1H MR imaging of anatomical compartments within the finger flexor muscles of the human forearm

Intramuscle Synergies: Their Place in the Neural Control Hierarchy

We accept a definition of synergy introduced by Nikolai Bernstein and develop it for various actions, from those involving the whole body to those involving a single muscle. Furthermore, we use two major theoretical developments in the field of motor control—the idea of hierarchical control with spatial referent coordinates and the uncontrolled manifold hypothesis—to discuss recent studies of synergies within spaces of individual motor units (MUs) recorded within a single muscle. During the accurate finger force production tasks, MUs within hand extrinsic muscles form robust groups, with parallel scaling of the firing frequencies. The loading factors at individual MUs within each of the two main groups link them to the reciprocal and coactivation commands. Furthermore, groups are recruited in a task-specific way with gains that covary to stabilize muscle force. Such force-stabilizing synergies are seen in MUs recorded in the agonist and antagonist muscles but not in the spaces of MUs combined over the two muscles. These observations reflect inherent trade-offs between synergies at different levels of a control hierarchy. MU-based synergies do not show effects of hand dominance, whereas such effects are seen in multifinger synergies. Involuntary, reflex-based, force changes are stabilized by intramuscle synergies but not by multifinger synergies. These observations suggest that multifinger (multimuscle synergies) are based primarily on supraspinal circuitry, whereas intramuscle synergies reflect spinal circuitry. Studies of intra- and multimuscle synergies promise a powerful tool for exploring changes in spinal and supraspinal circuitry across patient populations.

Understanding and Synergy: A Single Concept at Different Levels of Analysis?

Biological systems differ from the inanimate world in their behaviors ranging from simple movements to coordinated purposeful actions by large groups of muscles, to perception of the world based on signals of different modalities, to cognitive acts, and to the role of self-imposed constraints such as laws of ethics. Respectively, depending on the behavior of interest, studies of biological objects based on laws of nature (physics) have to deal with different salient sets of variables and parameters. Understanding is a high-level concept, and its analysis has been linked to other high-level concepts such as "mental model" and "meaning". Attempts to analyze understanding based on laws of nature are an example of the top-down approach. Studies of the neural control of movements represent an opposite, bottom-up approach, which starts at the interface with classical physics of the inanimate world and operates with traditional concepts such as forces, coordinates, etc. There are common features shared by the two approaches. In particular, both assume organizations of large groups of elements into task-specific groups, which can be described with only a handful of salient variables. Both assume optimality criteria that allow the emergence of families of solutions to typical tasks. Both assume predictive processes reflected in anticipatory adjustments to actions (motor and non-motor). Both recognize the importance of generating dynamically stable solutions. The recent progress in studies of the neural control of movements has led to a theory of hierarchical control with spatial referent coordinates for the effectors. This theory, in combination with the uncontrolled manifold hypothesis, allows quantifying the stability of actions with respect to salient variables. This approach has been used in the analysis of motor learning, changes in movements with typical and atypical development and with aging, and impaired actions by patients with various neurological disorders. It has been developed to address issues of kinesthetic perception. There seems to be hope that the two counter-directional approaches will meet and result in a single theoretical scheme encompassing biological phenomena from figuring out the best next move in a chess position to activating motor units appropriate for implementing that move on the chessboard.

Reciprocal and coactivation commands at the level of individual motor units in an extrinsic finger flexor-extensor muscle pair

We explored the synergic organization of motor units in extrinsic finger muscles, flexor digitorum superficialis (FDS), and extensor digitorum communis (EDC). Healthy subjects produced accurate cyclical force by pressing with the middle phalanges of one of the three fingers (Index, Middle, and Ring) and all three together. Two wireless sensor arrays were used to record and identify motor unit action potentials in FDS and EDC. Stable motor unit groups were identified within each muscle and across both muscles. Analysis of motor units combined over the two muscles showed one of the first two motor unit groups with consistently opposite signs of the loading factors for the FDS and EDC motor units, and the other group with consistently same signs of the loading factors for the two muscles. We interpret the two motor unit groups as reflections of the reciprocal and co-activation commands within the theory of control with spatial referent coordinates. Force changes within the cycle were primarily associated with the modulation of the co-activation motor unit group. Analysis of inter-cycle variance within the spaces of motor unit groups defined for FDS and EDC separately showed force-stabilizing synergies across both single-finger and three-finger tasks. In contrast, analysis within the motor unit groups defined across both muscles failed to show force-stabilizing synergies. We interpret these results as a reflection of the trade-off across levels within a hierarchical control system.

