MRI measures of perfusion-related changes in human skeletal muscle during progressive contractions

Exercise pressor responses are exaggerated relative to force production during, but not following, thirty-minutes of rhythmic handgrip exercise

PURPOSE

This study tested the hypothesis that blood pressure responses would increase relative to force production in response to prolonged bouts of muscular work.

METHODS

Fifteen individuals performed two minutes of static handgrip (SHG; 35% MVC), followed by three minutes of post-exercise-cuff-occlusion (PECO), before and after thirty minutes of rest (control), or rhythmic handgrip exercise (RHG) of the contralateral and ipsilateral forearms. Beat-by-beat recordings of mean arterial pressure (MAP), heart rate (HR), and handgrip force (kg) were averaged across one-minute periods at baseline, and minutes 5, 10, 15, 20, 25, and 30 of RHG. MAP was also normalized to handgrip force, providing a relative measure of exercise pressor responses (mmHg/kg). Hemodynamic responses to SHG and PECO were also compared before and after contralateral RHG, ipsilateral RHG, and control, respectively. Similar to the RHG trial, areas under the curve were calculated for MAP (blood pressure index; BPI) and normalized to the time tension index (BPInorm).

RESULTS

HR and MAP significantly increased during RHG (15.3 ± 1.4% and 20.4 ± 3.2%, respectively, both p < 0.01), while force output decreased by up to 36.6 ± 8.0% (p < 0.01). This resulted in a 51.6 ± 9.4% increase in BPInorm during 30 min of RHG (p < 0.01). In contrast, blood pressure responses to SHG and PECO were unchanged following RHG (all p ≥ 0.07), and only the mean HR (4.2 ± 1.5%, p = 0.01) and ΔHR (67.2 ± 18.1%, p < 0.01) response to SHG were exaggerated following ipsilateral RHG.

CONCLUSIONS

The magnitude of exercise pressor responses relative to force production progressively increases during, but not following, prolonged bouts of muscular work.

Measurements of in vivo skeletal muscle oxidative capacity are lower following sustained isometric compared with dynamic contractions

Human skeletal muscle oxidative capacity can be quantified non-invasively using 31-phosphorus magnetic resonance spectroscopy (31P-MRS) to measure the rate constant of phosphocreatine (PCr) recovery (kPCr) following contractions. In the quadricep muscles, several studies have quantified kPCr following 24-30 s of sustained maximal voluntary isometric contraction (MVIC). This approach has the advantage of simplicity but is potentially problematic because sustained MVICs inhibit perfusion, which may limit muscle oxygen availability or increase the intracellular metabolic perturbation, and thus affect kPCr. Alternatively, dynamic contractions allow reperfusion between contractions, which may avoid limitations in oxygen delivery. To determine whether dynamic contraction protocols elicit greater kPCr than sustained MVIC protocols, we used a cross-sectional design to compare quadriceps kPCr in 22 young and 11 older healthy adults following 24 s of maximal voluntary: (1) sustained MVIC and (2) dynamic (MVDC; 120°·s-1, 1 every 2 s) contractions. Muscle kPCr was ∼20% lower following the MVIC protocol compared with the MVDC protocol (p ≤ 0.001), though this was less evident in older adults (p = 0.073). Changes in skeletal muscle pH (p ≤ 0.001) and PME accumulation (p ≤ 0.001) were greater following the sustained MVIC protocol, and pH (p ≤ 0.001) and PME (p ≤ 0.001) recovery were slower. These results demonstrate that (i) a brief, sustained MVIC yields a lower value for skeletal muscle oxidative capacity than an MVDC protocol of similar duration and (ii) this difference may not be consistent across populations (e.g., young vs. old). Thus, the potential effect of contraction protocol on comparisons of kPCr in different study groups requires careful consideration in the future.

Blood Pressure Responses to Postexercise Circulatory Occlusion Are Attenuated After Exercise-Induced Muscle Weakness

PURPOSE

Exercise blood pressure (BP) responses are thought to be determined by relative exercise intensity (percent maximal voluntary contraction (MVC) strength). However, cross-sectional studies report that during a static contraction, higher absolute force is associated with greater BP responses to relative intensity exercise and subsequent muscle metaboreflex activation with postexercise circulatory occlusion (PECO). We hypothesized that a bout of unaccustomed eccentric exercise would reduce knee extensor MVC and subsequently attenuate BP responses to PECO.

METHODS

Continuous BP, heart rate, muscle oxygenation, and knee extensor electromyography were recorded in 21 young healthy individuals (female, n = 10) during 2 min of 20% MVC static knee extension exercise and 2 min of PECO, performed before and 24 h after 300 maximal knee extensor eccentric contractions to cause exercise-induced muscle weakness. As a control, 14 participants repeated the eccentric exercise 4 wks later to test whether BP responses were altered when exercise-induced muscle weakness was attenuated via the protective effects of the repeated bout effect.

RESULTS

Eccentric exercise reduced MVC in all participants (144 ± 43 vs 110 ± 34 N·m, P < 0.0001). BP responses to matched relative intensity static exercise (lower absolute force) were unchanged after eccentric exercise ( P > 0.99) but were attenuated during PECO (systolic BP: 18 ± 10 vs 12 ± 9 mm Hg, P = 0.02). Exercise-induced muscle weakness modulated the deoxygenated hemoglobin response to static exercise (64% ± 22% vs 46% ± 22%, P = 0.04). When repeated after 4 wks, exercise-induced weakness after eccentric exercise was attenuated (-21.6% ± 14.3% vs -9.3 ± 9.7, P = 0.0002) and BP responses to PECO were not different from control values (all, P > 0.96).

CONCLUSIONS

BP responses to muscle metaboreflex activation, but not exercise, are attenuated by exercise-induced muscle weakness, indicating a contribution of absolute exercise intensity on muscle metaboreflex activation.

Eccentric exercise-induced muscle weakness abolishes sex differences in fatigability during sustained submaximal isometric contractions

BACKGROUND

Females are typically less fatigable than males during sustained isometric contractions at lower isometric contraction intensities. This sex difference in fatigability becomes more variable during higher intensity isometric and dynamic contractions. While less fatiguing than isometric or concentric contractions, eccentric contractions induce greater and longer lasting impairments in force production. However, it is not clear how muscle weakness influences fatigability in males and females during sustained isometric contractions.

METHODS

We investigated the effects of eccentric exercise-induced muscle weakness on time to task failure (TTF) during a sustained submaximal isometric contraction in young (18-30 years) healthy males (n = 9) and females (n = 10). Participants performed a sustained isometric contraction of the dorsiflexors at 35° plantar flexion by matching a 30% maximal voluntary contraction (MVC) torque target until task failure (i.e., falling below 5% of their target torque for ≥2 s). The same sustained isometric contraction was repeated 30 min after 150 maximal eccentric contractions. Agonist and antagonist activation were assessed using surface electromyography over the tibialis anterior and soleus muscles, respectively.

RESULTS

Males were ∼41% stronger than females. Following eccentric exercise both males and females experienced an ∼20% decline in maximal voluntary contraction torque. TTF was ∼34% longer in females than males prior to eccentric exercise-induced muscle weakness. However, following eccentric exercise-induced muscle weakness, this sex-related difference was abolished, with both groups having an ∼45% shorter TTF. Notably, there was ∼100% greater antagonist activation in the female group during the sustained isometric contraction following exercise-induced weakness as compared to the males.

CONCLUSION

This increase in antagonist activation disadvantaged females by decreasing their TTF, resulting in a blunting of their typical fatigability advantage over males.

Post-fatigue ability to activate muscle is compromised across a wide range of torques during acute hypoxic exposure

The purpose of this study was to assess how severe acute hypoxia alters the neural mechanisms of muscle activation across a wide range of torque output in a fatigued muscle. Torque and electromyography responses to transcranial and motor nerve stimulation were collected from 10 participants (27 years ± 5 years, 1 female) following repeated performance of a sustained maximal voluntary contraction that reduced torque to 60% of the pre‐fatigue peak torque. Contractions were performed after 2 h of hypoxic exposure and during a sham intervention. For hypoxia, peripheral blood oxygen saturation was titrated to 80% over a 15‐min period and remained at 80% for 2 h. Maximal voluntary torque, electromyography root mean square, voluntary activation and corticospinal excitability (motor evoked potential area) and inhibition (silent period duration) were then assessed at 100%, 90%, 80%, 70%, 50% and 25% of the target force corresponding to the fatigued maximal voluntary contraction. No hypoxia‐related effects were identified for voluntary activation elicited during motor nerve stimulation. However, during measurements elicited at the level of the motor cortex, voluntary activation was reduced at each torque output considered (P = .002, η_p² = .829). Hypoxia did not impact the correlative linear relationship between cortical voluntary activation and contraction intensity or the correlative curvilinear relationship between motor nerve voluntary activation and contraction intensity. No other hypoxia‐related effects were identified for other neuromuscular variables. Acute severe hypoxia significantly impairs the ability of the motor cortex to voluntarily activate fatigued muscle across a wide range of torque output.

