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Departments of 1Radiological Sciences, 2Physiological Sciences, and 3Neurobiology, University of California, Los Angeles, California; and 4Department of Biology of Physical Activity, University of Jyväskylä, Jyväskylä, Finland
Submitted 6 September 2005 ; accepted in final form 17 January 2006
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ABSTRACT |
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 TOP ABSTRACT METHODSRESULTSDISCUSSIONGRANTSACKNOWLEDGMENTSREFERENCES |
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atrophy; Achilles tendon; strain; unilateral lower limb suspension; magnetic resonance imaging
Force produced by the muscle fibers is transmitted to the skeletal system via transmembrane proteins of skeletal muscle fibers and connective tissue structures, including the interfiber matrix, aponeuroses, and tendon. Current geometric MTC models (23, 47) suggest that the morphological arrangement of tendinous structures within an MTC plays an important role in force transmission to the bony skeletal system (24). Undoubtedly, the passive force-transmitting elastic (40, 43, 49) properties as well as the viscoelastic properties of musculotendinous structures play important roles in energy storage and recycling in enhancing joint performance and efficient power production (3, 10, 41). It has been shown, for example, that the elasticity of tendon has an effect on the optimum length of MTC in the isokinetic torque-angle relationship (27). We hypothesize that these complex interactions between muscle and tendon are modified following chronic absence of weight bearing. Such changes have significant implications for the functions of the MTC as well as the involved joint(s) performance.
It has been demonstrated that structural stiffness and failure load decreased significantly following chronic hindlimb unloading in rat and rabbit Achilles tendons (ATs) (4). Similar results were reported in ultrasound studies for in vivo human vastus lateralis (29) and gastrocnemius tendinous structures (39). A more recent study by Reeves et al. (39) using ultrasound imaging technique demonstrated that the stiffness of the gastrocnemius tendinous structure decreased from 124 to 52 N/mm (58% decrease) following a 90-day bed rest along with 28% decrease in maximum voluntary isometric force, with, however, no change in the tendon cross-sectional area (CSA). Although these studies provided valuable information, they were limited by several major factors: nonphysiological conditions in in vitro and in situ animal studies do not mimic or translate easily to in vivo conditions, and limited visibility of the ultrasound imaging technique requires further improvement to simultaneously visualize the whole tendinous structure in a MTC.
The nonuniform characteristics, particularly the heterogeneous distribution of strain within the aponeurosis along the length of the muscle (19, 48) and heterogeneous mechanical properties of aponeurosis and tendon (17, 31, 33), suggest the importance of studying large segments of musculotendinous structures in understanding in vivo muscle function in humans, as can be done with MRI (6, 11–14, 44).
The purpose of this study was to quantify the changes in the structural and mechanical properties of the whole aponeurosis and tendon in human skeletal muscle in vivo following chronic limb unloading and atrophy, induced using a 4-wk unilateral lower limb suspension (ULLS) model (8). The changes in the volume of the different muscle compartments and the strain properties of the AT and aponeurosis were quantified using a combined method of high-resolution and velocity-encoded, phase-contrast (VE-PC) MRI and magnetic resonance (MR)-compatible dynamometry techniques.
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METHODS |
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TOPABSTRACTMETHODSRESULTSDISCUSSIONGRANTSACKNOWLEDGMENTSREFERENCES |
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Healthy subjects underwent 4-wk ULLS (8) to induce localized atrophy and reduced functional capability in the lower leg muscles. The baseline measurements were taken 1 day before the start of ULLS (Pre-ULLS). We determined the maximum ankle plantar flexion torque [before maximum voluntary contraction (MVCpre)] for the assessment of muscle strength, static morphological MRI of the lower leg for the determination of CSA and muscle volume, and VE-PC MRI of the lower leg during submaximal ankle plantar flexion for the estimation of strain of tendinous structures (aponeurosis and tendon) of the soleus (Sol). Immediately after the ULLS (Post-ULLS), we obtained the same parameters as in the baseline measurements. Following the ULLS, subjects underwent 6 wk of standard rehabilitation therapy besides normal ambulatory activities, and the return to normalcy was confirmed by measuring muscle strength at the end of the rehabilitation period.
