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Department of Biological Sciences, Marquette University, Milwaukee, Wisconsin 53201
Submitted 13 June 2002 ; accepted in final form 19 June 2003
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ABSTRACT |
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 TOP ABSTRACT METHODSRESULTSDISCUSSIONDISCLOSURESREFERENCES |
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soleus; peak power; electromyography
Gains in muscle hypertrophy and strength are proportional to the intensity of the resistance-training protocol (18). Studies investigating the molecular events underlying mechanotransduction support these findings (6, 23). In particular, intracellular signals known to promote cell growth have been shown to increase early and in a dose-dependent manner with muscle overload or increased tension. We and others have observed that exercise countermeasures employing high-resistance exercise (ladder climbing or uphill running) provided the greatest protection against atrophy and loss of strength during HU (5, 17, 22, 29, 32). The efficacy of these countermeasures is further improved if exercise is administered in brief, multiple daily bouts rather than one long bout (7, 17, 25, 29).
Slow-tonic and fast-phasic muscles are likely to require different exercise countermeasures to optimally protect against muscle atrophy and the loss of function associated with spaceflight. We hypothesize that the slow soleus muscle would be best protected by maintained isometric contractions rather than short-duration, high-velocity isotonic contractions. The primary role of the soleus is to maintain posture against gravity by tonically contracting isometrically. During normal weight bearing, with the knee and ankle flexed, the soleus fibers are at optimal length (Lo), and the motoneuron drive to the soleus is facilitated, whereas that to the gastrocnemius is inhibited (4, 26). An isometric resistance exercise countermeasure that maintained these joint angles should maximally recruit the soleus at its Lo, yielding higher force contractions and thus the greatest degree of protection during HU. In contrast, isotonic contractions of the calf muscle that evoked maximal power would be obtained at a velocity of 20-30% of the maximal velocity of the fast-fiber population, a speed near or at the maximal velocity of the slow-fiber population (33). Consequently, the force output and power developed by the slow fibers would be minimal. Although eccentric contractions elicit the highest force, exercise employing them may be too brief in duration to provide optimal protection for the normally tonically active soleus muscle. In addition, HU further exacerbates the high risk of stretch-induced damage during exercise involving eccentric contractions (2, 30).
To date, there is little information regarding the effectiveness of isometric exercise (IE) as a countermeasure to HU-induced muscle atrophy, and no data regarding whether contractile function is preserved (9). Consequently, the objective of this study was to test the hypothesis that an exercise countermeasure consisting of multiple daily bouts of high-intensity isometric contractions would prevent the atrophy and decline in contractile properties of the soleus during 7 and 14 days of HU. Furthermore, this protection should be greater than that observed in the fast-twitch plantaris and gastrocnemius muscles.
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METHODS |
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TOPABSTRACTMETHODSRESULTSDISCUSSIONDISCLOSURESREFERENCES |
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Exercise-training protocol. IE was performed by placing rats in a 45.72-cm-long, 0.635-cm-thick Plexiglas tube with an inside diameter of 8.89 cm and positioning the tube vertically (Fig. 1). To keep from falling from the tube, the rat was required to wedge itself in the tube by pushing against the inside wall with its hind legs. A 0.318-cm-thick latex mat was secured on one-half of the inside of the tubing to provide the rat with a better grip. The inside diameter of the tube was of the appropriate size to require that the rat's knees be flexed and the ankle dorsiflexed while the rat pushed against the inside wall. This hindlimb posture was chosen to maximally recruit the soleus muscle (4, 26). To document that the contractions were isometric, we filmed each session. At no time during the applied load did we observe any movement of the hindlimb. There was no movement at the ankle joint. To increase the effort by the rat, weight was hung from its tail harness.
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IE was performed three times per day at 8:00 AM, 12:00 PM, and 4:00 PM. Each training bout lasted 15 min, for a total of 45 min/day. For each bout, the rat was removed from HU and placed in the tube while it was horizontal. At minute 0, the tube was placed vertically, and weight was hung from the rat's tail for 5 s. Five-second contractions were elicited based on the observation that in situ tetanic force of the soleus begins to decrease significantly after 
5 s of stimulation at physiological frequency (data not shown). Therefore, contractions were stopped at the time when maximal force was anticipated to drop. Furthermore, rats were unable to maintain maximum effort for >7 s. Immediately after the weight was removed from the rat's tail, the tube was placed horizontally. This procedure was repeated every 90 s, totaling 10 repetitions. The rat was removed from the tube while the tube was still vertical. The remaining 40 s were given to allow the rat time to groom itself before being returned to HU. Therefore, a total of 150 s of contractile activity, above normal weight bearing, was performed per day. The weight hung from the rat's tail was increased progressively from 100% body weight on day 1 up to 400% of body weight by day 13.