Evaluation of Local Skeletal Muscle Blood Flow in Manipulative Therapy by Diffuse Correlation Spectroscopy

Manipulative therapy (MT) is applied to motor organs through a therapist’s hands. Although MT has been utilized in various medical treatments based on its potential role for increasing the blood flow to the local muscle, a quantitative validation of local muscle blood flow in MT remains challenging due to the lack of appropriate bedside evaluation techniques. Therefore, we investigated changes in the local blood flow to the muscle undergoing MT by employing diffuse correlation spectroscopy, a portable and emerging optical measurement technology that non-invasively measures blood flow in deep tissues. This study investigated the changes in blood flow, heart rate, blood pressure, and autonomic nervous activity in the trapezius muscle through MT application in 30 volunteers without neck and shoulder injury. Five minutes of MT significantly increased the median local blood flow relative to that of the pre-MT period (p &lt; 0.05). The post-MT local blood flow increase was significantly higher in the MT condition than in the control condition, where participants remained still without receiving MT for the same time (p &lt; 0.05). However, MT did not affect the heart rate, blood pressure, or cardiac autonomic nervous activity. The post-MT increase in muscle blood flow was significantly higher in the participants with muscle stiffness in the neck and shoulder regions than in those without (p &lt; 0.05). These results suggest that MT could increase the local blood flow to the target skeletal muscle, with minimal effects on systemic circulatory function.

Blood Pressure Responses to Static and Dynamic Knee Extensor Exercise between Sexes: Role of Absolute Contraction Intensity

PURPOSE

Males have larger blood pressure (BP) responses to relative-intensity static handgrip exercise compared with females. Controlling for absolute load (maximal voluntary contraction (MVC)) abolishes these differences. Whether similar observations exist during large muscle mass exercise or dynamic contractions, and the mechanisms involved, remains unknown.

METHODS

BP, heart rate, muscle oxygenation (near-infrared spectroscopy), and rectus femoris EMG were recorded in 28 males and 17 females during 10% and 30% MVC static (120 s) and isokinetic dynamic (180 s; 1:2 work-to-rest ratio; angular velocity, 60°·s-1) knee extensor exercise. Static and dynamic exercises were completed on separate visits, in a randomized order. Sex differences were examined with and without statistical adjustment of MVC (ANCOVA).

RESULTS

Males had larger systolic BP responses (interaction, P < 0.0001) and muscle deoxygenation (interaction, P < 0.01) than did females during 10% static exercise, with no difference in EMG (interaction, P = 0.67). Peak systolic BP was correlated with MVC (r = 0.55, P = 0. 0001), and adjustment for MVC abolished sex differences in systolic BP (interaction, P = 0.3). BP, heart rate, muscle oxygenation/deoxygenation, and EMG responses were similar between sexes during 30% static exercise (interaction; all, P > 0.2), including following adjustment for MVC (all, P > 0.1). Males had larger systolic BP responses during dynamic exercise at 10% and 30% (interaction; both, P = 0.01), which were abolished after adjustment for MVC (interaction; both, P > 0.08). Systolic BP responses were correlated with absolute MVC and stroke volume responses during 10% (r = 0.31, P = 0.04; r = 0.61, P < 0.0001, respectively) and 30% (r = 0.48, P = 0.001; r = 0.59, P < 0.0001, respectively).

CONCLUSIONS

Absolute contraction intensity can influence systolic BP responses to 10% but not 30% MVC static, as well as 10% and 30% MVC dynamic knee extensor exercise, and should be considered in cross-sectional comparisons of exercise BP.

Intravoxel Incoherent Motion Magnetic Resonance Imaging in Skeletal Muscle: Review and Future Directions

Throughout the body, muscle structure and function can be interrogated using a variety of noninvasive magnetic resonance imaging (MRI) methods. Recently, intravoxel incoherent motion (IVIM) MRI has gained momentum as a method to evaluate components of blood flow and tissue diffusion simultaneously. Much of the prior research has focused on highly vascularized organs, including the brain, kidney, and liver. Unique aspects of skeletal muscle, including the relatively low perfusion at rest and large dynamic range of perfusion between resting and maximal hyperemic states, may influence the acquisition, postprocessing, and interpretation of IVIM data. Here, we introduce several of those unique features of skeletal muscle; review existing studies of IVIM in skeletal muscle at rest, in response to exercise, and in disease states; and consider possible confounds that should be addressed for muscle-specific evaluations. Most studies used segmented nonlinear least squares fitting with a b-value threshold of 200 sec/mm² to obtain IVIM parameters of perfusion fraction (f), pseudo-diffusion coefficient (D*), and diffusion coefficient (D). In healthy individuals, across all muscles, the average ± standard deviation of D was 1.46 ± 0.30 × 10^-3  mm² /sec, D* was 29.7 ± 38.1 × 10^-3  mm² /sec, and f was 11.1 ± 6.7%. Comparisons of reported IVIM parameters in muscles of the back, thigh, and leg of healthy individuals showed no significant difference between anatomic locations. Throughout the body, exercise elicited a positive change of all IVIM parameters. Future directions including advanced postprocessing models and potential sequence modifications are discussed. LEVEL OF EVIDENCE: 2 TECHNICAL EFFICACY: Stage 2.

Sex Differences in Muscle Metaboreflex Activation following Static Handgrip Exercise

PURPOSE

Larger blood pressure (BP) responses to relative-intensity static exercise in males vs. females is thought to involve altered muscle metaboreflex activation, but whether this is due to an intrinsic sex difference in metabolite production or to differences in muscle strength and absolute load is unknown.

METHODS

Continuous BP and heart rate were recorded in 200 healthy young males and females (females: n = 109) during 2 minutes of static handgrip exercise at 30% of maximal voluntary contraction (MVC), followed by 2 minutes of post-exercise circulatory occlusion (PECO). Muscle sympathetic nerve activity (MSNA) was recorded in a subset of participants (n = 39; female n = 21), permitting calculation of signal-averaged resting sympathetic transduction (MSNA-diastolic BP). Sex differences were examined with and without statistical adjustment for MVC. Multivariate regression analyses were performed to identify predictors of BP responses.

RESULTS

Males had larger systolic BP responses (interactions, P < 0.0001) to static handgrip exercise (24 ± 10 vs. 17 ± 9 mmHg [mean ± SD], P < 0.0001) and PECO (20 ± 11 vs. 16 ± 9 mmHg, P < 0.0001). Adjustment for MVC abolished these sex differences in BP (interactions, P > 0.7). In the subset with MSNA, neither burst frequency or incidence responses to static handgrip exercise or PECO differed between males and females (interactions, P > 0.2). Resting sympathetic transduction was also similar (P = 0.8). Multivariate analysis showed that MVC, the change in MSNA, and sympathetic transduction were predictors of the systolic BP response to static handgrip but only MVC was associated with responses during PECO.

CONCLUSION

Sex differences in absolute contraction load contribute to differences in BP responses during muscle metaboreflex isolation using PECO. These data do not support an intrinsic effect of sex as being responsible for exercise BP differences between males and females.

Microvascular blood flow changes of the abductor pollicis brevis muscle during sustained static exercise

A practical assessment of the general health and microvascular function of the palm muscle, abductor pollicis brevis (APB), is important for the diagnosis of different conditions. In this study, we have developed a protocol and a probe to study microvascular blood flow using near-infrared diffuse correlation spectroscopy (DCS) in APB during and after thumb abduction at 55% of maximum voluntary contraction (MVC). Near-infrared time resolved spectroscopy (TRS) was also used to characterize the baseline optical and hemodynamic properties. Thirteen (n=13) subjects were enrolled and subdivided in low MVC (N=6, MVC<2.3 kg) and high MVC (N=7, MVC≥2.3 kg) groups. After ruling out significant changes in the systemic physiology that influence the muscle hemodynamics, we have observed that the high MVC group showed a 56% and 36% decrease in the blood flow during exercise, with respect to baseline, in the long and short source-detector (SD) separations (p=0.031 for both). No statistical differences were shown for the low MVC group (p=1 for short and p=0.15 for long SD). These results suggest that the mechanical occlusion, due to increased intramuscular pressure, exceeded the vasodilation elicited by the higher metabolic demand. Also, blood flow changes during thumb contraction negatively correlated (R=-0.7, p<0.01) with the absolute force applied by each subject. Furthermore, after the exercise, muscular blood flow increased significantly immediately after thumb contractions in both high and low MVC groups, with respect to the recorded values during the exercise (p=0.031). An increase of 251% (200%) was found for the long (short) SD in the low MVC group. The high MVC groups showed a significant 90% increase in blood flow only after 80 s from the start of the protocol. For both low and high MVC groups, blood flow recovered to baseline values within 160 s from starting the exercise. In conclusion, DCS allows the study of the response of a small muscle to static exercise and can be potentially used in multiple clinical conditions scenarios for assessing microvascular health.

Impact of a submaximal mono-articular exercise on the skeletal muscle function of patients with sickle cell disease

PURPOSE

Sickle cell disease (SCD) patients exhibit a limited exercise tolerance commonly attributed to anaemia, as well as hemorheological and cardio-respiratory abnormalities, but the functional status of skeletal muscle at exercise is unknown. Moreover, the effect of SCD genotype on exercise tolerance and skeletal muscle function has been poorly investigated. The aim of this study was to investigate skeletal muscle function and fatigue during a submaximal exercise in SCD patients.

METHODS

Nineteen healthy individuals (AA), 28 patients with sickle cell anaemia (SS) and 18 with sickle cell-haemoglobin C disease (SC) performed repeated knee extensions exercise (FAT). Maximal isometric torque (Tmax) was measured before and after the FAT to quantify muscle fatigability. Electromyographic activity and oxygenation by near-infrared spectroscopy of the Vastus Lateralis were recorded.

