscle contraction

The purposes of this study were to create a three-dimensional representation of strain during isometric contraction in vivo and to interpret it with respect to the muscle fiber direction. Diffusion tensor MRI was used to measure the muscle fiber direction of the tibialis anterior (TA) muscle of seven healthy volunteers. Spatial-tagging MRI was used to measure linear strains in six directions during separate 50% maximal isometric contractions of the TA. The strain tensor (E) was computed in the TAs deep and superficial compartments and compared with the respective diffusion tensors. Diagonalization of E revealed a planar strain pattern, with one nonzero negative strain (ε(N)) and one nonzero positive strain (ε(P)); both strains were larger in magnitude (P < 0.05) in the deep compartment [ε(N) = -40.4 ± 4.3%, ε(P) = 35.1 ± 3.5% (means ± SE)] than in the superficial compartment (ε(N) = -24.3 ± 3.9%, ε(P) = 6.3 ± 4.9%). The principal shortening direction deviated from the fiber direction by 24.0 ± 1.3° and 39.8 ± 6.1° in the deep and superficial compartments, respectively (P < 0.05, deep vs. superficial). The deviation of the shortening direction from the fiber direction was due primarily to the lower angle of elevation of the shortening direction over the axial plane than that of the fiber direction. It is concluded that three-dimensional analyses of strain interpreted with respect to the fiber architecture are necessary to characterize skeletal muscle contraction in vivo. The deviation of the principal shortening direction from the fiber direction may relate to intramuscle variations in fiber length and pennation angle.

Absence of a significant extravascular contribution to the skeletal muscle BOLD effect at 3 T.

Blood oxygenation level dependent (BOLD) contrast in skeletal may reflect the contributions of both intravascular and extravascular relaxation effects. The purpose of this study was to determine the significance of the extravascular BOLD effect in skeletal muscle at 3 T. In experiments, R2* was measured before and during arterial occlusion under the following conditions: ( 1 ) the leg extended and rotated (to vary the capillary orientation with respect to the amplitude of static field) and ( 2 ) with the bloods signal nulled using a multiecho vascular space occupancy experiment. In the leg rotation protocol, 3 min of arterial occlusion decreased oxyhemoglobin saturation from 67% to 45% and increased R2* from 34.2 to 36.6 sec−1, but there was no difference in the R2* response to occlusion between the extended and rotated positions. Numerical simulations of intra‐ and extravascular BOLD effects corresponding to these conditions predicted that the intravascular BOLD contribution to the R2* change was always > 50 times larger than the extravascular BOLD contribution. Blood signal nulling eliminated the change in R2* caused by arterial occlusion. These data indicate that under these experimental conditions, the contribution of the extravascular BOLD effect to skeletal muscle R2* was too small to be practically important. Magn Reson Med, 2010.

Image-based calculation of perfusion and oxyhemoglobin saturation in skeletal muscle during submaximal isometric contractions

The relative oxygen saturation of hemoglobin and the rate of perfusion are important physiological quantities, particularly in organs such as skeletal muscle, in which oxygen delivery and use are tightly coupled. The purpose of this study was to demonstrate the image‐based calculation of the relative oxygen saturation of hemoglobin and quantification of perfusion in skeletal muscle during isometric contractions. This was accomplished by establishing an empirical relationship between the rate of radiofrequency‐reversible dephasing and near‐infrared spectroscopy–observed oxyhemoglobin saturation (relative oxygen saturation of hemoglobin) under conditions of arterial occlusion and constant blood volume. A calibration curve was generated and used to calculate the relative oxygen saturation of hemoglobin from radiofrequency‐reversible dephasing changes measured during contraction. Twelve young healthy subjects underwent 300 s of arterial occlusion and performed isometric contractions of the dorsiflexors at 30% of maximal contraction for 120 s. Muscle perfusion was quantified during contraction by arterial spin labeling and measures of muscle T1. Comparisons between the relative oxygen saturation of hemoglobin values predicted from radiofrequency‐reversible dephasing and that measured by near‐infrared spectroscopy revealed no differences between methods (P = 0.760). Muscle perfusion reached a value of 34.7 mL 100 g−1 min−1 during contraction. These measurements hold future promise in measuring muscle oxygen consumption in healthy and diseased skeletal muscle. Magn Reson Med, 2010.