In situ procedure. In situ twitch, tetanus, force-frequency, slack test, and force-velocity experiments were performed in that order on the soleus. After the soleus experiments, the plantaris muscle was then connected to the force-position motor, and the experimental protocol was repeated. All contractile measurements were made at a muscle temperature of 34°C. Core temperature was held at 37°C.
The soleus was studied in situ, as described previously, with the following modifications (24). A polyethylene tracheotomy tube (PE90, Intramedic, Franklin Lakes, NJ) was surgically inserted into the trachea, and dry gas, 95% O2-5% CO2, (Praxair, Milwaukee, WI) was blown across the end of the tube. The soleus and plantaris muscles were isolated by using care to leave the blood and nerve supply intact. A silk thread (2-0) was secured to the distal tendon, and a loop was tied and used to attach the muscle to a small hook on the end of the arm of a combination position and force transducer. The soleus was analyzed by using either transducer model 352 (Cambridge Technology, Cambridge, MA) or 309B (Aurora Scientific, Aurora, Ontario), whereas the plantaris muscle was analyzed by using model 309B transducer. A rat Ringer drip, maintained at 41°C, kept the muscles moist during the experiment. The sciatic nerve was drawn into a suction electrode, and supramaximal square-wave-positive and biphasic pulses were administered to elicit twitch and tetanic contractions, respectively. The output from the transducer was amplified and sampled at 3.3 kHz by a Pentium computer by using custom-made software.
Twitch and tetanus. Lo was determined for each muscle by eliciting twitches (pulse duration: 500 µs) and lengthening the muscle until peak twitch force (Pt) plateaued. The amount of passive tension at Lo was displayed on a Tektronix oscilloscope (model RM200). Contraction time (CT) was measured as the time from the onset of force development to peak force. One-half relaxation time (
RT) was the time required for force to decay to one-half of the maximum twitch value. Peak tetanic tension (Po) was elicited by stimulation at 120 Hz for >500 ms for the soleus and 160 Hz for >300 ms for the plantaris. For both the twitch and tetanus, the maximum rate of force development (+dP/dt) and decay (-dP/dt) were determined by measuring the peak slope of the contraction and relaxation, respectively. A rest period of 1.5 min was given between twitches and 2.0 min between tetanus stimulations.
Force frequency. The stimulation frequencies used to establish the force-frequency relationship were 5, 10, 20, 40, 60, 80, 100, 120, 140, and 160 Hz for the soleus and 10, 20, 40, 60, 80, 100, 120, 140, 160, 180, and 200 Hz for the plantaris. The rest interval between contractions was 2.0 min.
Maximal unloaded shortening velocity. Maximal unloaded shortening velocity (Vo) was determined by using the slack test method. After the development of peak isometric force (Po), the muscle was rapidly slacked to a predetermined length such that force dropped to zero. The muscle then shortened under zero load, until tension redeveloped. The time of unloaded shortening was determined by computer analysis. At least five different length steps were used, none that were >20% of fiber length (FL). The duration of unloaded shortening was plotted as a function of the slack distance, and a best-fit line was determined. Muscle velocity was calculated from the slope of the line and expressed as muscle lengths (ML) per second (ML/s). Velocity in FL per second (FL/s) was calculated for the soleus by dividing the velocity in ML/s by 0.58, the FL-to-ML ratio of the soleus muscle (34).
Force-velocity and power curves. After the slack test, the force-velocity relationship was determined, as described previously (33). At the plateau of peak tetanic contraction, the computer switched the ergometer from length to force control, and the load on the muscle was rapidly stepped to maintain three loads less than peak force. Force was held at each load for 100 ms for the soleus and 40 ms for the plantaris, during which time the change in ML was monitored. The computer calculated the velocity of shortening from the slope of the length change during the last one-half of each load step (see Fig. 6, inset). This procedure was repeated four to six times, such that 12-18 different loads were studied for each muscle. Loads were expressed as a percentage of peak force. Vmax was calculated by the straight-line form of the Hill equation (Po - P)/V = P/b + a/b by using loads 
30% of Po, where P is force, V is velocity, and a and b are constants with dimensions of force and velocity, respectively. A hyperbolic curve was fit to the data by using the Hill equation (P + a)(V + b) = (Po + a)b (19).