Synergies at the level of motor units in single-finger and multi-finger tasks

We explored the organization of motor units recorded in the flexor digitorum superficialis into stable groups (MU-modes) and force-stabilizing synergies in spaces of MU-modes. Young, healthy participants performed one-finger and three-finger accurate cyclical force production tasks. Two wireless sensor arrays (Trigno Galileo, Delsys, Inc.) were placed over the proximal and distal portions of the muscle for surface recording and identification of motor unit action potentials. Principal component analysis with Varimax rotation and factor extraction was used to identify MU-modes. The framework of the uncontrolled manifold hypothesis was used to analyze inter-cycle variance in the space of MU-modes and compute the index of force-stabilizing synergy. Multiple linear regression between the first MU-mode in the three-finger task and the first MU-modes in the three single-finger tasks showed no differences between the data recorded by the two electrodes suggesting that MU-modes were unlikely to be synonymous with muscle compartments. Multi-MU-mode synergies stabilizing task force were documented across all tasks. In contrast, there were no force-stabilizing synergies in the three-finger task analyzed in the space of individual finger forces. Our results confirm the synergic organization of motor units in single-finger tasks and, for the first time, expand this result to multi-finger tasks. We offer an interpretation of the findings within the theoretical scheme of control with spatial referent coordinates expanded to the analysis of individual motor units. The results confirm trade-offs between synergies at different hierarchical levels and expand this notion to intra-muscle synergies.

On Primitives in Motor Control

The concept of primitives has been used in motor control both as a theoretical construct and as a means of describing the results of experimental studies involving multiple moving elements. This concept is close to Bernstein’s notion of engrams and level of synergies. Performance primitives have been explored in spaces of peripheral variables but interpreted in terms of neural control primitives. Performance primitives reflect a variety of mechanisms ranging from body mechanics to spinal mechanisms and to supraspinal circuitry. This review suggests that primitives originate at the task level as preferred time functions of spatial referent coordinates or at mappings from higher level referent coordinates to lower level, frequently abundant, referent coordinate sets. Different patterns of performance primitives can emerge depending, in particular, on the external force field.

Magnetic resonance elastography: a review

Magnetic resonance elastography (MRE) is a rapidly developing technology for quantitatively assessing the mechanical properties of tissue. The technology can be considered to be an imaging-based counterpart to palpation, commonly used by physicians to diagnose and characterize diseases. The success of palpation as a diagnostic method is based on the fact that the mechanical properties of tissues are often dramatically affected by the presence of disease processes, such as cancer, inflammation, and fibrosis. MRE obtains information about the stiffness of tissue by assessing the propagation of mechanical waves through the tissue with a special magnetic resonance imaging technique. The technique essentially involves three steps: (1) generating shear waves in the tissue, (2) acquiring MR images depicting the propagation of the induced shear waves, and (3) processing the images of the shear waves to generate quantitative maps of tissue stiffness, called elastograms. MRE is already being used clinically for the assessment of patients with chronic liver diseases and is emerging as a safe, reliable, and noninvasive alternative to liver biopsy for staging hepatic fibrosis. MRE is also being investigated for application to pathologies of other organs including the brain, breast, blood vessels, heart, kidneys, lungs, and skeletal muscle. The purpose of this review article is to introduce this technology to clinical anatomists and to summarize some of the current clinical applications that are being pursued.

Vibration imaging for localization of functional compartments of the extrinsic flexor muscles of the hand

PURPOSE

To develop and test an MRI-based imaging technique for the localization of the functional compartments of the functionally finger-specific, yet anatomically indistinct, flexor muscles of the hand.

MATERIALS AND METHODS

A total of six normal healthy volunteers were involved in five studies in which individual fingers were vibrated with mechanical actuators and the resultant motion within the corresponding functional compartments of the flexor muscles, mechanically transferred through the structurally connected tendons, was imaged with a phase-contrast MR imaging technique that is highly sensitive to cyclic motion. The motion amplitude and relative phase relationship between the functional compartments of various muscles and fingers were obtained and analyzed from these images as a means to differentiate the various subcompartments.

RESULTS

The results show that this technique provides a detailed mapping of the regions of the complex flexor muscle compartments that correspond to each digit for both the flexor digitorum profundus and the flexor digitorum superficialis. The results also demonstrate the presence of mechanical interdependence between the flexor muscles.

CONCLUSION

It is concluded from the results that localization of the finger-specific subcompartments of the forearm flexor muscles can be performed with this technique.

Comparisons of the Skeletal Muscle Metabolic Abnormalities in the Arm and Leg Muscles of Patients With Chronic Heart Failure

BACKGROUND

It has been suspected for some time that patients with chronic heart failure (CHF) have abnormal muscle metabolism, so in the present study the muscle metabolism of the arm and leg were compared by (31)P magnetic resonance spectroscopy ((31)P-MRS) to examine the relationship to exercise tolerance.