Subjects
Eight healthy subjects (5 men and 3 women; 28 ± 7 yr; 171.8 ± 8.6 cm; 73.7 ± 10.3 kg) were recruited for this study. Subjects had no history of lower extremity orthopedic pathology or cardiovascular disease and were able to perform normal ambulatory activities. At the time of recruitment, we interviewed volunteers and excluded those who were involved in vigorous physical training at the level of competitive athletics for the previous 3 mo. Once recruited, subjects were instructed to refrain from any type of vigorous weight-bearing exercise before the start of the experiment. Before the experiment, detailed information of the experimental procedures and possible risks of the procedures were explained to the subjects, and written, informed consent was obtained. Experimental protocols used in this study were approved by the Institutional Review Board of the University of California, Los Angeles.
ULLS
ULLS (8) is designed to minimize weight bearing and activities in one of the subject's lower legs. The duration of the ULLS intervention was chosen to be 4 wk, based on the observations in previous studies that significant muscle atrophy occurred following 3
4 wk of unloading (8, 42). One of the subject's legs, which was chosen freely by the subject, was raised from the ground with the knee joint maintained at 120° using an adjustable sling, which was anchored at the subject's waist and linked to the foot of the suspended leg. Extra lifting of the suspended leg was produced by a high-platform (5 cm) shoe worn on the contralateral leg. All ambulatory activities during the ULLS were achieved using only the contralateral leg, together with the provided crutches. The subjects' compliance was monitored by daily telephone interview and a weekly journal that reported detailed activities and any symptom of complications. Subjects' physical health was monitored weekly by a designated physician, to preempt any possible ULLS-related complications, such as deep venous thrombosis (9).
Muscle Strength Measurement
To estimate isometric ankle plantar flexion torque, a custom-designed apparatus was constructed using a fiberglass cast equipped with an optical Fabry-perot interferometer strain gauge system (Fiberscan 2000, Luna Innovation, Blacksburg, VA) (Fig. 1). Fiberglass was molded to accommodate each subject's posterior half of the lower leg and the sole of the foot and to immobilize the ankle joint at 90°. The foot and ankle regions of the cast were reinforced by soaking the cast in epoxy resin. The strain gauge was embedded in the sole of the cast for the measurement of ankle plantar flexion torque. Each cast was calibrated with known weights. The strain gauge signal was digitally sampled at 200 Hz.
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Acquisition of MRI
In this study, the subjects were scanned in two batches: in the first (n = 4), MR images were acquired on GE 1.5-T LX system (General Electronics, Milwaukee, WI), with FastCard 9.3 software, whereas for the second series (n = 4), a Siemens 3.0-T Trio (Siemens, Malvern, PA) research scanner was available for acquiring the scan, with higher quality of images (because of the higher magnetic field). Both the detailed protocols as well as the explanatory physics behind the MR techniques are described in previous publications (18, 19, 44) and are only briefly described here.
Preparation. Before the acquisition of MRIs, the subject was asked to remain in a supine position for at least 30 min to minimize fluid shifts of the lower extremity, which may affect muscle volume (15, 21, 38). After the fluid stabilization procedure, the testing leg was placed on the muscle strength measurement apparatus and tightly secured with nonelastic surgical tape. Then, the subject was placed on a MR scanner bed in a feet-first, supine position. The leg to be tested was placed inside the head coil in the GE scanner and the spine coil in the Siemens scanner and aligned with the long axis of the coil using the laser crosshair positioning facility of the MR scanner. The ankle joint angle was set to 90° in the muscle strength apparatus, and the knee joint was fixed in a fully extended position. The center of the patella bone and a midpoint of malleoli of the ankle were used as external anatomical landmarks to align the leg, thereby ensuring consistency of the lower leg position in different scanning sessions. Once properly aligned, the leg was tightly secured to the bed using nonelastic straps, sponge wedges, and sand bags.
Static morphological image.