RESULTS

FAT caused a reduction in Tmax in SS (- 17.0 ± 12.1%, p < 0.001) and SC (- 21.5 ± 14.5%, p < 0.05) but not in AA (+ 0.58 ± 29.9%). Root-mean-squared value of EMG signal (RMS) decreased only in SS after FAT, while the median power frequency (MPF) was unchanged in all groups. Oxygenation kinetics were determined in SS and AA and were not different.

CONCLUSION

These results show skeletal muscle dysfunction during exercise in SCD patients, and suggest different fatigue aetiology between SS and SC. The changes in EMG signal and oxygenation kinetics during exercise suggest that the greater skeletal muscle fatigue occurring in SCD patients would be rather due to intramuscular alterations modifications than decreased tissue oxygenation. Moreover, SS patients exhibit greater muscle fatigability than SC.

Postcontractile blood oxygenation level-dependent (BOLD) response in Duchenne muscular dystrophy

Duchenne muscular dystrophy (DMD) is characterized by a progressive replacement of muscle by fat and fibrous tissue, muscle weakness, and loss of functional abilities. Impaired vasodilatory and blood flow responses to muscle activation have also been observed in DMD and associated with mislocalization of neuronal nitric oxide synthase mu (nNOSμ) from the sarcolemma. The objective of this study was to determine whether the postcontractile blood oxygen level-dependent (BOLD) MRI response is impaired in DMD and correlated with established markers of disease severity in DMD, including MRI muscle fat fraction (FF) and clinical functional measures. Young boys with DMD (n = 16, 5-14 yr) and unaffected controls (n = 16, 5-14 yr) were evaluated using postcontractile BOLD, FF, and functional assessments. The BOLD response was measured following five brief (2 s) maximal voluntary dorsiflexion contractions, each separated by 1 min of rest. FFs from the anterior compartment lower leg muscles were quantified via chemical shift-encoded imaging. Functional abilities were assessed using the 10 m walk/run and the 6-min walk distance (6MWD). The peak BOLD responses in the tibialis anterior and extensor digitorum longus were reduced (P < 0.001) in DMD compared with controls. Furthermore, the anterior compartment peak BOLD response correlated with function (6MWD ρ = 0.87, P < 0.0001; 10 m walk/run time ρ = -0.78, P < 0.001) and FF (ρ = -0.52, P = 0.05). The reduced postcontractile BOLD response in DMD may reflect impaired microvascular function. The relationship observed between the postcontractile peak BOLD response and functional measures and FF suggests that the BOLD response is altered with disease severity in DMD.NEW & NOTEWORTHY This study examined the postcontractile blood oxygen level-dependent (BOLD) response in boys with Duchenne muscular dystrophy (DMD) and unaffected controls, and correlated this measure to markers of disease severity. Our findings indicate that the postcontractile BOLD response is impaired in DMD after brief muscle contractions, is correlated to disease severity, and may be valuable to implement in future studies to evaluate treatments targeting microvascular function in DMD.

The finger flexors occlusion threshold in sport-climbers: an exploratory study on its indirect approximation

Blood flow partially determines specific climbing endurance (SCE) as it mediates oxygen bio-availability in the finger flexors. Blood flow is related to occlusion threshold (OT), which is defined as the contraction intensity at which intramuscular pressure exceeds perfusion blood pressure resulting in the cessation of local blood flow. The OT is represented as an inflection point on a force-time graph when isometric force is registered and applied through maximal and continuous tests. Endurance time (ET) to exhaustion is influenced by the relative isometric applied force and is different for each climber. The aim of this study was to explore whether an approximation of the finger flexoŕs OT in sport climbers through records of ET to exhaustion at different isometric relative intensities was possible. We measured maximum finger hang ETs at 6 intensities ranging from 85% to 35% maximal force in 34 sport climbers of advanced and elite level. The values obtained were analysed by two different methods in an attempt to determine a change in the shape of the curve in the intensity-ET relationship graphs that approximated the OT for each climber. The results suggest that the finger flexoŕs OT could be different among climbers, regardless of their strength and ability level. The presented methods do not accurately reflect the OT, but could indicate the intensity at which blood flow is restored in the active muscles. This is the first study to indirectly approximate the finger flexors OT in sport-climbers, a parameter that could be essential to assess SCE.

Adapting a fatigue model for shoulder flexion fatigue: Enhancing recovery rate during intermittent rest intervals

Although the rotator cuff muscles are susceptible to fatigue, shoulder fatigue studies reporting torque decline during intermittent tasks are relatively uncommon in the literature. A previous modification to the three-compartment controller (3CC) fatigue model incorporated a rest recovery multiplier (3CC-r model) to represent augmented blood flow to muscle during rest intervals (Looft et al., 2018). A rest recovery value of r = 15 was optimal for ankle, knee, and elbow joint regions, whereas r = 30 was better for hand/grip muscles. However, shoulder torque decline data was unavailable in the literature for comparison. Thus, the purpose of this study was to collect fatigue data for two different intermittent, isometric shoulder flexion fatiguing tasks and assess the 3CC-r model with r = 15 or 30 compared to the original 3CC model. Twenty healthy participants (9 M) completed two fatigue tasks: 50% maximum voluntary contraction (MVC) with 50% duty cycle (DC) and 70% MVC with 70% DC. MVCs were assessed at discrete time points (1, 3, 5, 10, and 15 min) until endurance time (MET). Mean observed percent torque decline (%TD) for the two tasks were compared to three model estimates: 3CC-r (using r = 15 and r = 30) and 3CC. Using these data, we confirmed that the addition of a rest multiplier (r = 15 somewhat better than r = 30) substantially improved predictions of shoulder fatigue using a previously validated analytical fatigue model (3CC). The relatively large reduction in model errors over the original model suggests the importance of representing augmented recovery during rest periods.

Behavior of oxygen saturation and blood filling in the venous capillary system of the biceps brachii muscle during a fatiguing isometric action

The objective of the study was to develop a better understanding of the capillary circulation in contracting muscles. Ten subjects were measured during a submaximal fatiguing isometric muscle action by use of the O2C spectrophotometer. In all measurements the capillary-venous oxygen saturation of hemoglobin (SvO2) decreased immediately after the start of loading and leveled off into a steady state. However, two different patterns (type I and type II) emerged. They differed in the extent of deoxygenation (-10.37 ±2.59 percent points (pp) vs. -33.86 ±17.35 pp, p = .008) and the behavior of the relative hemoglobin amount (rHb). Type I revealed a positive rank correlation of SvO2 and rHb (ρ = 0.735, p <.001), whereas a negative rank correlation (ρ = -0.522, p <.001) occurred in type II, since rHb decreased until a reversal point, then increased averagely 13% above the baseline value and leveled off into a steady state. The results reveal that a homeostasis of oxygen delivery and consumption during isometric muscle actions is possible. A rough distinction in two types of regulation is suggested.

The Impact of Cycling Cadence on Respiratory and Hemodynamic Responses to Exercise

PURPOSE

The physiological consequences of freely chosen cadence during cycling remains poorly understood. We sought to determine the effect of cadence on the respiratory and hemodynamic response to cycling exercise.

METHODS

Eleven cyclists (10 males, 1 female; age, 27 ± 6 yr; V˙O2max = 60.8 ± 3.7 mL·kg·min) completed four, 6-min constant-load cycling trials at 10% below their previously determined gas exchange threshold (i.e., 63% ± 5% peak power) while pedaling at 60, 90, and 120 rpm, and a freely chosen cadence (94.3 ± 6.9 rpm) in randomized order. Standard cardiorespiratory parameters were measured and an esophageal electrode balloon catheter was used to assess electromyography of the diaphragm (EMGdi) and the work of breathing (Wb). Leg blood flow index (BFI) was determined on four muscles using near-infrared spectroscopy with indocyanine green dye injections.

RESULTS

Oxygen uptake (V˙O2) increased as a function of increasing cadence (all pairwise comparisons, P < 0.05). The EMGdi and Wb were significantly greater at 120 rpm compared with all other conditions (all P < 0.01). Vastus medialis and semitendinosus BFI were significantly greater at 120 rpm compared with 60 and 90 rpm (all P < 0.05). Gastrocnemius BFI was higher at 120 rpm compared with all other cadences (all P < 0.01). No difference in BFI was found in the vastus lateralis (P = 0.06). Blood flow index was significantly correlated with the increase in V˙O2 with increasing cadence in the medial gastrocnemius (P < 0.001) and approached significance in the vastus lateralis (P = 0.09), vastus medialis (P = 0.06), and semitendinosus (P = 0.09). There was no effect of cadence on Borg 0-10 breathing or leg discomfort ratings (P > 0.05).

CONCLUSIONS

High cadence cycling at submaximal exercise intensities is metabolically inefficient and increases EMGdi, Wb, and leg muscle blood flow relative to slower cadences.

Aerobic Exercise Improves Microvascular Function in Older Adults

Microvascular function is reduced with age, disease, and inactivity. Exercise is well known to improve vascular health and has the potential to improve microvascular function in aging and disease.

PURPOSE

The study aimed to assess changes in peripheral microvascular function in sedentary older adults after aerobic exercise training.