METHODS AND RESULTS

The study group comprised 13 patients and 11 normal controls. Calf-plantar and forearm-wrist flexion were performed to evaluate the metabolic capacity assessed as the phosphocreatine breakdown rate (PCr-slope) and muscle pH at a submaximal (70% peak) work rate (submax-pH). Exercise of both the arm and leg resulted in an earlier decrease in PCr and muscle pH in patients with CHF compared with controls. There were significant correlations between peak oxygen uptake (peak V(O2)) and the PCr-slope in both limbs in patients with CHF (forearm: r=0.63, p<0.05; calf: r=0.60, p<0.05), but no correlations in normal controls. There was a close correlation between the ventilatory anaerobic threshold (AT) and the PCr-slope in the calf (r=0.85, p<0.01), but not in the forearm in patients with CHF. Submax-pH in both upper and lower limbs was not significantly correlated to peak V(O2) or AT in either patients with CHF or controls.

CONCLUSIONS

Although metabolic abnormalities during exercise are seen in both arms and legs, leg muscle abnormalities, in particular, are closely related to systemic exercise intolerance in patients with CHF.

Incomplete functional subdivision of the human multitendoned finger muscle flexor digitorum profundus: an electromyographic study

The human flexor digitorum profundus (FDP) sends tendons to all 4 fingers. One might assume that this multitendoned muscle consists of 4 discrete neuromuscular compartments each acting on a different finger, but recent anatomical and physiological studies raise the possibility that the human FDP is incompletely subdivided. To investigate the functional organization of the human FDP, we recorded electromyographic (EMG) activity by bipolar fine-wire electrodes simultaneously from 2 or 4 separate intramuscular sites as normal human subjects performed isometric, individuated flexion, and extension of each left-hand digit. Some recordings showed EMG activity during flexion of only one of the 4 fingers, indicating that the human FDP has highly selective core regions that act on single fingers. The majority of recordings, however, showed a large amount of EMG activity during flexion of one finger and lower levels of EMG activity during flexion of an adjacent finger. This lesser EMG activity during flexion of adjacent fingers was unlikely to have resulted from recording motor units in neighboring neuromuscular compartments, and instead suggests incomplete functional subdivision of the human FDP. In addition to the greatest agonist EMG activity during flexion of a given finger, most recordings also showed EMG activity during extension of adjacent fingers, apparently serving to stabilize the given finger against unwanted extension. Paradoxically, the functional organization of the human FDP-with both incomplete functional subdivision and highly selective core regions-may contribute simultaneously to the inability of humans to produce completely independent finger movements, and to the greater ability of humans (compared with macaques) to individuate finger movements.

Finger interactions studied with transcranial magnetic stimulation during multi-finger force production tasks

OBJECTIVE

We used transcranial magnetic stimulation (TMS) to investigate interactions among neuromuscular complexes involved in force production by individual fingers of a hand during single- and multi-finger tasks.

METHODS

Subjects were asked to press with the fingertips at various levels of force using different finger combinations. TMS was applied over the M1 cortical hand area during constant force production. TMS-induced increments in fingertip forces were analyzed, as well as motor evoked potentials (MEPs) in flexor digitorum superficialis.

RESULTS

Both MEP size and individual force increments had inverted U-shaped dependences on the background force (peak responses were seen at about 50% of the maximal force). Similar relationships were obtained when subjects were asked to produce different forces with the same finger combination or the same total force with different finger combinations. The relationships were similar when the force was produced by explicitly instructed fingers or by other fingers of the hand. Effects of TMS on the force of a finger showed a strong dependence on the background force produced by this finger and minimal or no dependence on forces produced by other fingers of the hand.

CONCLUSIONS

Overall, TMS applied over M1 revealed little interaction among fingers. This supports the notion of digit-specific compartments in multi-digit extrinsic muscles of the hand and suggests that these compartments possess a high degree of physiological independence.

New parameters reducing the interindividual variability of metabolic changes during muscle contraction in humans. A (31)P MRS study with physiological and clinical implications

Interindividual variations in skeletal muscle metabolism make comparative analyses difficult. In this study, we have addressed the issue of capturing the variability of metabolic performance observed during muscle exercise in humans by using an original method of normalization.Metabolic changes induced by various kinds of exercise were investigated using 31P magnetic resonance spectroscopy (MRS) at 4.7 T in 65 normal subjects (23 women and 42 men) and 12 patients with biopsy-proven muscular disorders. Large variations in the extent of PCr breakdown and intracellular acidosis were recorded among subjects and exercise protocols. For all the data pooled, the amplitude of mechanical performance accounts for 50% of these variations. When scaled to the work output, variations of PCr consumption account for 65% of pH changes through a linear relationship. This linear relationship was substantially improved (90%) when both variables were scaled to the square of work output performed (P1 and P2). By capturing most of the initial interindividual variability (90%), P1 vs. P2 relationship represents an ideal standardization procedure, independent of any anthropometric measurements. This relationship also discloses a significant link between the extent of PCr breakdown and intracellular acidosis regardless of exercise protocol. Moreover, changes in the slope of the P1 vs. P2 regression curve, as measured in old subjects and in selected patients, directly reflect alterations of energy production in muscle.

