averaged and this average was used to compare preflight values with those on landing day and during the postflight period. skeletal-muscle strength was defined as the peak torque generated throughout a range of motion from three consecutive voluntary contractions for flexion and extension. eccentric contractions are actions of the muscle in which force is generated while the muscle is lengthening, as opposed to the concentric actions in which the muscle is shortening (contracting) while generating force. skeletal-muscle endurance was defined as the total work generated during 25 repetitions of concentric knee exercise, as determined from the area under the torque curve for a complete exercise set. work also was compared between the first 8 and last 8 repetitions. endurance parameters were measured during concentric knee flexion and extension activity only. on r+0, significant decreases in concentric and eccentric strength were shown in the back and abdomen when compared to the preflight means (table 6-1). table 6-1. mean percent change on landing day from preflight mean, for skeletal muscle concentric and eccentric strength of various muscle groups. muscle group test mode concentric eccentric back −23 (±4)* −14 (±4)* abdomen −10 (±2)* −8 (±2)* quadriceps −12 (±3)* −7 (±3) hamstrings −6 (±3) −1 (±0) tibialis anterior −8 (±4) −1 (±2) gastroc/soleus 1 (±3) 2 (±4) deltoids 1 (±5) −2 (±2) pecs/lats 0 (±5) −6 (±2)* biceps 6 (±6) 1 (±2) triceps 0 (±2) 8 (±6) *preflight >r+0 (p < ); n=17 landing day (r+0) versus average of 3 preflight measures. from reference (14) concentric back extension and eccentric dorsiflexion remained significantly less than preflight values on r+7. recovery (an increase in peak torque from r+0 to r+7) was demonstrated for the eccentric abdomen and the concentric and eccentric back extensors. however, the data depicted in table 6-1 may be somewhat misleading because in some cases there were tremendous differences in strength between crewmembers who exercised during flight versus those who did not. for example, some crewmembers who exercised during flight actually gained in isokinetically measured strength in the ankle extensor/flexor muscles (anterior versus posterior calf muscles, that is m. tibialis anterior versus the gastrocnemius/soleus complex) compared to crewmembers who did not exercise and who actually showed a decrease in isokinetically measured strength in these muscles (figure 6-6). figure 6-6. percent change in isokinetic strength in ankle extensor and flexor muscles for crewmembers who exercised during flight versus those who did not. †preflight < r+0 (p < 0.05). from reference. with respect to endurance, a majority of the decrease in the total quadriceps work occurred on r+0. this likely reflects significant loss in the first third of the exercise bout (−11%). the declines in peak torque at the faster endurance test velocities are consistent with changes seen at the slower angular velocity used during the strength tests. torque for the quadriceps at 75° per second was 15% less than preflight values but for the hamstrings was 12% less than the preflight mean at 60° per second. endurance data showed little difference between preflight and r+7 tests, suggesting that crewmembers had returned to baseline by 1 week after landing. additionally, subjects who did exercise during flight compared to those who did not had significantly greater (p < 0.05) losses within 5 hours of landing in concentric strength of the back, concentric and eccentric strength of the quadriceps (30° per second), and eccentric strength of the hamstrings, relative to the respective preflight values (data not shown here). according to greenisen et al., non-exercisers also had significantly less concentric strength of the quadriceps at 75° per second and lower total work extension, work first-third flexion, and work last-third extension, immediately after landing, than before flight. the conclusions reached by the investigators were that the data indicate that muscles are less able to maintain endurance and resist fatigue after spaceflight, and that exercise may avert decrements in these aspects of endurance. conversely, crewmembers who exercised during flight had greater losses in trunk muscles strength as measured at landing than did the non-exercising group (figure 6-7). however, preflight strength in trunk flexion and extension was substantially greater in the exercising group than in the non-exercising group. apparently treadmill exercise did not prevent decrements in trunk strength after 9 to 11 days of spaceflight, and the investigators proffered the explanation that preservation of muscle function may be limited only to those muscles that are effectively used as part of the exercise regimen. figure 6-7. percent change in isokinetic strength in trunk muscles in crewmembers who exercised during flight versus those who did not. †pre > r+0 (p < 0.05). from reference. the specific aim of dso 475, "direct assessment of muscle atrophy before and after short spaceflight" was to define the morphologic and biochemical effects of spaceflight on skeletal fibers. to obtain myofiber biomechanical and morphological data from space shuttle crewmembers, biopsies were conducted once before flight (l – > 21 days) and again on landing day (r+0). the subjects were eight crewmembers, three