METHODS

Twenty-three sedentary older adults (67 ± 5 yr, body mass index = 29 ± 5, mean ± SD) successfully completed a randomized 12-wk graded treadmill walking intervention. The exercise group (EX) performed 40 min of uphill walking 4 d·wk at 70% heart rate reserve. The control group (CON) maintained a sedentary lifestyle for 12 wk. Blood oxygen level-dependent (BOLD) responses of the soleus measured by magnetic resonance imaging were used to evaluate microvascular function; brief (1 s) maximal plantarflexion contractions were performed. Separately, blood flow in the popliteal artery was measured by ultrasound after brief contraction. Phosphorus magnetic resonance spectroscopy of the calf was used to examine muscle oxidative capacity, and whole-body peak oxygen consumption (V˙O2peak) was used to confirm training-induced cardiorespiratory adaptations.

RESULTS

Peak postcontraction BOLD response increased by 33% in EX (PRE, 3.3% ± 1.0%; POST, 4.4% ± 1.4%) compared with CON (PRE, 3.0% ± 1.3%; POST, 3.2% ± 1.5%), P < 0.05. EX with hypertension tended to show a blunted peak BOLD increase (n = 6, 15%) compared with EX normotensive (n = 7, 50%), P = 0.056. Peak postcontraction blood flow increased by 39% in EX (PRE, 217 ± 88 mL·min; POST, 302 ± 167 mL·min) compared with CON (PRE, 188 ± 54 mL·min; POST, 184 ± 44 mL·min), P < 0.05. EX muscle oxidative capacity (kPCr) improved by 40% (PRE, 1.60 ± 0.57 min; POST, 2.25 ± 0.80 min) compared with CON (PRE, 1.69 ± 0.28 min; POST, 1.76 ± 0.52 min), P < 0.05. V˙O2peak increased by 9% for EX (PRE, 19.0 ± 3.1 mL·kg·min; POST, 20.8 ± 2.9 mL·kg·min) compared with a 7% loss in CON (PRE, 21.9 ± 3.6 mL·kg·min; POST, 20.4 ± 3.5 mL·kg·min), P < 0.05.

CONCLUSION

Moderate aerobic exercise significantly improved microvascular function of the leg in older adults.

Effects of PDE5 inhibition on dystrophic muscle following an acute bout of downhill running and endurance training

Lack of sarcolemma-localized neuronal nitric oxide synthase mu (nNOSμ) contributes to muscle damage and fatigue in dystrophic muscle. In this study, we examined the effects of compensating for lack of nNOSμ with a phosphodiesterase type 5 (PDE5) inhibitor in mdx mice following downhill running and endurance training. Dystrophic mice (mdx) were treated with sildenafil citrate and compared with untreated mdx and wild-type mice after an acute bout of downhill running and during a progressive low-intensity treadmill running program (5 days/wk, 4 wk). Magnetic resonance imaging (MRI) and spectroscopy (MRS) transverse relaxation time constant (T₂) of hindlimb and forelimb muscles were measured as a marker of muscle damage after downhill running and throughout training. The MRI blood oxygenation level dependence (BOLD) response and ³¹phosphorus MRS (³¹P-MRS) data were acquired after stimulated muscle contractions. After downhill running, the increase in T₂ was attenuated (P < 0.05) in treated mdx and wild-type mice compared with untreated mdx. During training, resting T₂ values did not change in wild-type and mdx mice from baseline values; however, the running distance completed during training was greater (P < 0.05) in treated mdx (>90% of target distance) and wild-type (100%) than untreated mdx (60%). The post-contractile BOLD response was greater (P < 0.05) in treated mdx that trained than untreated mdx, with no differences in muscle oxidative capacity, as measured by ³¹P-MRS. Our findings indicate that PDE5 inhibition reduces muscle damage after a single bout of downhill running and improves performance during endurance training in dystrophic mice, possibly because of enhanced microvascular function. NEW & NOTEWORTHY This study examined the combined effects of PDE5 inhibition and exercise in dystrophic muscle using high-resolution magnetic resonance imaging and spectroscopy. Our findings demonstrated that sildenafil citrate reduces muscle damage after a single bout of downhill running, improves endurance-training performance, and enhances microvascular function in dystrophic muscle. Collectively, the results support the combination of exercise and PDE5 inhibition as a therapeutic approach in muscular dystrophies lacking nNOSμ.

Task and sex differences in muscle oxygenation during handgrip fatigue development

The purpose of this study was to examine task and sex differences in forearm muscle oxygenation, measured using near infrared spectroscopy, during sustained submaximal handgrip exercises. Forty-eight adults (50% males) performed fatiguing handgrip exercises at 20, 40, 60 and 80% of their maximum handgrip strength. While males and females exhibited similar levels of relative fatigability, forearm oxygenation was found to be task (i.e. contraction intensity and phase of fatigue development) and sex dependent. Higher contraction intensities were associated with greater desaturation over time. Compared to females, males exhibited greater desaturation as fatigue progressed and this was augmented at higher contraction intensities. These may be likely affected by sex differences in muscle mass, morphology and strength differences during exercises at relative intensities. Future work that explores sex differences in muscle oxygenation during absolute force intensities are needed, which may have implications for muscle fatigue development and potential fatigue mitigation strategies. Practitioner Summary: Muscle oxygenation impacts fatigue development that can in turn affect worker health and productivity. Males exhibit greater forearm desaturation than females at higher relative work intensities, despite similar fatigue levels. Females may be predisposed to greater muscle delivery and oxygenation challenges that can increase their fatigability during work at absolute load levels.

Modification of a three-compartment muscle fatigue model to predict peak torque decline during intermittent tasks

This study aimed to test whether adding a rest recovery parameter, r, to the analytical three-compartment controller (3CC) fatigue model (Xia and Frey Law, 2008) will improve fatigue estimates during intermittent contractions. The 3CC muscle fatigue model uses differential equations to predict the flow of muscle between three muscle states: Resting (MR), Active (MA), and Fatigued (MF). This model uses a feedback controller to match the active state to target loads and two joint-specific parameters: F, fatigue rate controlling flow from active to fatigued compartments) and R, the recovery rate controlling flow from the fatigued to the resting compartments. This model does well to predict intensity-endurance time curves for sustained isometric tasks. However, previous studies find when rest intervals are present that the model over predicts fatigue. Intermittent rest periods would allow for the occurrence of subsequent reactive vasodilation and post-contraction hyperemia. We hypothesize a modified 3CC-r fatigue model will improve predictions of force decay during intermittent contractions with the addition of a rest recovery parameter, r, to augment recovery during rest intervals, representing muscle re-perfusion. A meta-analysis compiling intermittent fatigue data from 63 publications reporting decline in peak torque (% torque decline) were used for comparison. The original model over-predicted fatigue development from 19 to 29% torque decline; the addition of a rest multiplier significantly improved fatigue estimates to 6-10% torque decline. We conclude the addition of a rest multiplier to the three-compartment controller fatigue model provides a physiologically consistent modification for tasks involving rest intervals, resulting in improved estimates of muscle fatigue.

Triggered intravoxel incoherent motion MRI for the assessment of calf muscle perfusion during isometric intermittent exercise

The main aim of this paper was to propose triggered intravoxel incoherent motion (IVIM) imaging sequences for the evaluation of perfusion changes in calf muscles before, during and after isometric intermittent exercise. Twelve healthy volunteers were involved in the study. The subjects were asked to perform intermittent isometric plantar flexions inside the MRI bore. MRI of the calf muscles was performed on a 3.0 T scanner and diffusion-weighted (DW) images were obtained using eight different b values (0 to 500 s/mm² ). Acquisitions were performed at rest, during exercise and in the subsequent recovery phase. A motion-triggered echo-planar imaging DW sequence was implemented to avoid movement artifacts. Image quality was evaluated using the average edge strength (AES) as a quantitative metric to assess the motion artifact effect. IVIM parameters (diffusion D, perfusion fraction f and pseudo-diffusion D*) were estimated using a segmented fitting approach and evaluated in gastrocnemius and soleus muscles. No differences were observed in quality of IVIM images between resting state and triggered exercise, whereas the non-triggered images acquired during exercise had a significantly lower value of AES (reduction of more than 20%). The isometric intermittent plantar-flexion exercise induced an increase of all IVIM parameters (D by 10%; f by 90%; D* by 124%; fD* by 260%), in agreement with the increased muscle perfusion occurring during exercise. Finally, IVIM parameters reverted to the resting values within 3 min during the recovery phase. In conclusion, the IVIM approach, if properly adapted using motion-triggered sequences, seems to be a promising method to investigate muscle perfusion during isometric exercise.

Influence of group III/IV muscle afferents on small muscle mass exercise performance: a bioenergetics perspective

KEY POINTS

This investigation assessed the influence of group III/IV muscle afferents on small muscle mass exercise performance from a skeletal muscle bioenergetics perspective. Group III/IV muscle afferent feedback was attenuated with lumbar intrathecal fentanyl during intermittent isometric single-leg knee-extensor all-out exercise, while ³¹ P-MRS was used to assess skeletal muscle bioenergetics. Attenuation of group III/IV muscle afferent feedback improved exercise performance during the first minute of exercise, due to an increase in total ATP production with no change in the ATP cost of contraction. However, exercise performance was not altered during the remainder of the protocol, despite a sustained increase in total ATP production, due to an exacerbated ATP cost of contraction. These findings reveal that group III/IV muscle afferents directly limit exercise performance during small muscle mass exercise, but, due to their critical role in maintaining skeletal muscle contractile efficiency, with time, the benefit of attenuating the muscle afferents is negated.