Central mechanisms of finger interaction during one- and two-hand force production at distal and proximal phalanges

In this study we used changes in the relative involvement of different muscle groups during force production at the distal (DT) and proximal (PR) phalanges to test and modify a hypothesis on the central organization of multi-finger control for tasks involving non-homologous elements in the two hands. Ten subjects produced maximal force with different finger combinations. Two symmetrical (PR/PR and DT/DT) and two asymmetrical (PR/DT and DT/PR) combinations of force application sites in the two hands were used. During one-hand tasks, higher forces were produced at the PR site. In multi-finger tasks, total peak force was smaller than the sum of peak forces in single-finger tasks by the involved fingers (force deficit). Force production by some fingers of a hand was accompanied by involuntary force production by other fingers (enslaving). Force deficit and enslaving were both higher at the PR site. Two-hand tasks were accompanied by an additional drop in the force of individual fingers, i.e., bilateral deficit (BD). When symmetrical sites of force production were used in the two hands, BD was lower for symmetrical finger groups than for asymmetrical groups. During tests at asymmetrical sites, BD was higher and did not depend on symmetry of involved finger groups. We conclude that within-a-hand force deficit and enslaving are likely to be of a central, neural origin. An earlier introduced hypothesis has been expanded assuming that excitatory projections to contralateral finger representations exist only for homologous elements (sub-synergies) of a multi-finger force production synergy, while only inhibitory projections connect non-homologous elements.

Regional difference of muscle oxygen saturation and blood volume during exercise determined by near infrared imaging device

Using a near infrared (NIR) imaging device, we tested the hypothesis that regional differences in oxygen status could be detected in the gastrocnemius muscle during exercise and recovery. Six healthy subjects performed the standing plantar flexion exercises for 2 min; the frequency was one contraction per second. The NIR imaging device was placed over the medial head of the right gastrocnemius muscle and the signals from two optical sensors situated on the middle proximal and middle distal portions were used. The NIR-O(2) saturation (difference between deoxygenated and oxygenated Hb signals) and NIR-blood volume (sum of the oxygenated and deoxygenated Hb signals) were calculated in optical density units. Plantar flexion resulted in more deoxygenation during exercise and more reoxygenation during recovery in the distal portion compared with the proximal portion. The changes in NIR-O(2) between rest and a 2 min exercise, and between a 2 min exercise and a 3 min recovery were 0.11 and -0.23, respectively, in the distal portion, which were significantly larger than proximal values (0.05 and -0.10, p < 0.05). Plantar flexion resulted in lower NIR-blood volumes during exercise and greater recovery of blood after exercise in the distal portion compared with the proximal portion. The changes in NIR blood volume between rest and a 2 min exercise and between a 2 min exercise and a 3 min recovery were -0.19 and 0.31, respectively, in the distal portion, significantly larger than proximal values (-0.07 and 0.12, p < 0.05 for all comparisons). These findings indicate that the distal portion of the medial gastrocnemius had larger changes in NIR-O(2) saturation and NIR-blood volume than the proximal portion had. This is consistent with the distal portion having a greater impairment of blood flow possibly because of the higher intramuscular pressure during exercise.

IN CONCLUSION

(1) regional differences in oxygen status in the gastrocnemius muscle were detected with exercise, with the distal portion having greater NIR-O(2) saturation and NIR-blood volume changes, and (2) the NIR imaging device might be a useful method to detect the regional differences of oxygen status in the muscle.

Functional activation of the extensor carpi radialis muscles in humans

OBJECTIVES

To assess activity of radial wrist extensors caused by isometric radial deviation and extension by using magnetic resonance imaging (MRI) and to assess measures that might be used to normalize T2-weighted data.

DESIGN

Two-way analysis of variance (ANOVA) design.

SETTING

Laboratory and children's hospital.

PARTICIPANTS

Three healthy volunteers.

INTERVENTIONS

Ten repetitions of 10-second randomly ordered 30% or 60% of maximum voluntary isometric contractions toward wrist extension or radial deviation.