Oxygen cost of dynamic or isometric exercise relative to recruited muscle mass

BackgroundOxygen cost of different muscle actions may be influenced by different recruitment and rate coding strategies. The purpose of this study was to account for these strategies by comparing the oxygen cost of dynamic and isometric muscle actions relative to the muscle mass recruited via surface electrical stimulation of the knee extensors.MethodsComparisons of whole body pulmonary Δ V˙MathType@MTEF@5@5@+=feaafiart1ev1aaatCvAUfKttLearuWrP9MDH5MBPbIqV92AaeXatLxBI9gBaebbnrfifHhDYfgasaacH8akY=wiFfYdH8Gipec8Eeeu0xXdbba9frFj0=OqFfea0dXdd9vqai=hGuQ8kuc9pgc9s8qqaq=dirpe0xb9q8qiLsFr0=vr0=vr0dc8meaabaqaciaacaGaaeqabaqabeGadaaakeaacuWGwbGvgaGaaaaa@2DEA@O2 were made in seven young healthy adults (1 female) during 3 minutes of dynamic or isometric knee extensions, both induced by surface electrical stimulation. Recruited mass was quantified in T2 weighted spin echo magnetic resonance images.ResultsThe Δ V˙MathType@MTEF@5@5@+=feaafiart1ev1aaatCvAUfKttLearuWrP9MDH5MBPbIqV92AaeXatLxBI9gBaebbnrfifHhDYfgasaacH8akY=wiFfYdH8Gipec8Eeeu0xXdbba9frFj0=OqFfea0dXdd9vqai=hGuQ8kuc9pgc9s8qqaq=dirpe0xb9q8qiLsFr0=vr0=vr0dc8meaabaqaciaacaGaaeqabaqabeGadaaakeaacuWGwbGvgaGaaaaa@2DEA@O2 for dynamic muscle actions, 242 ± 128 ml • min-1 (mean ± SD) was greater (p = 0.003) than that for isometric actions, 143 ± 99 ml • min-1. Recruited muscle mass was also greater (p = 0.004) for dynamic exercise, 0.716 ± 282 versus 0.483 ± 0.139 kg. The rate of oxygen consumption per unit of recruited muscle (V˙O2RMMathType@MTEF@5@5@+=feaafiart1ev1aaatCvAUfKttLearuWrP9MDH5MBPbIqV92AaeXatLxBI9gBaebbnrfifHhDYfgasaacH8akY=wiFfYdH8Gipec8Eeeu0xXdbba9frFj0=OqFfea0dXdd9vqai=hGuQ8kuc9pgc9s8qqaq=dirpe0xb9q8qiLsFr0=vr0=vr0dc8meaabaqaciaacaGaaeqabaqabeGadaaakeaacuqGwbGvgaGaaiabb+eapnaaBaaaleaacqaIYaGmdaahaaadbeqaaiabbkfasjabb2eanbaaaSqabaaaaa@32B0@) was similar in dynamic and isometric exercise (346 ± 162 versus 307 ± 198 ml • kg-1 • min-1; p = 0.352), but the V˙O2RMMathType@MTEF@5@5@+=feaafiart1ev1aaatCvAUfKttLearuWrP9MDH5MBPbIqV92AaeXatLxBI9gBaebbnrfifHhDYfgasaacH8akY=wiFfYdH8Gipec8Eeeu0xXdbba9frFj0=OqFfea0dXdd9vqai=hGuQ8kuc9pgc9s8qqaq=dirpe0xb9q8qiLsFr0=vr0=vr0dc8meaabaqaciaacaGaaeqabaqabeGadaaakeaacuqGwbGvgaGaaiabb+eapnaaBaaaleaacqaIYaGmdaahaaadbeqaaiabbkfasjabb2eanbaaaSqabaaaaa@32B0@ calculated relative to initial knee extensor torque was significantly greater during dynamic exercise 5.1 ± 1.5 versus 3.6 ± 1.6 ml • kg-1 • Nm-1 • min-1 (p = 0.019).ConclusionThese results are consistent with the view that oxygen cost of dynamic and isometric actions is determined by different circumstances of mechanical interaction between actin and myosin in the sarcomere, and that muscle recruitment has only a minor role.