ABSTRACT

The direct influence of group III/IV muscle afferents on exercise performance remains equivocal. Therefore, all-out intermittent isometric single-leg knee-extensor exercise and phosphorous magnetic resonance spectroscopy (³¹ P-MRS) were utilized to provide a high time resolution assessment of exercise performance and skeletal muscle bioenergetics in control conditions (CTRL) and with the attenuation of group III/IV muscle afferent feedback via lumbar intrathecal fentanyl (FENT). In both conditions, seven recreationally active men performed 60 maximal voluntary quadriceps contractions (MVC; 3 s contraction, 2 s relaxation), while knee-extensor force and ³¹ P-MRS were assessed during each MVC. The cumulative integrated force was significantly greater (8 ± 6%) in FENT than CTRL for the first minute of the all-out protocol, but was not significantly different for the second to fifth minutes. Total ATP production was significantly greater (16 ± 21%) in FENT than CTRL throughout the all-out exercise protocol, due to a significantly greater anaerobic ATP production (11 ± 13%) in FENT than CTRL with no significant difference in oxidative ATP production. The ATP cost of contraction was not significantly different between FENT and CTRL for the first minute of the all-out protocol, but was significantly greater (29 ± 34%) in FENT than in CTRL for the second to fifth minutes. These findings reveal that group III/IV muscle afferents directly limit exercise performance during small muscle mass exercise, but, due to their critical role in maintaining skeletal muscle contractile efficiency, with time, the benefit from muscle afferent attenuation is negated.

Bioenergetics and ATP Synthesis during Exercise: Role of Group III/IV Muscle Afferents

PURPOSE

The purpose of this study was to investigate the role of the group III/IV muscle afferents in the bioenergetics of exercising skeletal muscle beyond constraining the magnitude of metabolic perturbation.

METHODS

Eight healthy men performed intermittent isometric knee-extensor exercise to task failure at ~58% maximal voluntary contraction under control conditions (CTRL) and with lumbar intrathecal fentanyl to attenuate group III/IV leg muscle afferents (FENT). Intramuscular concentrations of phosphocreatine (PCr), inorganic phosphate (Pi), diprotonated phosphate (H2PO4), adenosine triphosphate (ATP), and pH were determined using phosphorous magnetic resonance spectroscopy (P-MRS).

RESULTS

The magnitude of metabolic perturbation was significantly greater in FENT compared with CTRL for [Pi] (37.8 ± 16.8 vs 28.6 ± 8.6 mM), [H2PO4] (24.3 ± 12.2 vs 17.9 ± 7.1 mM), and [ATP] (75.8% ± 17.5% vs 81.9% ± 15.8% of baseline), whereas there was no significant difference in [PCr] (4.5 ± 2.4 vs 4.4 ± 2.3 mM) or pH (6.51 ± 0.10 vs 6.54 ± 0.14). The rate of perturbation in [PCr], [Pi], [H2PO4], and pH was significantly faster in FENT compared with CTRL. Oxidative ATP synthesis was not significantly different between conditions. However, anaerobic ATP synthesis, through augmented creatine kinase and glycolysis reactions, was significantly greater in FENT than in CTRL, resulting in a significantly greater ATP cost of contraction (0.049 ± 0.016 vs 0.038 ± 0.010 mM·min·N).

CONCLUSION

Group III/IV muscle afferents not only constrain the magnitude of perturbation in intramuscular Pi, H2PO4, and ATP during small muscle mass exercise but also seem to play a role in maintaining efficient skeletal muscle contractile function in men.

Advanced MRI Techniques for Muscle Imaging

Advanced magnetic resonance imaging (MRI) techniques can evaluate a wide array of muscle pathologies including acute or chronic muscle injury, musculotendinous response to injury, intramuscular collections and soft tissue masses, and others. In recent years, MRI has played a more important role in muscle disease diagnosis and monitoring. MRI provides excellent spatial and contrast resolution and helps direct optimal sites for muscle biopsy. Whole-body MRI now helps identify signature patterns of muscular involvement in large anatomical regions with relative ease. Quantitative MRI has advanced the evaluation and disease tracking of muscle atrophy and fatty infiltration in entities such as muscular dystrophies. Multivoxel magnetic resonance spectroscopy (MRS) now allows a more thorough, complete evaluation of a muscle of interest without the inherent sampling bias of single-voxel MRS or biopsy. Diffusion MRI allows quantification of muscle inflammation and capillary perfusion as well as muscle fiber tracking.

Sex-Specific Impact of Ischemic Preconditioning on Tissue Oxygenation and Maximal Concentric Force

Prior peripheral hypoxia induced via remote ischemic preconditioning (IPC) can improve physical performance in male athletes through improved O₂ delivery and utilization. Since females may have an innate protective mechanism against ischemia-reperfusion injury, and since muscle metabolism during contraction differs between sexes, it is relevant to examine the impact of sex in response to IPC to determine whether it is also ergogenic in females. In a randomized, crossover, single-blind study, we investigated muscle performance, hemodynamic and O₂ uptake in strength-trained males (n = 9) and females (n = 8) performing five sets of 5 maximum voluntary knee extensions on an isokinetic dynamometer, preceded by either IPC (3 × 5-min ischemia/5-min reperfusion cycles at 200 mmHg) or SHAM (20 mmHg). Changes in deoxy-hemoglobin (Δ[HHb], expressed in percentage of arterial occlusion and considered an index of O₂ extraction), and total hemoglobin (Δ[THb]) concentrations of the vastus lateralis muscle were continuously monitored by near-infrared spectroscopy. The metabolic efficiency of the contractions was calculated as the average force/Δ[HHb]_avg ratio. Cohen's effect sizes (ES) ± 90% confidence limits were used to estimate IPC-induced changes and sex differences. IPC increased total muscular force in males only (13.0%, ES 0.64, 0.37;0.90), and this change was greater than in females (10.4% difference, ES 0.40, 0.10;0.70). Percent force decrement was only attenuated in females (-19.8%, ES -0.38, -0.77;0.01), which was clearly different than males (sex difference: ES 0.45, -0.16;1.07). IPC also induced different changes between sexes for average muscle O₂ uptake in set 2 (males: 6.4% vs. females: -16.7%, ES 0.21, -0.18;0.60), set 3 (males: 7.0% vs. females: -44.4%, ES 0.56, -0.17;1.29), set 4 (males: 9.1% vs. females: -40.2%, ES 0.51, -0.10;1.13), and set 5 (males: 10.2% vs. females: -40.4%, ES 0.52, -0.04;1.09). However, metabolic efficiency was not meaningfully different between conditions and sexes. IPC increased muscle blood volume (↑[THb]) at rest and during recovery between sets, to the same extent in both sexes. Despite a similar IPC-induced initial increase in O₂ delivery in both sexes, males displayed greater peripheral O₂ extraction and greater strength enhancement. This ergogenic effect appears to be mediated in part via an up regulated oxidative function in males. We conclude that strength-trained males might benefit more from IPC than their female counterparts during repeated, maximal efforts.

Muscle oxygenation during dynamic plantar flexion exercise: combining BOLD MRI with traditional physiological measurements

Blood-oxygen-level-dependent magnetic resonance imaging (BOLD MRI) has the potential to quantify skeletal muscle oxygenation with high temporal and high spatial resolution. The purpose of this study was to characterize skeletal muscle BOLD responses during steady-state plantar flexion exercise (i.e., during the brief rest periods between muscle contraction). We used three different imaging modalities (ultrasound of the popliteal artery, BOLD MRI, and near-infrared spectroscopy [NIRS]) and two different exercise intensities (2 and 6 kg). Six healthy men underwent three separate protocols of dynamic plantar flexion exercise on separate days and acute physiological responses were measured. Ultrasound studies showed the percent change in popliteal velocity from baseline to the end of exercise was 151 ± 24% during 2 kg and 589 ± 145% during 6 kg. MRI studies showed an abrupt decrease in BOLD signal intensity at the onset of 2 kg exercise, indicating deoxygenation. The BOLD signal was further reduced during 6 kg exercise (compared to 2 kg) at 1 min (-4.3 ± 0.7 vs. -1.2 ± 0.4%, P < 0.001). Similarly, the change in the NIRS muscle oxygen saturation in the medial gastrocnemius was -11 ± 4% at 2 kg and -38 ± 11% with 6 kg (P = 0.041). In conclusion, we demonstrate that BOLD signal intensity decreases during plantar flexion and this effect is augmented at higher exercise workloads.