MAIN OUTCOME MEASURES

Average T2 values from T2-weighted MR images of the extensor carpi radialis brevis (ECRB) and the extensor carpi radialis longus (ECRL), flexor digitorum profundus (FDP), and radius marrow were determined across 7 sections and 4 exercise bouts and a preexercise condition.

RESULTS

Significant differences across task and across sections were determined. Post hoc analysis revealed differences in activity between proximal and distal ECRB and ECRL during an exercise and differential activation of the same muscle across the 2 exercise tasks. Bone marrow and FDP did not show task-related changes. The range of average T2 values of bone marrow across sections was greater than a muscle (FDP) that was not the target of the exercise protocol. However, FDP did show small but significant differences across sections.

CONCLUSIONS

T2-weighted MR images can be used to study muscle activation at 30% and 60% of maximum voluntary contractions. The use of inactive muscle and bone marrow for normalizing data requires further investigation.

Correlation of the near-infrared spectroscopy signals with signal intensity in T(2)-weighted magnetic resonance imaging of the human masseter muscle

The purpose of this study was to compare and contrast blood volume changes transcutaneously measured using near-infrared (NIR) spectroscopy against water signal intensity changes taken from a transverse T(2)-weighted MR image of the masseter muscle in healthy human subjects before, during and after contraction. Eight healthy non-smoking males with no history of chronic muscle pain or vascular headaches participated (mean age: 23.9+/-0.6 years). The MRI data were gathered using a turbo spin echo sequence (TR: 2300 ms; TE: 90 ms; FOV: 188x300 mm; scanning time: 30 s; slice thickness: 10 mm) and the slice level was set at the mid-point between the origin and insertion of the masseter. Intramuscular haemoglobin (Hb) levels and water content of the right masseter muscle were continuously monitored for 2 min before, 30 s during and 15 min after a maximum voluntary clenching (MVC) task. Both the near-infrared and MRI data were baseline-corrected and normalized and mean levels were established and plotted. Plots of the data showed that both near-infrared-based total Hb and T(2)-weighted MRI-based signal-intensity levels clearly decreased during contraction and a clear post-contraction rebound response was evident after the contraction. The near-infrared data were found to be highly correlated with MRI-based signal-intensity data (Pearson's r=0.909, P<0.0001). In conclusion, these data provide powerful evidence that near-infrared data (total Hb), transcutaneously taken from the masseter muscle in humans, will reflect the intramuscular water signal intensity changes seen using a T(2)-weighted MRI imaging method.

Tension distribution to the five digits of the hand by neuromuscular compartments in the macaque flexor digitorum profundus

The macaque flexor digitorum profundus (FDP) consists of a muscle belly with four neuromuscular regions and a complex insertion tendon that divides to serve all five digits of the hand. To determine the extent to which compartments within FDP act on single versus multiple digits, we stimulated the primary nerve branch innervating each neuromuscular region while recording the tension in all five distal insertion tendons. Stimulation of each primary nerve branch activated a distinct region of the muscle belly, so that each primary nerve branch and the muscle region innervated can be considered a neuromuscular compartment. Although each neuromuscular compartment provided a distinct distribution of tension across the five distal tendons, none acted on only one digital tendon. Most of the distribution of tension to multiple digits could be attributed to passive biomechanical interactions in the complex insertion tendon, although for the larger compartments a wider distribution resulted from the broad insertion of the muscle belly. Nerve ligations excluded contributions of spinal reflexes or distal axon reflexes to the distribution of tension to multiple digits. We conclude that the macaque FDP consists of four neuromuscular compartments, each of which provides a distinct distribution of tension to multiple digits.

The relationship between creatine kinase kinetics and exercise intensity in human forearm is unchanged by age

Using (31)P magnetic resonance spectroscopy, creatine kinase (CK) reaction kinetics was assessed in the forearm flexor digitorum profundus muscle of healthy young (n = 11, age 34.7 +/- 5 yr) and older (n = 20, age 73.5 +/- 8 yr) subjects at rest, intermittent exercise at 20% maximum voluntary contraction (MVC), and 40% MVC. Exercise resulted in a significant increase in the average ratio of inorganic phosphate (P(i)) to phosphocreatine (PCr) from resting values of 0.073 +/- 0.031 (young) and 0.082 +/- 0.037 (older) to 0. 268 +/- 0.140 (young, P < 0.01) and 0.452 +/- 0.387 (older, P < 0. 01) at 40% MVC. At 40% MVC, intracellular pH decreased significantly, from resting values of 7.08 +/- 0.08 (young) and 7.08 +/- 0.11 (older) to 6.84 +/- 0.19 (young, P < 0.05) and to 6.75 +/- 0.25 (older, P < 0.05). Average values of the pseudo-first-order reaction rate k((PCr-->ATP)) at rest were 0.07 +/- 0.04 s(-1) in the young and 0.07 +/- 0.03 s(-1) in the older group. At both exercise levels, the reaction rate constant increased compared with the resting value, but only the difference between the resting value and the 20% MVC value, which showed an 86% higher reaction rate constant in both groups, reached statistical significance (P < 0.05). No difference in the reaction rate constant between the young and older groups was observed at either exercise level. As with k((PCr-->ATP)), the average phosphorus flux through the CK reaction increased during exercise at 20% MVC (P < 0.05 in the older group) but decreased toward resting values at 40% MVC in both groups. The data in our study suggest that normal aging does not significantly affect the metabolic processes associated with the CK reaction.

