Purpose The strain rate (SR) tensor measures the principal directions and magnitude of the instantaneous deformation; this study aims to track age related changes in the 2D SR tensor in the medial gastrocnemius during passive joint rotation and active isometric contraction. Methods SR tensors were derived from velocity encoded magnetic resonance phase-contrast images in nine young (28 yrs) and eight senior (78 yrs) women. Strain rates along and in the cross-section of the fiber were calculated from the SR tensor and used to derive the out-plane SR. Age related and regional differences in the SR eigenvalues, orientation, and the angle between the SR and muscle fiber (SR-fiber angle) were statistically analyzed. Results SR along the fiber was significantly different between the cohorts during isometric contraction with higher values in the young (P<0.05). The SR-fiber angle was larger in the young for both motion types but this difference was not statistically significant. Significant regional differences in the SR indices was seen in passive joint rotation (P<0.05) for both cohorts. Conclusion SR mapping reflects age related and regional differences during active and passive motion respectively; this may arise from differences in contractility (active motion) and elastic properties (active and passive motion).

The purpose of the present study was to investigate recruitment patterns of the thigh muscles during maximal sprint cycling by muscle functional magnetic resonance imaging (mfMRI). Twelve healthy men participated in this study and performed 2, 5, and 10 sets of 6-s supramaximal cycling with a load of 7.5 % of their body weight with 0.5 min of rest between the sets. Before and immediately after the exercise, T2-weighted MR images, i.e. mfMRI, of the right-thigh were taken to calculate T2 of eleven thigh muscles. Vastus lateralis, semitendinosus, and sartorius were the highest activated, i. e. had the greatest T2 change, among the quadriceps, hamstring, and adductors, respectively, compared with other muscles. Total power output during 2, 5, and 10 sets of sprint cycling was correlated with percent change in T2 in the quadriceps correlated (r (2) = 0.507 to 0.696, p < 0.01), the hamstring (r (2) = 0.162 to 0.335, p < 0.05 approximately 0.001), and the adductor muscles (r (2) = 0.162 to 0.473, p < 0.05 approximately 0.0001). With use of stepwise regression analysis, total power output was significantly correlated with % change in T2 of the vastus medialis (VM) (p < 0.0001) and vastus intermedius (VI) (p < 0.05) (r (2) = 0.698, p < 0.0001). We concluded that eleven thigh muscles were activated non-uniformly, and that the VM and VI play a key role during maximal sprint cycling.

The purposes of this study were 1) to quantify the volume of activated parts within a whole muscle and 2) to examine activated area distributions along the length of muscle. Seven male subjects performed five sets of 10 repetitions of a single-leg calf-raise exercise with the knee fully extended. Transverse relaxation time (T2)-weighted spin echo images were acquired before and immediately after the exercise. A range of pixels with a T2 greater than the mean +1 SD of the region of interest (ROI) from the preexercise image and pixels with a T2 lower than the mean + SD of the ROI from the postexercise image were defined as "active" muscle. The active muscle images were three dimensionally reconstructed, from which the volume of the activated muscle was determined for individual triceps surae (TS) muscles. Our data indicate that approximately 46% of the medial gastrocnemius (MG) muscle was activated during the exercise, with activation of the lateral gastrocnemius (LG) and soleus (Sol) muscles being approximately 35%. In the MG, distal portions had a greater percentage area of activated muscle than the proximal portions (P < 0.05), which was consistent with the results regarding electromyogram activity. In contrast, regional activation differences were not observed in the LG and Sol. These findings suggest that the amounts of activated muscle and its distribution would be different among TS muscles.

These results suggest that 1) mfMRI signals and iEMG activity correlate with workload in individual TS muscles, 2) mfMRI signals are associated with neuromuscular activity reflected in iEMG in the MG and SOL but not in the LG, and 3) these relationships are associated with neuromuscular and metabolic factors during exercise.

ton VR, Sinha S. Phase-contrast MRI reveals mechanical behavior of superficial and deep aponeuroses in human medial gastrocnemius during isometric contraction. J Appl Physiol 105: 1312-1320, 2008. First published August 14, 2008 doi:10.1152/japplphysiol.90440.2008.-The behavior of the entire medial gastrocnemius (MG) superficial and deep aponeurosis structure was investigated with velocity-encoded phasecontrast, spin-tag, and three-dimensional morphometric magnetic resonance imaging. The displacements and strain of both these aponeuroses, muscle length, and the cross-sectional segment length of the deep aponeurosis were measured during isometric plantarflexion at 20% and 40% of maximal voluntary contraction (MVC). The length of the entire MG shortened during 20% and 40% MVC. All regions of interest in both aponeuroses moved proximally. Positive strain (lengthening) occurred in both ends of the deep aponeurosis and in the proximal region of the superficial aponeurosis. In contrast, negative strain (shortening) was observed in the middle region of the deep aponeurosis and in the distal region of the superficial aponeurosis. Consistent with this shortening of the deep aponeurosis length along the proximal-distal axis was expansion of the aponeuroses in the medial-lateral and anterior-posterior directions in the cross-sectional plane. It is concluded that at low to moderate force levels of isometric contraction, regional differences in strain occur along the proximal-distal axis of both aponeuroses, and some regions of both aponeuroses shorten. strain; skeletal muscle; morphology; velocity-encoded phase-contrast magnetic resonance imaging; aponeurosis DURING HUMAN MOVEMENT, force produced by muscle fibers is transmitted through tendinous tissue, that is, tendons and aponeuroses, to bones since skeletal muscle is attached to the bones via tendinous tissues at both proximal and distal ends of the muscle. These tendinous tissues play an important role as series elastic components and can store elastic energy during the movement (1, 4). Ultrasonographic techniques have suggested that during an isometric contraction the superficial and deep aponeuroses of medial gastrocnemius (MG) are homogeneously stretched along their lengths in opposite directions; the superficial aponeurosis is stretched distally, whereas the deep aponeurosis is stretched proximally (13,14). A rather different conclusion was drawn in other references in the literature (16,24,25), which pointed out that the strain properties of aponeurosis may be more complex than the ultrasound data suggest. Direct measurements of aponeurosis strain in contracting rat gastrocnemius muscle indicate inhomogeneous strain (25), and several models of contracting unipennate muscle also indicate inhomogeneous strain in the aponeuroses (16,24).Phase-contrast (PC) magnetic resonance imaging (MRI) has been used to investigate the mechanical behavior of the muscle and tendinous tissue during muscle contraction (5)(6)(7)20). The PC MRI technique has the advantage of superior sof...