Ischemic preconditioning increases muscle perfusion, oxygen uptake, and force in strength-trained athletes

Muscle ischemia and reperfusion induced by ischemic preconditioning (IPC) can improve performance in various activities. However, the underlying mechanisms are still poorly understood. The purpose of this study was to examine the effects of IPC on muscle hemodynamics and oxygen (O2) uptake during repeated maximal contractions. In a cross-over, randomized, single-blind study, 10 strength-trained men performed 5 sets of 5 maximal voluntary knee extensions of the right leg on an isokinetic dynamometer, preceded by either IPC of the right lower limb (3×5-min compression/5-min reperfusion cycles at 200 mm Hg) or sham (20 mm Hg). Changes in deoxyhemoglobin, expressed as a percentage of arterial occlusion, and total hemoglobin ([THb]) concentrations of the vastus lateralis muscle were monitored continuously by near-infrared spectroscopy. Differences between IPC and sham were analyzed using Cohen’s effect size (ES) ± 90% confidence limits, and magnitude-based inferences. Compared with sham, IPC likely increased muscle blood volume at rest (↑[THb], 46.5%; ES, 0.56; 90% confidence limits for ES, –0.21, 1.32). During exercise, peak force was almost certainly higher (11.8%; ES, 0.37; 0.27, 0.47), average force was very likely higher (12.6%; ES, 0.47; 0.29, 0.66), and average muscle O2 uptake was possibly increased (15.8%; ES, 0.36; –0.07, 0.79) after IPC. In the recovery periods between contractions, IPC also increased blood volume after sets 1 (23.6%; ES, 0.30; –0.05, 0.65) and 5 (25.1%; ES, 0.32; 0.09, 0.55). Three cycles of IPC immediately increased muscle perfusion and O2 uptake, conducive to higher repeated force capacity in strength-trained athletes. This maneuver therefore appears relevant to enhancing exercise training stimulus.

Skeletal muscle microvascular function in girls with Turner syndrome

BACKGROUND

Exercise intolerance is prevalent in individuals with Turner Syndrome (TS). We recently demonstrated that girls with TS have normal aerobic but altered skeletal muscle anaerobic metabolism compared to healthy controls (HC). The purpose of this study was to compare peripheral skeletal muscle microvascular function in girls with TS to HC after exercise. We hypothesized that girls with TS would have similar muscle blood-oxygen level-dependent (BOLD) magnetic resonance imaging (MRI) signal responses during recovery from exercise compared to HC.

METHODS

Thirteen TS participants and 8 HC completed testing. BOLD MRI was used to measure skeletal muscle microvascular response during 60 second recovery, following 60 s of exercise at 65% of maximal workload. Exercise and recovery were repeated four times, and the BOLD signal time course was fit to a four-parameter sigmoid function.

RESULTS

Participants were 13.7 ± 3.1 years old and weighed 47.9 ± 14.6 kg. The mean change in BOLD signal intensity following exercise at the end of recovery, the mean response time of the function/the washout of deoxyhemoglobin, and the mean half-time of recovery were similar between the TS and HC groups.

CONCLUSIONS

Our results demonstrate that compared to HC, peripheral skeletal muscle microvascular function following exercise in girls with TS is not impaired.

GENERAL SIGNIFICANCE

This study supports the idea that the aerobic energy pathway is not impaired in children with TS in response to submaximal exercise. Other mechanisms are likely responsible for exercise intolerance in TS; this needs to be further investigated.

Eccentric exercise slows in vivo microvascular reactivity during brief contractions in human skeletal muscle

Unaccustomed exercise involving eccentric contractions results in muscle soreness and an overall decline in muscle function, however, little is known about the effects of eccentric exercise on microvascular reactivity in human skeletal muscle. Fourteen healthy men and women performed eccentric contractions of the dorsiflexor muscles in one leg, while the contralateral leg served as a control. At baseline, and 24 and 48 h after eccentric exercise, the following were acquired bilaterally in the tibialis anterior muscle: 1) transverse relaxation time (T2)-weighted magnetic resonance images to determine muscle cross-sectional area (mCSA) and T2; 2) blood oxygen level-dependent (BOLD) images during and following brief, maximal voluntary contractions (MVC) to monitor the hyperemic responses with participants positioned supine in a 3T magnet; 3) muscle strength; and 4) pain pressure threshold. Compared with the control leg, eccentric exercise resulted in soreness, decline in strength (∼20%), increased mCSA (∼7%), and prolonged T2 (∼7%) at 24 and 48 h ( P < 0.05). The BOLD response to a brief MVC was altered 24 and 48 h after eccentric exercise, such that time-to-peak (∼35%, P < 0.05) and time-to-half-recovery (∼23%, P < 0.05) were prolonged. The altered contraction-induced hyperemic response suggests slowed microvascular reactivity and altered matching of O2 delivery to O2 utilization within muscle tissue showing signs of muscle damage. These changes in microvascular regulation after eccentric exercise may impede rapid adjustments in muscle blood flow at exercise onset and during activities involving brief bursts of muscle activation, which may impair O2 delivery and contribute to reduced muscle function after eccentric exercise.

Mechanical compression during repeated sustained isometric muscle contractions and hyperemic recovery in healthy young males

BACKGROUND

An elevated intramuscular pressure during a single forearm isometric muscle contraction may restrict muscle hyperemia. However, during repeated isometric exercise, it is unclear to what extent mechanical compression and muscle vasodilatation contribute to the magnitude and time course of beat-to-beat limb hemodynamics, due to alterations in leg vascular conductance (LVC).

METHODS

In eight healthy male subjects, the time course of both beat-to-beat leg blood flow (LBF) and LVC in the femoral artery was determined between repeated 10-s isometric thigh muscle contractions and 10-s muscle relaxation (a duty cycle of 20 s) for steady-state 120 s at five target workloads (10, 30, 50, 70, and 90% of maximum voluntary contraction (MVC)). The ratio of restricted LBF due to mechanical compression across workloads was determined by the formula (relaxation LBF--contraction LBF)/relaxation LBF (%).

RESULTS

The exercise protocol was performed completely by all subjects (≤ 50% MVC), seven subjects (≤ 70% MVC), and two subjects (≤ 90% MVC). During a 10-s isometric muscle contraction, the time course in both beat-to-beat LBF and LVC displayed a fitting curve with an exponential increase (P < 0.001, r (2) ≥ 0.956) at each workload but no significant difference in mean LBF across workloads and pre-exercise. During a 10-s muscle relaxation, the time course in both beat-to-beat LBF and LVC increased as a function of workload, followed by a linear decline (P < 0.001, r (2) ≥ 0.889), that was workload-dependent, resulting in mean LBF increasing linearly across workloads (P < 0.01, r (2) = 0.984). The ratio of restricted LBF can be described as a single exponential decay with an increase in workload, which has inflection point distinctions between 30 and 50% MVC.

CONCLUSIONS

In a 20-s duty cycle of steady-state repeated isometric muscle contractions, the post-contraction hyperemia (magnitude of both LBF and LVC) during muscle relaxation was in proportion to the workload, which is in agreement with previous findings. Furthermore, time-dependent beat-to-beat muscle vasodilatation was seen, but not restricted, during isometric muscle contractions through all target workloads. Additionally, the relative contribution of mechanical obstruction and vasodilatation to the hyperemia observed in the repeated isometric exercise protocol was non-linear with regard to workload. In combination with repeated isometric exercise, the findings could potentially prove to be useful indicators of circulatory adjustment by mechanical compression for muscle-related disease.

Blood flow and muscle oxygenation during low, moderate, and maximal sustained isometric contractions

A reduction of blood flow to active muscle will precipitate fatigue, and sustained isometric contractions produce intramuscular and compartmental pressures that can limit flow. The present study explored how blood flow and muscle oxygenation respond to isometric contractions at low, moderate, and maximal intensities. Over two visits, 10 males (26 ± 2 yr; means ± SD) performed 1-min dorsiflexion contractions at 30, 60, and 100% of maximal voluntary contraction (MVC) torque. Doppler ultrasound of the anterior tibial artery was used to record arterial diameter and mean blood velocity and to calculate absolute blood flow. The tissue oxygenation index (TOI) of tibialis anterior was acquired with near-infrared spectroscopy (NIRS). There was a progressive increase in blood flow at 30% MVC (peak of 289 ± 139% resting value), no change from rest until an increase in the final 10 s of exercise at 60% MVC (peak of 197 ± 102% rest), and an initial decrease (59 ± 30% resting value) followed by a progressive increase at 100% MVC (peak of 355 ± 133% rest). Blood flow was greater at 30 and 100% than 60% MVC during the last 30 s of exercise. TOI was ∼63% at rest and, within 30 s of exercise, reached steady-state values of ∼42%, ∼22%, and ∼22% for 30, 60, and 100% MVC, respectively. Even maximal contraction of the dorsiflexors is unable to cause more than a transient decrease of flow in the anterior tibial artery. Unlike dynamic or intermittent isometric exercise, our results indicate blood flow is not linearly graded with intensity or directly coupled with oxygenation during sustained isometric contractions.

Advanced Noncontrast MR Imaging in Musculoskeletal Radiology

Multiple nonmorphologic magnetic resonance sequences are available in musculoskeletal imaging that can provide additional information to better characterize and diagnose musculoskeletal disorders and diseases. These sequences include blood-oxygen-level-dependent (BOLD), arterial spin labeling (ASL), diffusion-weighted imaging (DWI), and diffusion-tensor imaging (DTI). BOLD and ASL provide different methods to evaluate skeletal muscle microperfusion. The BOLD signal reflects the ratio between oxyhemoglobin and deoxyhemoglobin. ASL uses selective tagging of inflowing blood spins in a specific region for calculating local perfusion. DWI and DTI provide information about the structural integrity of soft tissue including muscles and fibers as well as pathologies.