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Muscle power was calculated from the fitted force-velocity parameters and the maximum isometric force that was developed during the experiment (33). Absolute power was defined as the product of force (in mN) and shortening velocity for the plantaris (ML/s) and for the soleus (FL/s), yielding a final value of milli-Newtons per ML per second and milli-Newtons per FL per second for the plantaris and soleus muscles, respectively. Normalized power of the soleus muscle was defined as the product of force per cross-sectional area (CSA) (kN·m2) times velocity (FL/s), yielding final units of kilo-Newtons per square meter per FL per second. Mean force-velocity and force-power curves for each group were constructed by calculating muscle velocity and power values at 1% intervals between 0 and 100% of Po (see Figs. 6 and 7). After each in situ experiment, the length of the soleus and plantaris muscles at Lo was measured by using calipers.
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Single-fiber diameter. Single fibers from the soleus and the deep (primarily type IIa fiber) red and superficial (primarily type IIx and IIb fibers) white regions of the lateral and medial head of the gastrocnemius, respectively, were mounted between a force transducer and motor arm, as described previously (5). Sarcomere length was adjusted to 2.5 µm by using an eyepiece micrometer. A photograph was taken of the fiber while it was briefly suspended in air. The diameter was measure at three points along the fiber, and the mean value was used as the fiber diameter.
Muscle electromyography electrodes and implantation. In a subgroup of five rats from the I14 group, electromyography (EMG) electrodes were implanted on the surface of the soleus (n = 4) and either the lateral gastrocnemius (n = 3) or plantaris (n = 3). EMG electrodes were constructed from one 13-cm-long and four 21-cm-long segments of Teflon-coated, seven-stranded, stainless steel wire that was 0.001 in. bare (A-M Systems). The 13-cm wire, which acted as a ground wire, was soldered to the center two leads of a 7.5-cm-long, six-conductor laminated cable (AWM style 20566, Parlex), which was embedded in a 3-cm x 1.5-cm x 3-mm dental acrylic mold. The four 21-cm-long wires were soldered to the four remaining leads of the laminated cable and acted as the bipolar surface electrodes. The solder joints were then dialectically sealed in silicone rubber cement (General Electric). Conductivity of the solder joints was tested by immersion in rat Ringer and checking for short or open connections. A 3-cm-long segment of the Teflon on the ends of the four bipolar leads was etched by using Tetra-Etch (WL Gore & Associates, Dundee, UK), and the ends were bent into a "U" shape, 2 mm wide by 2 mm long. The wires were then glued to a 1-cm-square piece of nylon mesh by using Super Glue and run parallel to each other, 1 mm apart, with the hooked ends of the "U" facing away from each other. Thus two bipolar electrodes were constructed from the four wires. After the glue dried, a 2-mm segment of each wire was scraped clean of glue and Teflon, exposing the electrode. Care was taken to ensure that the ends of the wire remained sealed in the Teflon insulation. Conductivity of the leads was again verified.
To implant electrodes, rats were anesthetized with pento-barbital sodium (50 mg/kg body wt), and regions on the upper and lower back and medial and lateral portions of the lower right leg were shaved. Incisions, 2 cm long, running parallel to the axis of the rat, were made in the upper and lower portions of the exposed skin on the back. A 2-cm incision was then made in the exposed skin on the lateral portion of the lower leg. If the rat was to receive a plantaris electrode, another 2-cm incision was made on the medial side of the lower leg. A glass tube, 1 cm in diameter, was inserted through the top back incision and run subcutaneously to the lower back incision. The five electrode wires were threaded down the tube, and the tube was then removed. A 5-cm segment of Teflon was stripped from the ground wire, and the wire was secured to the muscle fascia of the back by using 6-0 suture. The glass tube was then run from the lower back incision to the incision on the lateral portion of the leg, and the four wires were threaded down and exposed through the leg incision. To implant the soleus electrode, a 1-cm length of the aponeuroses between the gastrocnemius and the anterior portion of the leg was cut, and the connective tissue was blunt dissected, exposing the ventral surface of the soleus. The electrode was placed over the center of the muscle such that it was implanted parallel to the muscle fibers. Suture (6-0) was used to secure the Teflon mesh to the epimysium to prevent electrode movement. To implant the gastrocnemius electrode, a portion of the lateral gastrocnemius was dissected free from the biceps femoris. The electrode was then inserted and secured in the same fashion as for the soleus. The plantaris electrode was implanted by running the electrode wires around the anterior portion of the lower leg to the incision made on the medial side. A 5-mm length of the biceps femoris muscle was cut between the gastrocnemius and plantaris muscles 1 cm proximal to the heel. The dorsal surface of the plantaris muscle was exposed, and the electrode was implanted as described for the soleus muscle. The incisions on the leg and lower back were then closed, and the dental acrylic mold was implanted under the skin on the upper back such that the end of the laminated cable exited the skin. Nylon mesh, glued to the top surface of the acrylic mold, facilitated growth of dermal connective tissue to the mold anchoring the laminated cable, as it exited the skin. All cuts in muscle and connective tissue were closed by using 6-0 suture while skin incisions were closed by using either 4-0 (leg) or 2-0 (back) suture. Rats were allowed to recover for 2 days before EMG was recorded.