For a relaxed condition, static morphological, high-resolution MR, proton-density-weighted images, particularly suited to show differentiation between different muscle compartments, were first obtained to estimate CSA and to produce a model for the three-dimensional (3D) volume estimation of the triceps surae muscles. We acquired high-resolution axial MR images of the lower leg covering from the sole of the foot to the distal end of the femur. This was done by taking two separate shots: the distal shot covered from the foot to the insertion region of the medial gastrocnemius (MG), and the proximal shot included the region from the insertion region of the MG up to the knee. On the GE scanner, a fast-spin echo sequence was used, with echo (TE)-to-repetition time (TR) ratio of 17 to 4,500 ms, respectively, echo train length of 8, a matrix size of 256 by 256, two averages, 220-mm field of view (FOV), slice thickness of 7 mm separated by 1.5 mm, with 34 slices acquired in 5 min. For the higher 3-T Siemens scanner, because of the better signal-to-noise ratio (quality of image), these images were acquired with faster sequences called field echoes, with TE-to-TR ratio and flip angle of 2.65 to 140 ms and 45°, respectively, a matrix size of 256 by 192, two averages, 180-m by 135-mm FOV, 5.5-mm-thick contiguous slices, with 40 slices acquired in 2 min.
Dynamic VE-PC MRI.
Using the technique of VE-PC MRI, the mechanical behaviors of the musculoskeletal system have been investigated, both in terms of anatomical and dynamic characteristics during contractions and joint motions, in a noninvasive way and simultaneously over a relatively large FOV (7, 11–14, 18, 44). The detailed background of the VE-PC MRI was described elsewhere (19, 44) and is described very briefly here. Conventional MRI acquires magnitude images that exhibit excellent soft tissue contrast arising from the differences in the (proton) density of each different tissue and their microscopic magnetic relaxation properties (longitudinal relaxation time and transverse relaxation time), which reflect their microscopic physiochemical properties. The inherent flexibility of the physics of this technique further allows quantification of the motion or velocity of moving protons, such as those in arteries and veins, the heart, or, in our case, moving muscles of the lower leg during isometric contraction. This technique of VE-PC MRI sensitizes the motion of the protons in any or all of the three directions by means of velocity-encoding gradients. This results in mapping the quantitative value of the velocity to an angular phase degree, with 2
radians mapped to the maximum velocity one expects in the experiment. The movement of the different anatomical points of the muscles during different phases of the contraction cycle can be visualized by collecting about 20 images during the cycle. Unlike the almost instantaneous acquisition of images in the ultrasound technique, MRI unfortunately requires the acquisition of typically 256 views or "phase-encoding levels." In the case of imaging of moving anatomy, each of these levels has to be acquired during one contraction cycle and is initiated (or gated) by a chosen threshold point in the rising part of the torque curve, such that the total time required to collect the entire set would be 256 times the time period of contraction. This total time can be reduced by various modifications of the acquisition scheme.
For the acquisition of VE-PC MRI, the slice location in the midsagittal plane of the lower leg was chosen to bisect the AT and to visualize the Sol, MG, and flexor hallucis longus. The subjects performed repetitive isometric ankle plantar flexion-relaxation cycle with a rhythm of about 40 times/min, following an audio cue generated by an offline computer and fed via headphones. The torque level of submaximal ankle plantar flexion used in this study was 20% of MVCpre. To ensure that the subject reached the requisite target level of torque consistently during each of the contraction cycles, a visual feedback was provided to the subject by digitally projecting on the front panel of the MRI scanner, a trace of the force exerted by the subject during each cycle (which was sensed by the force transducer) and the target 20% torque level. The real-time torque signal was also used to gate the MR scanner during cine VE-PC MRI scanning. The imaging sequence used in this study to quantify muscle motion during isometric contraction was (on both scanners) a VE-PC field echo, retrospectively gated to a threshold in the rising part of the torque cycle, with four views per segment, requiring 70 repetitions of the contraction cycle to yield 20 phases per cycle with 20 phase or velocity images and 20 magnitude or morphological images. The velocity was encoded only in the superior/inferior with a maximum velocity of 10 cm/s (in other words, for the results reported here, our velocity images were sensitive to that component of the motion of the muscle that moved in the superior-inferior direction, with the maximum velocity presumed to be 10 cm/s, which was mapped to the maximum of 2 radians). In terms of the details of the parameters used, these images were acquired (on both the scanners), with TE of 4.9 ms (GE) and 7.6 ms (Siemens), TR of 60 ms (depending on how long each contraction cycle was), flip angle of 30°, a matrix size of 256 by 192, three averages, 320 mm by 240 mm FOV, 5-mm-thick contiguous slices, with acquisition time of 2 min (depending on the length of the contraction cycle).