Increased VO2 max with right-shifted Hb-O2 dissociation curve at a constant O2 delivery in dog muscle in situ

If the diffusive component of O2 transport in muscle is important in determining exercise capacity, an increased capillary-to-tissue PO2 difference should enhance gas exchange from blood to skeletal muscle during exercise. Thus a rightward shift in the O2 dissociation curve should theoretically increase O2 extraction and improve maximal O2 uptake (VO2 max). To test this hypothesis, we used the canine gastrocnemius muscle to study maximal exercise in eight dogs at a normal P50 (33.1 +/- 0.4 Torr) and with the O2 dissociation curve shifted to the right by an allosteric modifier of hemoglobin (Hb) (methylpropionic acid, RSR-13; P50 = 53.2 +/- 5.0 Torr). Four control dogs were also studied before and after infusion of vehicle. O2 (100%) was inspired during exercise to maintain arterial saturation in both conditions. The muscle was surgically isolated and electrically stimulated (tetanic train: 0.2-ms stimuli for 200-ms duration at 50 Hz, once per s). To maintain O2 delivery (pre-RSR-13 = 19.1 +/- 2.9; RSR-13 = 19.6 +/- 2.5 ml . 100 g-1 . min-1), the muscle was pump perfused. At a constant O2 delivery, RSR-13 significantly increased percent O2 extraction (pre-RSR-13 = 61 +/- 4.0; RSR-13 = 75.5 +/- 4.7) and muscle VO2 max (pre-RSR-13 = 11.8 +/- 2.1; RSR-13 = 14.2 +/- 1.5 ml . 100 g-1 . min-1). This improvement in VO2 max with increased P50 demonstrates its O2 supply dependence when P50 is normal and the importance of O2 diffusive transport to muscle at maximal exercise.

Skeletal muscle metabolism in maximal bicycle and treadmill exercise distinguished by using in vivo metabolic freeze method and phosphorus-31 magnetic resonance spectroscopy in normal men

This study indicates that skeletal muscle metabolism may affect the results of maximal bicycle and treadmill exercise differently, and that maximal bicycle exercise was limited by quadriceps muscle metabolism rather than by cardiopulmonary capacity. In contrast, maximal treadmill exercise was not limited, eliciting more cardiopulmonary reserve and attaining greater peak oxygen uptake than maximal bicycle exercise.

Different effects of exercise and edema on T2 relaxation in skeletal muscle

The hypothesis that increased muscle T2 after exercise is caused by increased extracellular fluid volume was tested by comparing the effects of exercise versus external leg negative pressure on muscle T2 relaxation in normal human subjects. T2 in lower leg muscles was measured by echo-planar imaging at 63 echo times from 24 to 272 ms, and the relaxation spectrum was calculated by using a non-negative least squares algorithm. T2 relaxation in anterior leg muscle before exercise was characterized by a single component with mean T2 = 29.3 +/- 0.7 (SE, n = 5). After ankle dorsiflexion exercise, this single component broadened, and mean T2 increased to 38.3 +/- 0.7 ms. In contrast, after leg negative pressure, which increased the total leg muscle cross-sectional area by 21% (range 12-32% n = 6), there was a variable appearance of much slower-relaxing components (60-500 ms). The results suggest that increased extracellular fluid can account for only a minor portion of the increase in muscle T2 observed during exercise.

Measurement of spin-lattice relaxation times and kinetic rate constants in rat muscle using progressive partial saturation and steady-state saturation transfer

31P spin-lattice relaxation times (T1) of metabolites in rat calf muscle at 1.9 Tesla and the forward rate through the creatine kinase (CK) reaction have been measured using a new method based on modeling progressive saturation explicitly incorporating the effect of chemical exchange. In a separate series of experiments, we compared our method with inversion recovery both in vitro and in vivo, finding agreement between the techniques. We found that the T1 values of phosphocreatine (PCr) (6.6 +/- 0.3 s), gamma-ATP (2.6 +/- 0.6 s), alpha-ATP (2.4 +/- 0.4 s) and beta-ATP (2.2 +/- 0.2 s) are unchanged by stimulation of sufficient intensity to induce a 32% drop in PCr level. The errors in T1 values which arise when chemical exchange is neglected are calculated. These are found to be on the order of 20% for PCr and 30-50% for gamma-ATP under typical conditions. Use of longer repetition times results in larger errors in measured values of T1. This source of error can be effectively eliminated by use of sufficiently short repetition times. We found that the rate constant of the forward CK reaction was increased 60% by stimulation, from 0.20 +/- 0.03 s-1 to 0.32 +/- 0.03 s-1, but that the phosphorus flux did not change.