EMG data collection. EMG data were collected at a rate of 2,000 Hz for 45 min, three to four times daily (135-180 min/day) for 14 days. The 45-min EMG recordings included the IE session (both horizontal standing and the isometric contractions), as well as the 15 min of HU directly before and after the IE. The raw EMG signal was fed into an alternating-current-coupled amplifier with a 30-Hz high-pass filter. The data were stored on a personal computer and analyzed by using a Spike 2 software program. A script was written in Spike 2 to rectify the raw EMG signal, identify trains, record the train duration and intertrain intervals, and calculate the mean EMG of each train. Parameters qualifying trains for each muscle were determined from work by Hennig and Lomo (16). For the soleus, a train was identified if at least seven consecutive spikes were no greater than 77 ms apart. Gastrocnemius and plantaris trains were identified if six consecutive spikes were no greater than 19 ms apart.
Representative EMG data from the soleus, plantaris, and gastrocnemius muscles during IE are shown in Fig. 2. This figure demonstrates that all three muscles were recruited during IE. The arrow indicates when weight was hung from the rat's tail.
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Statistical analysis. Differences between groups for all values except mean EMG and food consumption are reported by using an ANOVA with Newman-Keuls post hoc tests. Differences in mean EMG between standing and IE were analyzed by using a t-test. Reliability over time in days was analyzed with an intraclass R analysis utilizing data from a repeated-measures ANOVA table, with time as the repeated factor. A significant effect of time on food consumption was determined by using a repeated-measures ANOVA with time as the repeated factor. Significance was accepted at P < 0.05.
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RESULTS |
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TOPABSTRACTMETHODSRESULTSDISCUSSIONDISCLOSURESREFERENCES |
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Food consumption, body weight, and muscle weights. Food consumption between groups was not significantly different. Control values averaged 23.4 ± 0.9 g/day. However, there was a significant time effect on food consumption. Food consumption was the lowest on days 1 (19.3 ± 1.5 g) and 4 (22.5 ± 1.4 g) and greatest on days 8 (29.1 ± 1.1 g) and 10 (28.2 ± 1.0 g). The lengths of the soleus (29 ± 1 mm) and plantaris muscles (35 ± 1 mm) were not different between groups. The body weights for all groups on day 14 are shown in Table 1. The I14 group was the only group with a body weight significantly less than that of control: 13% (Table 1).
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After H7, the soleus wet mass was significantly less than that of the control group (18%) and continued to drop during the next 7 days, such that that of the H14 group was significantly less than that of the H7 and control groups by 24 and 37%, respectively (Table 1). The muscle-to-body weight ratio of the soleus was not significantly reduced after H7, but after H14 was significantly less than both the H7 and control groups by 24 and 31%, respectively (Table 1). Alterations in soleus wet weight were accompanied by a decline in fiber diameter of this muscle which, by H14, were significantly less than that of control. The passive tension of the soleus muscle was not significantly different among groups (Table 1).
The plantaris wet weight was significantly reduced after H7 and remained less than WB at H14 (Table 1). In contrast to the soleus, the muscle weight-to-body weight ratio of the plantaris was not significantly different from control at either H7 or H14 (Table 1). Passive tension of the plantaris muscle was not significantly different among groups (Table 1).