Data Treatment and Analysis
Muscle volume. Static morphological MRI was used for 3D volume rendering of the lower leg MTC in an offline image processing workstation (Vitrea 2, Vital Images, Minneapolis, MN). The axial morphological MR images were transferred to the workstation, the CSA of the target muscles was digitally traced in each image, and the muscle volume was estimated from 3D-rendered muscle models. We used the distal and proximal shots, as described in the previous section, to estimate the volume of the Sol and the MG and lateral gastrocnemii (LG).
Strain of aponeurosis and tendon. The VE-PC MRI yielded two sets of 20 images: one with anatomical information and the other with velocity information. The VE-PC MRI was analyzed using a custom-developed image processing program under LabView IMAQ environment (National Instrument, Austin, TX). The velocity images were first corrected after the calibration process using two steps: averaging the 20 velocity images and subtracting it from each image, and smoothing images by 3 x 3-pixel averaging (44).
Using the first anatomical image, we chose 3 x 3-pixel regions of interest (ROI) at myotendinous junctions, which were the positions on the muscle that were closest to the aponeurosis, along the posterior aponeurosis of the Sol, typically at every 10 pixels (12.5 mm) [see Fig. 1 in Finni et al. (18) and Fig. 2A]. By using this method, we estimated the aponeurosis movement. Then the movements of ROIs were digitized throughout the 20 images in the following manner. From the first velocity image, the velocities of ROIs were obtained. Then the locations in the second image were estimated by the product of the time interval between two images (75 ms) and the velocities of the ROIs, thereby allowing the relocation of the ROIs in the second image. When the relocated ROI position lay between the two pixels, we took the weighted velocity of two pixels in proportion to the areas of the relocated ROI in the subsequent calculations. This procedure allowed us to calculate muscle tissue movement with a subpixel resolution (<1.25 mm). From the estimated displacement of ROIs, we calculated the regional strain [(L – L0)/L0] by using the distance between two adjacent ROIs (L) at given temporal phase and the resting length (L0) during a cycle of the contraction-relaxation cycle. Following this procedure, we normalized the estimated data so that the 11 ROIs represent the whole aponeurosis below the MG insertion. The AT length was determined as the distance between the ROIs in the most distal ROI in the Sol and the calcaneus.
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While this study was not intended to test the effects of rehabilitation therapy, subjects were provided standard rehabilitation therapy subsequent to the ULLS intervention. They participated in a 30-min rehabilitation session 3 times/wk for 6 wk. The rehabilitation program included strength, balance, and stretch exercise. We did not restrict subjects from performing other types of exercise or ambulatory activities in addition to the provided rehabilitation therapy.
Statistics
Statistical significance of the measured variables was tested using nonparametric Friedman's test and Wilcoxon's sign test at the level of 
= 0.05. The group values are presented with means ± 1 SE.
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RESULTS |
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TOPABSTRACTMETHODSRESULTSDISCUSSIONGRANTSACKNOWLEDGMENTSREFERENCES |
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Maximum muscle strength of the triceps surae was estimated by measuring maximum ankle plantar flexion torque (n = 8). Immediately following 4-wk ULLS (Post-ULLS), MVC decreased to 51.9 ± 6.7% of MVCpre (P < 0.05) (Fig. 3). Following the 6-wk recovery period with uncontrolled standard rehabilitation therapy and normal ambulatory activities, MVC recovered to the baseline level (105.2 ± 7.6% MVCpre).
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Muscle volume was determined by using 3D-rendered muscle models (Fig. 4A). The 4-wk limb suspension decreased the triceps surae (Sol, MG, and LG combined) volume from 723.5 ± 56.1 to 679.3 ± 56.1 ml (P < 0.05: 6.2% loss, Fig. 4B). The relative volume of each muscle to the combined volume (Sol + MG + LG) altered following the ULLS (Fig. 4B). For individual muscles, the Sol volume decreased from 425.7 ± 26.5 to 403.8 ± 27.9 ml (P < 0.05: 5.5% loss), and the MG decreased from 191.1 ± 17.9 to 175.8 ± 16.7 ml (P < 0.05: 7.5% loss), but the LG volume did not change (Fig. 4C).
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The strain distribution of aponeurosis along the length of the posterior Sol was determined when torque reached the peak during the contraction-relaxation cycle at 20% MVCpre. Furthermore, the strain characteristics of two distinct regions, the mid- and distal aponeurosis, divided based on the morphology, and the strain distribution were examined (Figs. 2 and 5).