Change in chemical shift and splitting of 31P gamma-ATP signal in human skeletal muscle during exercise and recovery

Proton decoupled 31P in vivo NMR spectroscopy of the human finger flexor muscles was performed during exercise and recovery using a 1.5 T whole-body imager. Predominantly the gamma-ATP signal shows a splitting caused by different signal contributions with chemical shifts that vary independently. Studies on the human gastrocnemius and biceps femoris muscle were undertaken to investigate the appearance of the splitting in these muscles as well. In all cases more than one signal contribution was found which might represent the different muscle fibre types and their recruitment pattern following exercise. An analysis of the chemical shifts (delta) of ATP results in changes of up to 0.4 ppm and 0.1 ppm for delta gamma- and delta beta-ATP, respectively. Based solely on the chemical shifts of the ATP 31P signals the tissue pH value following exercise was determined. The result was in good agreement with the value derived from delta Pi.

In vivo measurement of diffusion and pseudo-diffusion in skeletal muscle at rest and after exercise

To investigate whether diffusion-related compartmentalization could be observed in skeletal muscle and whether this compartmentalization was affected by exercising, attenuation curves of signal against diffusion weighting were obtained in skeletal muscle of nine healthy volunteers at rest and after an exercise. Fifteen points were obtained for each diffusion curve with diffusion weightings ranging between approximately 0 and 560 x 10(6) s/m2. Data were fitted a biexponential model using three parameters to yield two apparent diffusion coefficients, a long one, ADCL, and a short one, ADCS, together with the fractional volume, fL, associated with the long one. At rest, values of parameters ADCL, ADCS, and fL were 46 x 10(-9) +/- 37 x 10(-9) m2/s, 1.74 x 10(-9) +/- 0.11 x 10(-9) m2/s, and 3.6 +/- 1.3%, respectively. After exercise, these values were 89 x 10(-9) +/- 37 x 10(-9) m2/s (p < .001 vs. rest), 1.94 x 10(-9) +/- 0.13 x 10(-9) m2/s (p < .001), and 5.2 +/- 1.3% (p < .05), respectively. These variations demonstrate significant changes in attenuation curves between rest and postexercise in skeletal muscle and may support an interpretation of the long and the short components in terms of a microvascular and an extra-microvascular compartments.

Serial water changes in human skeletal muscles on exercise studied with proton magnetic resonance spectroscopy and imaging

In vivo 1H-magnetic resonance imaging (MRI) enabled us to study the distribution of water in living tissues and to document changes in human skeletal muscles during physical exercise. The purpose of the present study was to determine the total muscle water changes after exercise using water in 1H-MR spectroscopy and to compare these changes to the signal intensity change on T2*-weighted images and/or to the T2 value change. Seven young male volunteers were positioned in a 1.5 T Philips MR imaging system. They were then asked to dorsiflex their ankle joint against a 2 kg weight once every 2 seconds for 2 minutes. The peak height of water declined according to the clearance curve after exercise in all seven cases with the 1H-MRS similar to the signal intensity. The increasing rate at peak height of total muscle water exceeded both the signal intensity and the T2 value because the water peak height on the 1H-MRS included the extracellular water. In addition, we measured the changes in signal intensity in both calf muscles after walking race exercise. The time intensity curves were used to draw a clearance curve for each muscle group after exercise. It was possible to discern which muscle was used most from the T2*-weighted image that was obtained once after exercise.

Experimental design of 31P MRS assessment of human forearm muscle function: restrictions imposed by functional anatomy

The restrictions imposed by the functional anatomy of the finger flexor muscles on the experimental design of 31P MRS assessment of human forearm muscle function employing surface coil localization and voluntary exercise were investigated. It was found that 31P MRS metabolic data of finger flexor muscle should be correlated with mechanical data of combined flexion of only the ring and little fingers, rather than all four fingers as has been commonly the case in previously reported studies.