No significant change in muscle weight occurred after H7 in the adductor longus (AL), gastrocnemius, extensor digitorum longus, or tibialis anterior (TA) muscles (Table 1). However, after H14, there was a significant decline in wet muscle mass in the AL, gastrocnemius, and TA (Table 1). After H14, the muscle weight-to-body weight ratio was significantly depressed for the AL and gastrocnemius muscles (Table 1). Despite no difference in wet muscle mass at H7, fiber diameters of red and white gastrocnemius were significantly depressed. (Data for red gastrocnemius only are presented in Table 1.) White gastrocnemius fiber diameters were as follows: WB, 78 ± 4; H7, 66 ± 4; and H14, 64 ± 3 µm. The red gastrocnemius fibers continued to atrophy, such that fiber diameters of H14 were 14% less than those of H7.
Isometric resistance exercise as a countermeasure to HU-induced atrophy was effective for the soleus and TA but not the plantaris, gastrocnemius, and AL (Table 1). IE attenuated 54% of the loss of soleus wet mass and 75% of the loss of relative (mg muscle/g body wt) mass, such that both measures of soleus muscle mass in the I14 group were significantly greater than the corresponding H14 values (Table 1). Although there was no significant drop in soleus fiber diameter at H7, IE significantly increased fiber diameter of I7 relative to H7. IE reduced the decline in soleus fiber diameter after H14, from 34 to 8% (Table 1).
In contrast to the soleus, exercise during H14 had no significant effect on gastrocnemius or plantaris wet mass. For the plantaris, both the I7 and I14 groups were significantly less than control by 15%, respectively (Table 1). The muscle-to-body weight ratio of the plantaris was not significantly altered by HU or HU plus exercise. The IE did protect the gastrocnemius relative mass; thus the I14 muscle-to-body weight ratio was significantly greater than that of the H14 and not different from that of the WB group. The partial protection of the gastrocnemius mass was also reflected at the single-fiber level, where fiber diameter for both the I7 and I14 groups was significantly greater than that for the H7 and H14 groups, respectively, but still significantly less than that for the WB group. Although the decline was attenuated at H7 (74 ± 2 µm), IE did not significantly affect white gastrocnemius fiber diameters at H14 (64 ± 2 µm). IE had no affect on AL wet mass after H14, but it did prevent the drop in TA wet mass (Table 1).
Twitch contractile properties. The 43% drop in soleus twitch force (N) after H14 was prevented by IE (Table 2, Fig. 3). The CT and RT of the soleus were unaffected after H7. However, CT was significantly reduced by 38% after 14 days, a change that was prevented by IE (Table 2, Fig. 3). No significant differences were observed between soleus WB twitch +dP/dt (23.045 ± 5.19 N/s) or -dP/dt (-5.14 ± 0.64 N/s) and the HU groups.
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In contrast to the soleus, neither H7 nor H14 or HU plus exercise had significant effects on plantaris twitch properties (Table 2, Fig. 3). The +dP/dt and -dP/dt values were 98.11 ± 12.00 and -21.37 ± 2.90 N/s, respectively.
Tetanic contractile properties. HU induced a significant drop in soleus Po at 14 days, and IE prevented 40% of this loss (Table 3, Fig. 4). Although the Po (N) of the I14 group remained 25% less than that of WB, the difference was not significant.
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The plantaris Po (N) of the H7 and H14 groups was significantly less than that of WB (Table 3, Fig. 4). Despite the lack of effect of IE on plantaris muscle mass, it prevented 46 and 13% of the decline in Po (N) at H7 and H14, respectively (Table 3). However, the Po of the I14 group remained significantly lower than that of WB.
HU had no effect on the peak rate of tetanic tension development (+dP/dt) or decline (-dP/dt) for either muscle. Control values for +dP/dt and -dP/dt were 25.6 ± 6.3 and -23.3 ± 7.6 N/s for the soleus and 102 ± 11 and -243 ± 11 N/s for the plantaris, respectively.
The force-frequency relationship of the soleus was unaffected by H7, but, after H14, the relationship was significantly shifted to the right (Fig. 5). The percent Po of the H14 group was significantly less than that of the WB at 20, 40, 60, and 80 Hz. IE completely prevented the shift in the soleus force-frequency curve during H14. Neither HU nor HU plus IE altered the force-frequency relationship of the plantaris (Fig. 5).
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Vo. The slack test data showed that neither HU nor HU plus IE affected soleus Vo. However, because of unexpected high variability, statistical conclusions about the data cannot be made (Table 4).