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DISCUSSION |
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TOPABSTRACTMETHODSRESULTSDISCUSSIONGRANTSACKNOWLEDGMENTSREFERENCES |
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Muscle Volume and Strength
The extent of decrease in muscle volume observed in this study was 6%, which was similar to a 7% decrease in a study that used a 21-day ULLS (40), but somewhat smaller than the 12–13% decrease observed in a bed-rest study with a similar unloading period (2, 29). Considering 48% decrease in isometric ankle plantar flexion torque, the loss of muscle volume (7% on average), at least for the 4-wk ULLS, is probably a minor factor influencing the decline in muscle strength (Fig. 6 ). The present study did not investigate neural changes and, therefore, is unable to provide direct evidence. However, there have been consistent observations that neural activation is affected more rapidly and to a greater degree than muscle atrophy in response to unloading. It has been showed that, in a 20-day bed-rest study, decrease in muscle activation, rather than physiological CSA, was closely related with the loss of muscle strength (26).
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28%) following 6-wk ULLS (21). However, the trend in differential decrease in muscle volume, without considering statistical significance, might be more pronounced with longer ULLS duration. In addition to different ULLS duration, the volume estimation method used in the study of Hather et al. (21) was averaging CSA of the muscles in 14 axial slices, while the present study used 3D volume rendering of the muscles using 20–24 axial slices, which probably influenced the difference in the results between two studies.
Four weeks of ULLS induced 48% reduction in ankle plantar flexor strength. The extent of the plantar flexor strength loss observed in this study was much greater than the 17% reduction obtained in a 21-day ULLS study (42) and 20% observed in other studies using similar unloading duration but within a bed-rest model (2, 30). The subject who showed the greatest loss in muscle strength (67%) also showed exceptionally high muscle atrophy (14%), illustrating significant extent of intersubject variability of muscle atrophy. However, this variation is probably due to typically large variations in the plantar flexor strength loss, as reported in studies following long-term unloading (see for a review Ref. 1).
Mechanical Properties of the Aponeurosis and the AT
Tissue velocity measures of the Sol MTC using VE-PC MRI enabled us to estimate the deformation of the aponeurosis of the Sol MTC during a submaximal contraction. One of the strengths of this technique compared with others, such as the ultrasound imaging technique, is that it allows one to visualize the entire length of the aponeurosis, tendon, and even the calcaneus. This allowed us to determine that chronic unloading significantly influence the strain distribution of the Sol aponeurosis.
In the normal condition (i.e., Pre-ULLS), the nonuniform strain pattern of the aponeurosis observed in the present study corresponds to that reported earlier by our group in a cross-sectional study (19). Positive strain (lengthening) was observed in the midregion of the Sol aponeurosis while the distal region shortened when torque reached peak during a submaximal contraction (Fig. 5). This is also in agreement with observations in other studies of nonuniform aponeurosis deformation reported for human tibialis anterior (32), human Sol (19), and rat MG (48). A contradictory observation (34) of uniform aponeurosis strain emphasizes the importance of the method of imaging (technique) and necessity of in vivo whole MTC observation. It is clear that the aponeurosis-AT complex possesses a complex structure (18). The strain distributions observed seemed to be linked morphologically in the border region, which is indicated with an arrow in Fig. 5, to the region where the anterior protrusion of the Sol aponeurosis emerges (Fig. 2), suggesting that the structural property is a possible source of the nonuniform strain distribution. In the present study, we were able to make measurements over the entire aponeurosis length, and this may be the explanation for the apparent difference with the uniform strain measurements made using ultrasound studies.
The strain distribution changing from positive to negative in the midportion of the aponeurosis following ULLS intervention might be explained by several mechanisms. There could have been a regional change in stiffness of the Sol aponeurosis. A decrease in stiffness of tendinous structures following chronic unloading has been observed in laboratory animal’s and human AT (4, 29). Chronic unloading may also change architectural parameters, such as muscle fiber length and pennation angle (36). The altered architectural changes might modulate the direction of force applied to the aponeurosis after ULLS. Changes in the extramuscular myofascia in muscle force transmission (25) also might influence the strain characteristics differentially in selected regions of the aponeurosis. Spatial changes in neural recruitment strategies in either a proximal-distal orientation or a mediolateral orientation could erratically contribute to the observed regionality (35).