Impact of age on exercise-induced ATP supply during supramaximal plantar flexion in humans

Currently, the physiological factors responsible for exercise intolerance and bioenergetic alterations with age are poorly understood due, at least in art, to the confounding effect of reduced physical activity in the elderly. Thus, in 40 healthy young (22 ± 2 yr) and old (74 ± 8 yr) activity-matched subjects, we assessed the impact of age on: 1) the relative contribution of the three major pathways of ATP synthesis (oxidative ATP synthesis, glycolysis, and the creatine kinase reaction) and 2) the ATP cost of contraction during high-intensity exercise. Specifically, during supramaximal plantar flexion (120% of maximal aerobic power), to stress the functional limits of the skeletal muscle energy systems, we used (31)P-labeled magnetic resonance spectroscopy to assess metabolism. Although glycolytic activation was delayed in the old, ATP synthesis from the main energy pathways was not significantly different between groups. Similarly, the inferred peak rate of mitochondrial ATP synthesis was not significantly different between the young (25 ± 8 mM/min) and old (24 ± 6 mM/min). In contrast, the ATP cost of contraction was significantly elevated in the old compared with the young (5.1 ± 2.0 and 3.7 ± 1.7 mM·min(-1)·W(-1), respectively; P < 0.05). Overall, these findings suggest that, when young and old subjects are activity matched, there is no evidence of age-related mitochondrial and glycolytic dysfunction. However, this study does confirm an abnormal elevation in exercise-induced skeletal muscle metabolic demand in the old that may contribute to the decline in exercise capacity with advancing age.

Multiparametric assessment of vascular function in peripheral artery disease: dynamic measurement of skeletal muscle perfusion, blood-oxygen-level dependent signal, and venous oxygen saturation

BACKGROUND

Endothelial dysfunction present in patients with peripheral artery disease may be better understood by measuring the temporal dynamics of blood flow and oxygen saturation during reactive hyperemia than by conventional static measurements.

METHODS AND RESULTS

Perfusion, Intravascular Venous Oxygen saturation, and T2* (PIVOT), a recently developed MRI technique, was used to measure the response to an ischemia-reperfusion paradigm in 96 patients with peripheral artery disease of varying severity and 10 healthy controls. Perfusion, venous oxygen saturation SvO2, and T2* were each quantified in the calf at 2-s temporal resolution, yielding a dynamic time course for each variable. Compared with healthy controls, patients had a blunted and delayed hyperemic response. Moreover, patients with lower ankle-brachial index had (1) a more delayed reactive hyperemia response time, manifesting as an increase in time to peak perfusion in the gastrocnemius, soleus, and peroneus muscles, and in the anterior compartment, (2) an increase in the time to peak T2* measured in the soleus muscle, and (3) a prolongation of the posterior tibial vein SvO2 washout time. Intrasession and intersession repeatability were also assessed. Results indicated that time to peak perfusion and time to peak T2* were the most reliable extracted time course metrics.

CONCLUSIONS

Perfusion, dynamic SvO2, and T2* response times after induced ischemia are highly correlated with peripheral artery disease severity. Combined imaging of peripheral microvascular blood flow and dynamics of oxygen saturation with Perfusion, intravascular SvO2, and T2* may be a useful tool to investigate the pathophysiology of peripheral artery disease.

Dietary nitrate accelerates postexercise muscle metabolic recovery and O2 delivery in hypoxia

We tested the hypothesis that the time constants (τ) of postexercise T2* MRI signal intensity (an index of O2 delivery) and muscle [PCr] (an index of metabolic perturbation, measured by (31)P-MRS) in hypoxia would be accelerated after dietary nitrate (NO3 (-)) supplementation. In a double-blind crossover design, eight moderately trained subjects underwent 5 days of NO3 (-) (beetroot juice, BR; 8.2 mmol/day NO3 (-)) and placebo (PL; 0.003 mmol/day NO3 (-)) supplementation in four conditions: normoxic PL (N-PL), hypoxic PL (H-PL; 13% O2), normoxic NO3 (-) (N-BR), and hypoxic NO3 (-) (H-BR). The single-leg knee-extension protocol consisted of 10 min of steady-state exercise and 24 s of high-intensity exercise. The [PCr] recovery τ was greater in H-PL (30 ± 4 s) than H-BR (22 ± 4 s), N-PL (24 ± 4 s) and N-BR (22 ± 4 s) (P < 0.05) and the maximal rate of mitochondrial ATP resynthesis (Qmax) was lower in the H-PL (1.12 ± 0.16 mM/s) compared with H-BR (1.35 ± 0.26 mM/s), N-PL (1.47 ± 0.28 mM/s), and N-BR (1.40 ± 0.21 mM/s) (P < 0.05). The τ of postexercise T2* signal intensity was greater in H-PL (47 ± 14 s) than H-BR (32 ± 10 s), N-PL (38 ± 9 s), and N-BR (27 ± 6 s) (P < 0.05). The postexercise [PCr] and T2* recovery τ were correlated in hypoxia (r = 0.60; P < 0.05), but not in normoxia (r = 0.28; P > 0.05). These findings suggest that the NO3 (-)-NO2 (-)-NO pathway is a significant modulator of muscle energetics and O2 delivery during hypoxic exercise and subsequent recovery.

Assessing the end-organ in peripheral arterial occlusive disease-from contrast-enhanced ultrasound to blood-oxygen-level-dependent MR imaging

Peripheral arterial occlusive disease (PAOD) is a result of atherosclerotic disease which is currently the leading cause of morbidity and mortality in the western world. Patients with PAOD may present with intermittent claudication or symptoms related to critical limb ischemia. PAOD is associated with increased mortality rates. Stenoses and occlusions are usually detected by macrovascular imaging, including ultrasound and cross-sectional methods. From a pathophysiological view these stenoses and occlusions are affecting the microperfusion in the functional end-organs, such as the skin and skeletal muscle. In the clinical arena new imaging technologies enable the evaluation of the microvasculature. Two technologies currently under investigation for this purpose on the end-organ level in PAOD patients are contrast-enhanced ultrasound (CEUS) and blood-oxygen-level-dependent (BOLD) MR imaging (MRI). The following article is providing an overview about these evolving techniques with a specific focus on skeletal muscle microvasculature imaging in PAOD patients.

Estimation and visualization of longitudinal muscle motion using ultrasonography: a feasibility study

Ultrasonography is a convenient and widely used technique to look into the longitudinal muscle motion as it is radiation-free and real-time. The motion of localized parts of the muscle, disclosed by ultrasonography, spatially reflects contraction activities of the corresponding muscles. However, little attention was paid to the estimation of longitudinal muscle motion, especially towards estimation of dense deformation field at different depths under the skin. Yet fewer studies on the visualization of such muscle motion or further clinical applications were reported in the literature. A primal-dual algorithm was used to estimate the motion of gastrocnemius muscle (GM) in longitudinal direction in this study. To provide insights into the rules of longitudinal muscle motion, we proposed a novel framework including motion estimation, visualization and quantitative analysis to interpret synchronous activities of collaborating muscles with spatial details. The proposed methods were evaluated on ultrasound image sequences, captured at a rate of 25 frames per second from eight healthy subjects. In order to estimate and visualize the GM motion in longitudinal direction, each subject was asked to perform isometric plantar flexion twice. Preliminary results show that the proposed visualization methods provide both spatial and temporal details and they are helpful to study muscle contractions. One of the proposed quantitative measures was also tested on a patient with unilateral limb dysfunction caused by cerebral infarction. The measure revealed distinct patterns between the normal and the dysfunctional lower limb. The proposed framework and its associated quantitative measures could potentially be used to complement electromyography (EMG) and torque signals in functional assessment of skeletal muscles.

Skeletal muscle fatigue

Skeletal muscle fatigue is defined as the fall of force or power in response to contractile activity. Both the mechanisms of fatigue and the modes used to elicit it vary tremendously. Conceptual and technological advances allow the examination of fatigue from the level of the single molecule to the intact organism. Evaluation of muscle fatigue in a wide range of disease states builds on our understanding of basic function by revealing the sources of dysfunction in response to disease.

Influence of fish oil on skeletal muscle mitochondrial energetics and lipid metabolites during high-fat diet

Omega-3 polyunsaturated fatty acids (n-3 PUFAs) enhance insulin sensitivity and glucose homeostasis in rodent models of insulin resistance. These beneficial effects have been linked with anti-inflammatory properties, but emerging data suggest that the mechanisms may also converge on mitochondria. We evaluated the influence of dietary n-3 PUFAs on mitochondrial physiology and muscle lipid metabolites in the context of high-fat diet (HFD) in mice. Mice were fed control diets (10% fat), HFD (60% fat), or HFD with fish oil (HFD+FO, 3.4% kcal from n-3 PUFAs) for 10 wk. Body mass and fat mass increased similarly in HFD and HFD+FO, but n-3 PUFAs attenuated the glucose intolerance that developed with HFD and increased expression of genes that regulate glucose metabolism in skeletal muscle. Despite similar muscle triglyceride levels in HFD and HFD+FO, long-chain acyl-CoAs and ceramides were lower in the presence of fish oil. Mitochondrial abundance and oxidative capacity were similarly increased in HFD and HFD+FO compared with controls. Hydrogen peroxide production was similarly elevated in HFD and HFD+FO in isolated mitochondria but not in permeabilized muscle fibers, likely due to increased activity and expression of catalase. These results support a hypothesis that n-3 PUFAs protect glucose tolerance, in part by preventing the accumulation of bioactive lipid mediators that interfere with insulin action. Furthermore, the respiratory function of skeletal muscle mitochondria does not appear to be a major factor in sphingolipid accumulation, glucose intolerance, or the protective effects of n-3 PUFAs.