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Force velocity, a/Po, percent force and maximal shortening velocity at peak power, velocity at peak power (FL/s), and force at peak power. The mean force-velocity relationships for WB, H14, and I14 are shown in Fig. 6. The soleus force at peak power (mN) was significantly reduced by H7 and H14, and the drop was prevented by IE (Table 5). In contrast, there was no significant difference among groups for normalized force (kN/m2) and velocity (FL/s) at peak power (Table 5). The soleus a/Po ratio of the I7 and I14 groups was significantly increased compared with that of the H7 and H14 groups by greater than three- and twofold, respectively. There were no significant differences among groups for any of these values for the plantaris muscle at either 7 or 14 days (Table 5).
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Peak muscle power. The effects of H7 and H14 and HU plus IE on soleus and plantaris peak power are shown in Table 6 and Fig. 7. The significant 54% drop in soleus peak power (mN·FL·s-1) at H14 was completely prevented by IE (Table 6, Fig. 7). For the plantaris, the ANOVA showed no significant differences in peak power among groups.
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DISCUSSION |
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TOPABSTRACTMETHODSRESULTSDISCUSSIONDISCLOSURESREFERENCES |
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The IE protocol was designed to maximally recruit the soleus, and this was verified by EMG data of the soleus, gastrocnemius, and plantaris (Fig. 2). Comparison of EMG records obtained during IE, standing, walking, and running verified that the soleus muscle was maximally activated during IE (data not shown). Based on muscle wet weights, the exercise protocol did not prevent soleus atrophy, but it did significantly reduce the HU-induced atrophy at 14 days from 63 to 83% of the control. In contrast, the IE was not successful in reducing atrophy (i.e., the decline in wet weight) of the fast gastrocnemius or plantaris muscles. This negative finding may have resulted from the less than optimal limb position for the development of peak force in these muscles (Fig. 1). It is clear that both the plantaris and the gastrocnemius muscles were recruited during the IE (Fig. 2), but neither showed a significant increase in the integrated EMG amplitude with the increase in tail load. This suggests that the increasing load over the 2 wk of HU was borne by regions of these muscles not monitored by the EMG electrode and/or other hindlimb muscles. This conclusion should be viewed with some caution, because the plantaris and gastrocnemius EMGs were recorded in only three rats to document muscle recruitment. The study was not designed to quantify the effect of the IE on the amplitude and frequency of the EMG signal.
The observation that IE either prevented (soleus) or reduced (gastrocnemius) the HU-induced decline in single-fiber diameter demonstrated that the IE protection of muscle mass was not simply the result of an increased tissue water due to exercise-induced damage (Table 1). Additional evidence that the fibers from the HU and HU plus IE groups were not damaged was that they showed uniform sarcomere lengths and generated forces per CSA that were similar to those of the control groups (unpublished observations).
The primary hypothesis of this study was that long isometric contractions would best protect slow antigravity muscles, such as the soleus, from HU-induced muscle atrophy and loss of function. The rationale was that high-resistance shortening contractions of the calf would be too short and fast to adequately load the slow-twitch soleus fibers. The rapid contractions would depend primarily on fast-twitch fibers, and the peak power of the movement would be obtained at 20% of the fast-fiber Vo (33). Whereas maximal protection of the fast-fiber population would occur, the shortening speeds would be at or near the Vmax of the slow-twitch fiber, such that the force and power developed would be low and unlikely to optimally protect the slow fiber. From the perspective of muscle mass, our data support the hypothesis. At H7, the IE program fully protected the soleus mass and soleus-to-body weight ratio. To our knowledge, this is the first exercise countermeasure to prevent atrophy of the soleus during the first week of HU. Table 7 shows the relative effectiveness of other paradigms that employed simple standing, walking, grid climbing, or electrical stimulation. It is difficult to directly compare these results because the duration of HU and daily exercise varied, but some generalizations can be made. For example, walking was clearly more effective in preventing soleus muscle atrophy than simple standing (15, 25, 31). Second, the IE appeared to be more effective than shortening contractions in protecting the mass of the slow-twitch soleus muscle (Table 7). The relative difference in soleus muscle mass between the HU and exercise groups was greater with IE at both 7 and 14 days than that observed after grid climbing or walking (15, 17, 25, 32). However, Diffee et al. (9) found no difference in the effectiveness of isometric and isovelocity contractions in preventing soleus muscle atrophy during 28 days of HU.