Endo MY, Kobayakawa M, Kinugasa R, Kuno S, Akima H, Rossiter HB, Miura A, Fukuba Y. Thigh muscle activation distribution and pulmonary V O2 kinetics during moderate, heavy, and very heavy intensity cycling exercise in humans. Am J Physiol Regul Integr Comp Physiol 293: R812-R820, 2007. First published April 25, 2007; doi:10.1152/ajpregu.00028.2007.-The mechanisms underlying the oxygen uptake (V O2) slow component during supra-lactate threshold (supra-LT) exercise are poorly understood. Evidence suggests that the V O2 slow component may be caused by progressive muscle recruitment during exercise. We therefore examined whether leg muscle activation patterns [from the transverse relaxation time (T2) of magnetic resonance images] were associated with supra-LT V O2 kinetic parameters. Eleven subjects performed 6-min cycle ergometry at moderate (80% LT), heavy (70% between LT and critical power; CP), and very heavy (7% above CP) intensities with breath-by-breath pulmonary V O2 measurement. T2 in 10 leg muscles was evaluated at rest and after 3 and 6 min of exercise. During moderate exercise, nine muscles achieved a steady-state T2 by 3 min; only in the vastus medialis did T2 increase further after 6 min. During heavy exercise, T2 in the entire vastus group increased between minutes 3 and 6, and additional increases in T2 were seen in adductor magnus and gracilis during this period of very heavy exercise. The V O2 slow component increased with increasing exercise intensity (being functionally zero during moderate exercise). The distribution of T2 was more diverse as supra-LT exercise progressed: T2 variance (ms) increased from 3.6 Ϯ 0.2 to 6.5 Ϯ 1.7 between 3 and 6 min of heavy exercise and from 5.5 Ϯ 0.8 to 12.3 Ϯ 5.4 in very heavy exercise (rest ϭ 3.1 Ϯ 0.6). The T2 distribution was significantly correlated with the magnitude of the V O2 slow component (P Ͻ 0.05). These data are consistent with the notion that the V O2 slow component is an expression of progressive muscle recruitment during supra-LT exercise. oxygen uptake; slow component; T2 time; muscle use patterns PULMONARY OXYGEN UPTAKE (V O 2 ) during moderate intensity, constant-load cycling exercise below the lactate threshold (LT) approaches a steady state with an exponential time course (phase II) after a short delay (phase I) and attains a steady state within 2-3 min. However, during exercise at supra-LT work rates, the V O 2 response is more complex, and the fundamental (phase II) kinetics are supplemented by an additional delayed phase that causes a secondary rise in V O 2 , termed the "excess" V O 2 or the V O 2 "slow component" (30,43). The consequence of this delayed V O 2 slow component is that V O 2 attains levels greater than those projected from the sub-LT V O 2 -work rate relationship (15). Additionally, these supra-LT V O 2 kinetics (as well as the responses of other physiological variables, such as blood lactate and ventilation) differ depending on whether exercise is undertaken below (heavy intensity exercise) or above (very heavy intensity ...

Purpose To design a computer-controlled, MR compatible foot pedal device that allows in vivo mapping of changes in morphology and in strain of different musculoskeletal components of the lower leg under passive, isometric, concentric and eccentric contractions. Materials and Methods A programmable servo-motor in the control room pumped hydraulic fluid to rotate a foot-pedal inside the magnet. Towards validating the performance of the device, six subjects were imaged with gated velocity-encoded phase-contrast (VE-PC) imaging to investigate dynamics of muscle and aponeurotic structures. Results Artifact-free VE-PC imaging clearly delineated different muscle compartments by differences in distribution of mechanical strains. High repeatability of contraction cycles allowed establishing that fascicles lengthened 6.1% more during passive compared to eccentric contractions. Aponeurosis separation during passive (range between three locations: −2.6~1.3 mm) and active (range: −2.4 ~1.6 mm) contractions were similar but significantly different from concentric (range: −0.9~3.3 mm), with proximal and distal regions showing mostly negative values for the first two modes, but positive for the last. Conclusion The device was sufficiently robust and artifact-free to accurately assess, using VE-PC imaging, physiologically important structure and dynamics of the musculo-tendon complex.

This study demonstrated that the values of the muscle torque per unit volume of both injured and uninjured sides of patients with ACL rupture were significantly lower than those of the control group, thereby providing indirect evidence of the hindrance of motor unit recruitment in these patients. The results of the present study also indicate that there may be bilateral QF weakness in patients with ACL rupture. Since persistent QF weakness is a significant barrier to effective rehabilitation in patients with ACL injuries, a better understanding of the underlying mechanisms will allow clinicians and scientists to develop more effective therapeutic strategies for patient rehabilitation.