Separate measures of ATP utilization and recovery in human skeletal muscle

1. The chemical changes during contractile activity were separated from recovery metabolism in the forearm flexor musculature in normal human subjects using 31P nuclear magnetic resonance (NMR) spectroscopy. Percutaneous, supramaximal twitch stimulation of the median and ulnar nerves was used in combination with temporary ischaemia of the forearm to characterize the summed ATPase activity. The recovery following restoration of blood flow provided a measure of oxidative ATP synthesis activity. These processes were measured based on the dynamics of creatine phosphate (PCr) content. 2. Muscle oxygen stores were depleted using ischaemia without stimulation as indicated by PCr breakdown after 250 +/- 33 s (mean +/- S.D.; n = 5), which provided a measure of the basal metabolic rate (0.008 +/- 0.002 mM s-1, n = 5). 3. The PCr breakdown rate during twitch stimulation of the oxygen-depleted muscle was constant at 1 Hz at 0.15 +/- 0.03 mM PCr per second or per twitch (n = 8). A constant cost per twitch was found from 0.5 to 2 Hz stimulation (depletion of PCr per twitch = 0.15 mM per twitch). 4. No net anaerobic recovery of PCr was found during a 2 min post-stimulation ischaemia. 5. Upon restoration of blood flow, PCr recovery followed an exponential time course with a time constant of 63 +/- 14 s (n = 8). From these recovery rates, the capacity for oxidative phosphorylation was estimated to be 0.4 mM s-1. 6. This experimental approach defines a non-invasive and quantitative measure of human muscle ATPase rate and ATP synthetase rate.

Development and characterization of an ergometer to study the bioenergetics of the human quadriceps muscle by 31P NMR spectroscopy inside a standard MR scanner

A description is given of an ergometer made of nonmagnetic materials which fits into a standard whole body MR magnet. T2-weighted images show that exercise is highly specific for quadriceps muscles. The ergometer permits the noninvasive study of T2-related changes in the proton images of the leg as well as changes occurring in muscle bioenergetics during exercise and recovery.

31P-NMR study of skeletal muscle metabolism in patients with chronic respiratory impairment

To evaluate the energy metabolism of peripheral skeletal muscle during exercise in patients with chronic respiratory impairment, the 31P-nuclear magnetic resonance (NMR) spectra of forearm muscle were investigated in nine patients and nine age-matched control subjects. We calculated the phosphocreatine (PCr) to PCr + inorganic phosphate (PI) ratio, the time constant of PCr recovery and the intracellular pH. The exercise consisted of repetitive hand grips against a 2-kg load every 3 s for 6 min (0.33 W). The patients showed a marked decrease in the PCr/(PCr + PI) ratio and pH in the muscle during exercise in contrast to the control subjects whose PCr/(PCr + PI) showed a minor decrease without any change in pH. The relationship between PCr utilization and pH demonstrated that anaerobic glycolysis switched on earlier in patients with chronic respiratory impairment. A split PI peak was observed in five of nine patients during exercise. The PCr/(PCr + PI) ratio during the last minute of exercise correlated significantly with the vital capacity (% predicted), with the FEV1/FVC, with the body weight, with the maximum strength of hand grip, and with the muscle mass. The results indicate impaired oxidative phosphorylation and the early activation of anaerobic glycolysis in the muscles of patients with chronic respiratory impairment. Several factors related to chronic respiratory impairment, such as disuse, malnutrition and dysoxia, would contribute to the metabolic changes observed in the muscles examined.

Regional 31P magnetic resonance spectroscopy of exercising human masseter muscle

Changes in fibre structure and function associated with exercise have been quantified ultrastructurally and biochemically in selected limb muscles, but the biochemical events associated with contraction are rarely studied in the human jaw muscles. Here, 31P NMR spectroscopy, or MRS, was used to examine the multipennate masseter in six adult men at rest and while performing isometric clenching exercises. NMR spectra were acquired from three locations within the muscle with a 2 x 3 cm, single-turn, copper receiver coil. The spectra, corrected for partial saturation effects, were quantified on the basis of relative peak area and position. The inorganic phosphate (Pi) to creatine phosphate (PCr) ratio (Pi/PCr), which has been shown to be indirectly related to the phosphate potential and hence the metabolic activity, as well as pH, were calculated for each site and exercise. The mean resting Pi/PCr ratio was greater for the deep part of the muscle than for the superficial and intermediate parts; these differences were significant to p less than 0.01. The mean pH was similar in all parts of the muscle at rest. During exercise, a significant increase in mean Pi/PCr was found in the superficial and intermediate parts of the muscle; both these differences were significant to p less than 0.05. An accompanying decrease in mean pH was observed in all parts of the muscle during exercise. In the superficial part, this decrease was significant to the p less than 0.05 level, and in the deep part, to the p less than 0.001 level. No significant differences were found for these measures between left and right molar clenching.(ABSTRACT TRUNCATED AT 250 WORDS)