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Although the force generated by the hindlimbs was not quantitated in this study, it is presumed that it was greater than that during the grid-climbing study of Herbert et al. (17), in which, on average, only 75% of body weight was hung from the rat's tail during exercise. Therefore, it is possible that the greater efficacy of this study for the soleus was due to the high-force component. This is supported by the data of Linderman et al. (22) and Widrick and Fitts (32). In the former study, the authors found grid climbing with 50% body weight on the tail to have no beneficial effects on soleus, gastrocnemius, or plantaris muscle mass. In contrast, in the Widrick and Fitts study, the load during grid climbing was 1.5 times body weight, and, at H14, the soleus mass showed a similar percentage of protection as that observed after only 7 days in the Herbert et al. (17) study. Furthermore, the protection of Po (measured at the single-fiber level) was somewhat better than that observed in this study (where Po was determined at the whole muscle level). Collectively, these data suggest that protection of the soleus increases with the load moved or tension developed by the muscle. We also hypothesize that the prolonged isometric component coupled with the high force were responsible for the greater protection of soleus muscle mass by IE compared with isotonic grid climbing. Relative to preliminary data on rats that only stood in the tube while horizontal (H14, n = 3; 112 ± 10 mg wet muscle mass; 0.324 ± 0.034 mg wet mass/g body wt), the 150 s of maximal contractions superimposed on the 12.5 min of standing accounted for 40% of the protection of IE on soleus mass.
The efficacy of IE on soleus wet mass dropped 25% during the second week of HU. The reason that IE was not as effective during the second week of HU might be due to overtraining. In a separate study, our laboratory found an inverse relationship between the number of isotonic contractions performed and gastrocnemius mass during H14 (unpublished observations). In the second week of this study, the daily increment of the weight added to the rat's tail was reduced compared with that during week 1. This observation suggests that the rats may have become overtrained in week 2.
In contrast to previous studies (12, 22), H7 did not induce significant atrophy of the gastrocnemius muscle. However, the discrepancy between this and the aforementioned studies may be due to a difference in the atrophic response to HU in different aged rats. Steffen et al. (28) found that HU-induced atrophy was greater in 1.5-mo-old, 200-g juvenile rats vs. 5-mo-old, 459-g adult rats.
No exercise countermeasure has significantly attenuated the loss of gastrocnemius wet mass during H7, and this was the first study to investigate the effects of exercise on the gastrocnemius beyond H7 (17, 22). The IE employed in this study had no significant effect on reversing the loss of gastrocnemius mass, but the relative mass (muscle-to-body wt ratio) was significantly higher than that in the H14 group. This result was similar to that observed by Linderman et al. (22) after 5 days of HU plus grid climbing. In contrast, Herbert et al. (17) found grid climbing to have no effect on either the absolute or relative decline in gastrocnemius mass with H7. The rather marginal effect of the IE on the gastrocnemius was likely a direct result of modest activation of this muscle. EMG records (Fig. 2) indicate that IE recruited the gastrocnemius; however, the mean EMG signal (mV/s) was approximately ninefold less than that of the soleus. The possibility exists that regional differences existed in the activation of the gastrocnemius. Because the electrodes were implanted on the surface of the lateral gastrocnemius, activation of the deep portions of the muscle may not have been recorded, and thus the EMG may not be indicative of the actual percentage of the gastrocnemius recruited during weight holding (20). Preliminary single-fiber data support this, because IE was most effective on fibers isolated from the deep region of the lateral head (red gastrocnemius), the portion of the gastrocnemius muscle farthest from the recording electrodes (unpublished observations).
A surprising finding was the lack of an effect of IE on the plantaris atrophy, given that the EMG records indicate that the plantaris activation was greater than that of the gastrocnemius (Fig. 2). Consistent with these findings, Diffee et al. (10) observed that synergist ablation plus treadmill walking failed to attenuate plantaris atrophy during HU. Similarly, the atrophy of the plantaris was not significantly attenuated during 5 days of HU plus grid climbing (22). A significant reduction of the loss of plantaris wet mass has been observed only in animals that were allowed to stand for 2 h/day during HU and not in rats that ran uphill on a treadmill for 1.5 h/day (29).
Twitch and tetanic properties. In contrast to the findings of other studies, there was no effect of H7 on soleus twitch parameters (12, 17, 25). This difference might be explained by the reduced atrophy of the soleus (18%) compared with that in previous studies (26-42%). The larger and somewhat older rats used in this study relative to other laboratories were apparently less affected by the HU protocol. In contrast, H14 soleus Pt (N) was significantly less than WB (Table 2). The drop in Pt at 14 days was most likely due to the loss of contractile protein, because the drop in magnitude (41%) was similar to the extent of soleus atrophy (37%), as determined from muscle wet weights (Table 1). Similarly, the protective effect of IE on Pt (N) was at least, in part, due to the attenuation of atrophy.