A method for detecting the temporal sequence of muscle activation during cycling using MRI

Surface electromyography (EMG) can assess muscle recruitment patterns during cycling, but has limited applicability to studies of deep muscle recruitment and electrically stimulated contractions. We determined whether muscle recruitment timing could be inferred from MRI-measured transverse relaxation time constant (T(2)) changes and a cycle ergometer modified to vary power as a function of pedal angle. Six subjects performed 6 min of single-leg cycling under two conditions (E0°-230° and E90°-230°), which increased the power from 0°-230° and 90-230° of the pedal cycle, respectively. The difference condition produced a virtual power output from 0-180° (V0°-180°). Recruitment was assessed by integrating EMG over the pedal cycle (IEMG) and as the (post-pre) exercise T(2) change (ΔT(2)). For E0°-230°, the mean IEMG for vastus medialis and lateralis (VM/VL; 49.3 ± 3.9 mV·s; mean ± SE) was greater (P < 0.05) than that for E90°-230° (17.9 ± 1.9 mV·s); the corresponding ΔT(2) values were 8.7 ± 1.0 and 1.4 ± 0.5 ms (P < 0.05). For E0°-230° and E90°-230°, the IEMG values for biceps femoris/long head (BF(L)) were 37.7 ± 5.4 and 27.1 ± 5.6 mV·s (P > 0.05); the corresponding ΔT(2) values were 0.9 ± 0.9 and 1.5 ± 0.9 ms (P > 0.05). MRI data indicated activation of the semitendinosus and BF/short head for E0°-230° and E90°-230°. For V0°-180°, ΔT(2) was 7.2 ± 0.9 ms for VM/VL and -0.6 ± 0.6 ms for BF(L); IEMG was 31.5 ± 3.7 mV·s for VM/VL and 10.6 ± 7.0 mV·s for BF(L). MRI and EMG data indicate VM/VL activity from 0 to 180° and selected hamstring activity from 90 to 230°. Combining ΔT(2) measurements with variable loading allows the spatial and temporal patterns of recruitment during cycling to be inferred from MRI data.

Comparison of muscle BOLD responses to arterial occlusion at 3 and 7 Tesla

The purpose of this study was to determine the feasibility of muscle BOLD (mBOLD) imaging at 7 Tesla (T) by comparing the changes in R2* of muscle at 3 and 7T in response to a brief period of tourniquet‐induced ischemia.

Matching of postcontraction perfusion to oxygen consumption across submaximal contraction intensities in exercising humans

Studying the magnitude and kinetics of blood flow, oxygen extraction, and oxygen consumption at exercise onset and during the recovery from exercise can lead to insights into both the normal control of metabolism and blood flow and the disturbances to these processes in metabolic and cardiovascular diseases. The purpose of this study was to examine the on- and off-kinetics for oxygen delivery, extraction, and consumption as functions of submaximal contraction intensity. Eight healthy subjects performed four 1-min isometric dorsiflexion contractions, with two at 20% MVC and two at 40% MVC. During one contraction at each intensity, relative perfusion changes were measured by using arterial spin labeling, and the deoxyhemoglobin percentage (%HHb) was estimated using the spin- and gradient-echo sequence and a previously published empirical calibration. For the whole group, the mean perfusion did not increase during contraction. The %HHb increased from ∼28 to 38% during contractions of each intensity, with kinetics well described by an exponential function and mean response times (MRTs) of 22.7 and 21.6 s for 20 and 40% MVC, respectively. Following contraction, perfusion increased ∼2.5-fold. The %HHb, oxygen consumption, and perfusion returned to precontraction levels with MRTs of 27.5, 46.4, and 50.0 s, respectively (20% MVC), and 29.2, 75.3, and 86.0 s, respectively (40% MVC). These data demonstrate in human subjects the varied recovery rates of perfusion and oxygen consumption, along with the similar rates of %HHb recovery, across these exercise intensities.