Skeletal muscle BOLD MRI: from underlying physiological concepts to its usefulness in clinical conditions

Blood oxygenation-level dependent (BOLD) MRI has gained particular attention in functional brain imaging studies, where it can be used to localize areas of brain activation with high temporal resolution. To a higher degree than in the brain, skeletal muscles show extensive but transient alterations of blood flow between resting and activation state. Thus, there has been interest in the application of the BOLD effect in studying the physiology of skeletal muscles (healthy and diseased) and its possible application to clinical practice. This review outlines the potential of skeletal muscle BOLD MRI as a diagnostic tool for the evaluation of physiological and pathological alterations in the peripheral limb perfusion, such as in peripheral arterial occlusive disease. Moreover, current knowledge is summarized regarding the complex mechanisms eliciting BOLD effect in skeletal muscle. We describe technical fundaments of the procedure that should be taken into account when performing skeletal muscle BOLD MRI, including the most often applied paradigms to provoke BOLD signal changes and key parameters of the resulting time courses. Possible confounding effects in muscle BOLD imaging studies, like age, muscle fiber type, training state, and drug effects are also reviewed in detail.

Clinical implications of skeletal muscle blood-oxygenation-level-dependent (BOLD) MRI

Blood-oxygenation-level-dependent (BOLD) contrast in magnetic resonance (MR) imaging of skeletal muscle mainly depends on changes of oxygen saturation in the microcirculation. In recent years, an increasing number of studies have evaluated the clinical relevance of skeletal muscle BOLD MR imaging in vascular diseases, such as peripheral arterial occlusive disease, diabetes mellitus, and chronic compartment syndrome. BOLD imaging combines the advantages of MR imaging, i.e., high spatial resolution, no exposure to ionizing radiation, with functional information of local microvascular perfusion. Due to intrinsic contrast provoked via changes in hemoglobin oxygen saturation, it is a safe and easy applicable procedure on standard whole-body MR devices. Therefore, BOLD MR imaging of skeletal muscle is a potential new diagnostic tool in the clinical evaluation of vascular, inflammatory, and muscular pathologies. Our review focuses on the current evidence concerning the use of BOLD MR imaging of skeletal muscle under pathological conditions and highlights ways for future clinical and scientific applications.

Postmaximal contraction blood volume responses are blunted in obese and type 2 diabetic subjects in a muscle-specific manner

The purpose of this study was to determine whether there are differences in postisometric contraction blood volume and oxygenation responses among groups of type 2 diabetes mellitus (T2DM), obese, and lean individuals detectable using MRI. Eight T2DM patients were individually matched by age, sex, and race to non-T2DM individuals with similar body mass index (obese) and lean subjects. Functional MRI was performed using a dual-gradient-recalled echo, echo-planar imaging sequence with a repetition time of 1 s and at two echo times (TE = 6 and 46 ms). Data were acquired before, during, and after 10-s isometric dorsiflexion contractions performed at 50 and 100% of maximal voluntary contraction (MVC) force. MRI signal intensity (SI) changes from the tibialis anterior and extensor digitorum longus muscles were plotted as functions of time for each TE. From each time course, the difference between the minimum and the maximum postcontraction SI (ΔSI) were determined for TE = 6 ms (ΔSI(6)) and TE = 46 ms (ΔSI(46)), reflecting variations in blood volume and oxyhemoglobin saturation, respectively. Following 50% MVC contractions, the mean postcontraction ΔSI(6) values were similar in the three groups. Following MVC only, and in the EDL muscle only, T2DM and obese participants had ∼56% lower ΔSI(6) than the lean individuals. Also following MVC only, the ΔSI(46) response in the EDL was lower in T2DM subjects than in lean individuals. These data suggest that skeletal muscle small vessel impairment occurs in T2DM and body mass index-matched subjects, in muscle-specific and contraction intensity-dependent manners.

Comparative determination of energy production rates and mitochondrial function using different 31P MRS quantitative methods in sedentary and trained subjects

Muscle energetics has been largely and quantitatively investigated using (31)P MRS. Various methods have been used to estimate the corresponding rate of oxidative ATP synthesis (ATP(ox)); however, potential differences among methods have not been investigated. In this study, we aimed to compare the rates of ATP production and energy cost in two groups of subjects with different training status using four different methods: indirect method (method 1), ADP control model (method 2) and phosphate potential control model (method 3). Method 4 was a modified version of method 3 with the introduction of a correction factor allowing for similar values to be obtained for the end-exercise oxidative ATP synthesis rate inferred from exercise measurements and the initial recovery phosphocreatine resynthesis rate. Seven sedentary and seven endurance-trained subjects performed a dynamic standardised rest-exercise-recovery protocol. We quantified the rates of ATP(ox) and anaerobic ATP synthesis (ATP(ana)) using (31)P MRS data recorded at 1.5 T. The rates of ATP(ox) over the entire exercise session were independent of the method used, except for method 4 which provided significantly higher values in both groups (p < 0.01). In addition, methods 1-3 were cross-correlated, thereby confirming their statistical agreement. The rate of ATP(ana) was significantly higher with method 1 (p < 0.01) and lower with method 4 (p < 0.01). As a result of the higher rate of ATP(ox), EC (method 4) calculated over the entire exercise session was higher and initial EC (method 1) was lower in both groups compared with the other methods. We showed in this study that the rate of ATP(ox) was independent of the calculation method, as long as no corrections (method 4) were performed. In contrast, results related to the rates of ATP(ana) were strongly affected by the calculation method and, more exactly, by the estimation of protons generated by ATP(ox). Although the absolute EC values differed between the methods, within- or between-subject comparisons are still valid given the tight relationships between them.

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 T(1). 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.

Intracellular energetics and critical PO2 in resting ischemic human skeletal muscle in vivo

During ischemia and some types of muscular contractions, oxygen tension (Po(2)) declines to the point that mitochondrial ATP synthesis becomes limited by oxygen availability. Although this critical Po(2) has been determined in animal tissue in vitro and in situ, there remains controversy concerning potential disparities between values measured in vivo and ex vivo. To address this issue, we used concurrent heteronuclear magnetic resonance spectroscopy (MRS) to determine the critical intracellular Po(2) in resting human skeletal muscle in vivo. We interleaved measurements of deoxymyoglobin using (1)H-MRS with measures of high-energy phosphates and pH using (31)P-MRS, during 15 min of ischemia in the tibialis anterior muscles of 6 young men. ATP production and intramyocellular Po(2) were quantified throughout ischemia. Critical Po(2), determined as the Po(2) corresponding to the point where PCr begins to decline (PCr(ip)) in resting muscle during ischemia, was 0.35 ± 0.20 Torr, means ± SD. This in vivo value is consistent with reported values ex vivo and does not support the notion that critical Po(2) in resting muscle is higher when measured in vivo. Furthermore, we observed a 4.5-fold range of critical Po(2) values among the individuals studied. Regression analyses revealed that time to PCr(ip) was associated with critical Po(2) and the rate of myoglobin desaturation (r = 0.83, P = 0.04) but not the rate of ATP consumption during ischemia. The apparent dissociation between ATP demand and myoglobin deoxygenation during ischemia suggests that some degree of uncoupling between intracellular energetics and oxygenation is a potentially important factor that influences critical Po(2) in vivo.

Repeatability of a dual gradient-recalled echo MRI method for monitoring post-isometric contraction blood volume and oxygenation changes

The purpose of this study was to assess the repeatability of a dual gradient-recalled echo (GRE) muscle functional MRI technique. On 2 days, subjects (n = 8) performed 10 s isometric dorsiflexion contractions under conditions of: (1) maximal voluntary contraction (MVC), (2) 50% MVC (50% MVC), or (3) 50% MVC with concurrent proximal arterial cuff occlusion (50% MVC(cuff)). Functional MRI data were acquired using single-slice dual GRE (TR/TE = 1000/6, 46 ms)-echo planar imaging for 20 s before, during, and for 180 s after each contraction. The mean signal intensity (SI) time courses at each TE (SI(6) and SI(46), reflecting variations in blood volume and %HbO(2), respectively) from the tibialis anterior (TA) and extensor digitorum longus (EDL) muscles were characterized with the post-contraction change in SI and the time-to-peak SI (DeltaSI and TTP, respectively). DeltaSI(6) following an MVC was 36% higher than that obtained after a 50% MVC (p = 0.048). For DeltaSI(6), the highest intraclass correlation coefficients (ICCs) were observed for the TA muscle in the 50% MVC and MVC conditions, with values of 0.83 (p = 0.01) and 0.88 (p = 0.005), respectively. Bland-Altman plots revealed repeatability coefficients (RCs) for the 50% MVC and MVC conditions in the TA muscle of 1.9 and 1.4, respectively. The most repeatable measures for DeltaSI(46) were obtained for the 50% MVC and MVC conditions in the EDL muscle (p = 0.01 and p = 0.04, respectively). Bland-Altman plots revealed RC's for 50% MVC and MVC conditions in the EDL muscle of 3.9 and 5.7, respectively. DeltaSI(6) and DeltaSI(46) increased as a function of the contraction intensity. The repeatability of the method depends on the muscle and contraction condition being evaluated, and in general, is higher following an MVC.