The decrease in CT with H14 was consistent with previous findings (13, 27) and was likely caused by an increased expression of fast-type sarcoplasmic reticulum pump protein (27). This would increase the rate of sarcoplasmic reticulum Ca2+ uptake and thus shorten CT. IE prevented the HU-induced decrease in both Pt and CT at H14 (Table 2 and Fig. 3). Previous countermeasure studies have been unsuccessful at preventing the altered twitch properties during HU (17, 25). For example, despite significant attenuation of mass, grid climbing failed to prevent the decline in Pt or the altered CT or RT (17). This suggests that sustained contractions during the IE may be important in preventing HU-induced changes in soleus Pt and CT. Interestingly, chronic stretching of the soleus during H14 was the only previous countermeasure to completely protect Pt, CT, and RT (21). As a result of the normalization of the CT, IE prevented the right shift in the soleus force-frequency relationship (Fig. 5).
The decline in soleus Po at H14 was consistent with our previous findings and that of other laboratories (13, 17). In contrast, the force per CSA was not significantly different from WB, a finding that differs from previous studies from this (13) and other laboratories (25). This suggests that the drop in Po (mN) with HU in the present study was primarily due to skeletal muscle atrophy. IE was equally effective at preventing the decline in the relative mass, fiber diameter, and Po in the H14 soleus.
The drop in Po (N) of the plantaris muscle at H7 and H14 was consistent with the findings of Diffee et al. (8), who reported that 28 days of HU significantly reduced plantaris Po (g) without a change in relative Po (g/cm2). Furthermore, myofibril yield (mg/g tissue) was not significantly different from control after 28-day HU (10). Similar to the effects on muscle mass, the IE program was not effective in preventing the HU-induced decline in plantaris Po. The drop in soleus peak power (mN·FL·s-1) at 14 days was reflective of the decline in Po (N) with HU (Table 6 and Fig. 7), because the velocity at peak power was not affected (Table 5). IE completely prevented the decline in soleus peak power. This effect can be attributed to a partial protection of muscle mass (I14 soleus weight significantly greater than H14, Table 1) and to a reduction in the curvature of the force-velocity relationship (increased a/Po). The mechanism for the increased a/Po with IE is unknown. As can be observed from the WB group in Table 5, fast muscles have higher a/Po, indicating less curvature in the force-velocity relationship compared with slow muscles. However, the increased a/Po in the soleus I14 group cannot be attributed to an increased distribution of fast fibers, because Vo and Vmax were both unaltered (Table 4).
The ANOVA showed no difference in plantaris peak power between groups. However, the data in Table 6 are suggestive of a HU-induced decline in plantaris peak power and protection by IE. A t-test analysis between WB and H14 showed a significant difference at P < 0.02, whereas no difference was observed between the WB and I14 groups.
In conclusion, the primary effect of HU was to induce muscle atrophy, which, in turn, caused the decline in peak force (Po) and power. Consequently, HU had no effect on relative force (kN/m2) or power (kN·m-2·FL·s-1). The IE countermeasure tested in this study was most effective on the soleus, the muscle most atrophied during HU. These findings are consistent with the mean EMG during IE that indicated that the soleus was preferentially recruited over the plantaris and gastrocnemius muscles. The amount of soleus atrophy prevented by the IE protocol was greater than that observed by previous studies using isotonic exercise (walking or grid climbing). Furthermore, IE provided protection against the HU-induced shortening of soleus twitch CT. The latter allowed the muscle to produce greater tension at physiological motoneuron firing frequencies. The partial protection of the soleus mass and the elevated a/Po induced by IE prevented the drop in the peak power, a parameter perhaps most indicative of muscle performance. EMG data indicated that the plantaris was recruited during IE, but Po and the mass of this muscle were not protected. The decline in effectiveness of the IE during the second week of HU may indicate that the volume of work performed was too great, leading to exhaustion or overtraining. The results indicate that IE is an important component of any countermeasure designed to protect antigravity slow-twitch muscle, but that it appears less effective in altering the HU effects on fast-twitch muscles. Future studies should focus on titrating the volume of IE needed to provide the maximum protection and evaluate countermeasures utilizing combinations of both isometric and isotonic resistance exercise.
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