Combined effects of the possible mechanisms mentioned above might be important factors to increase probability of injury of the atrophied triceps surae MTC. It has been generally referred that eccentric contraction induces muscle injury and that excessive loading with higher loading rate is the primary cause of tendon injury. Specifically, it has been suggested that the mechanisms of the AT injury include abrupt forced plantar flexion of the foot, unexpected dorsiflexion of the foot, and violent dorsiflexion of a plantar-flexed foot (5). When the atrophied triceps surae MTC encounters the situations requiring emergency maneuver, it is intuitively obvious that the atrophied triceps surae MTC with deteriorated function is much more prone to injury. Regarding this issue, further investigations will shed light into the development of the injury prevention program.
The AT strain was 2.1% at 20% MVCpre, both before and after the ULLS intervention. A recent study by Reeves at al. (39) reported that tendon strain at maximal plantar flexion changed from 5.5% at baseline to 7.0% following the 90-day bed rest, even with decreased tendon stress. The source of the difference compared with the present study seems to be merely the force level used to assess the tendon strain in the study of Reeves et al. Considering 2% of the tendon strain at 3 MPa (20%) stress level, shown in Fig. 5 of the study of Reeves et al., 2.1% strain at 20% MVCpre in the present study does not seem to be significantly different. However, the present study did not observe meaningful change in tendon stiffness following 4-wk ULLS, whereas Reeves et al. did following 90-day bed rest (17% increase in tendon strain), even with exercise intervention. This different effect of unloading on the tendon property is probably due to the different duration of unloading period and method. Kubo et al. (29) showed a decrease in tendon stiffness following 20-day bed rest. However, the experiment was performed on the vastus lateralis, and thus direct comparisons with the present study do not seem to account for the difference in observations.
Advantage and Limitation of MRI Technique
The present study with a FOV of 32 cm in the proximo-distal direction allowed us to image the entire length of the lower leg from the heel to the origin of the Sol. Furthermore, the movements of the whole aponeurosis and the AT could be analyzed with respect to the movements of the heel. However, the most obvious drawback of VE-PC MRI technique is the number of contractions required to acquire a set of cine MR images. This protracted exertion restricted the muscle contraction intensity, usually up to 40–50% MVC, and was particularly debilitating in the immediate Post-ULLS state. This disadvantage can conceivably be minimized in the future by the enhancements of the MRI software and hardware, which may allow much more rapid imaging, possibly even in a single shot (6). We did not utilize this image strategy in the present study because of the considerably lower time resolution, i.e., much less number of frames possible within the contraction cycle. A combination of higher magnetic field (3 T), parallel imaging, and dedicated leg coil is likely to enhance sensitivity in future studies.
In conclusion, the 4-wk ULLS induced a decrease in plantar flexor muscle volume and strength. However, it appears that the decrease in muscle size is unable to explain the much greater loss of muscle strength. We further observed that ULLS affected heterogeneously the mechanical behavior of the Sol aponeurosis: while the strain distribution was not changed in the distal aponeurosis, significant alteration was observed in the mid-aponeurosis after the ULLS. On the other hand, no change in AT strain was observed. These data demonstrate a clear and significant change in distribution of strain within the Sol aponeurosis during submaximal isometric contractions as a result of prolonged absence of weight-bearing. These results raise the issue as to whether there is an increase in the probability of strain injuries within the triceps surae following chronic unloading in those regions of this muscle complex in which unusual strains occur. Further studies will be necessary to determine whether this redistribution of strain is due to altered anatomical properties of active and passive tissues and/or active physiological properties of muscle and its neural control system.
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TOPABSTRACTMETHODSRESULTSDISCUSSIONGRANTSACKNOWLEDGMENTSREFERENCES |
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ACKNOWLEDGMENTS |
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TOPABSTRACTMETHODSRESULTSDISCUSSIONGRANTSACKNOWLEDGMENTSREFERENCES |
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Present address of S. Sinha; Dept. of Radiology, University of California San Diego, School of Medicine, 410 Dickinson St., San Diego, CA 92103-8756 (e-mail: shsinha{at}ucsd.edu).
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FOOTNOTES |
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The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
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