from a 5-day mission and five from an 11-day mission. biopsies of the mid-portion of the m. vastus lateralis were obtained by means of a 6-mm biopsy needle with suction assist. a one-tailed paired t-test was used to identify significant differences (p < 0.05) between the mean values of fiber cross-sectional area (csa), fiber distribution, and number of capillaries of all crewmembers before flight and the mean values for all crewmembers after flight. according to this report, csa of slow-twitch (type i) fibers in postflight biopsies was 15% less than in preflight biopsies; the csa of fast-twitch (type ii) fibers was 22% less after flight than before (figure 6-8). mean values did not reflect the considerable variation seen in the biopsies from the eight astronauts who participated. at least some of this variation likely resulted from differences in the types and quantities of preflight and in-flight countermeasures (exercise or lower body negative pressure) used by the different crewmembers. the relative proportions of type i and type ii fibers were different before and after the 11 day mission: the fiber distribution also seemed to follow the same trend after the 5 day mission (more type ii and fewer type i fibers after than before), but the sample size was too small to reach statistical significance. the number of capillaries per fiber was significantly reduced after 11 days of spaceflight. figure 6-8. percent change in csa of type i (slow twitch) and type ii (fast twitch) myofibers in postflight versus preflight muscle biopsies from 8 crewmembers. from reference. however, since the mean fiber size was also reduced, the number of capillaries per unit of csa of skeletal muscle tissue remained the same. atrophy of both major myofiber types, with atrophy of type ii > type i, is somewhat different from the more selective type i myofiber atrophy observed in unloaded sprague-dawley and wistar rat muscle representing an uncommon case in which difference exist between responses of human and murine skeletal muscle. the purpose of dso 606, "quantifying skeletal muscle size by magnetic resonance imaging (mri)," was to non-invasively quantify changes in size, water, and lipid composition in antigravity (leg) muscles after spaceflight. this experiment was the first attempt to measure limb volumes before and after flight since the less sophisticated methods of measuring limb girths during apollo and skylab programs were used. the subjects included a total of eight space shuttle crewmembers, five from a 7-day flight and three from a 9-day flight. all subjects completed one preflight and two postflight tests on either l-30 or l-16 and on r+2 and r+7. testing involved obtaining an mri scan of the leg (soleus and gastrocnemius) at the university of texas – houston health science center, hermann hospital. multi-slice axial images of the leg were obtained to identify and locate various muscle groups. changes in water and lipid content were measured, in addition to csa, to distinguish changes in fluid versus tissue volumes. multiple slices were measured by computerized planimetry. csa and volume of the total leg compartment, soleus, and gastrocnemius were evaluated to assess the degree of skeletal muscle atrophy. the volumes of all 3 compartments were significantly smaller (p < 0.05) after both the 7 and 9 day shuttle flights than they were before flight. volume decreased by 5.8% in the soleus, 4.0% in the gastrocnemius, and 4.3% in the total compartment. these losses were stated to represent the true level of skeletal muscle tissue atrophy and not changes associated with fluid shifts. no recovery was apparent by 7 days after landing (data not shown). this finding indicates that the losses were not due to fluid shifts, but the delay in recovery after these rather short flights is contrary to what was observed and documented during the skylab program of flights much longer in duration, albeit by less sophisticated methods during skylab. the space shuttle program and, in particular, edomp has provided a great deal of knowledge about the effects of spaceflight on human physiology and specifically on alterations in skeletal muscle mass, strength, and function. once again, losses of skeletal muscle mass, strength, and endurance were documented, in some cases in spite of exercise countermeasures. but some findings were encouraging, particularly indications that in-flight exercise does have a positive effect in countering losses in muscle strength at least in the legs (see table 6-1 and figure 6-6), as predicted from the results of the 84-day skylab 4 mission when multiple modesof exercise were used including a unique "treadmill" device (see figure 6-4). this unusual treadmill provided loads of sufficient magnitude to the legs in a fashion approaching resistance exercise. however, the data provided by mri volume studies indicate that not all crewmembers, despite utilization of various exercise countermeasures, escape the loss in muscle mass that has been documented during most of the history of u.s. human spaceflight since project mercury. this, additional research is needed to continue the development of countermeasures and equipment that will eventually provide a successful solution for all human space travelers. shuttle-mir and nasa-mir during the seven nasa-mir flights, seven u.s. astronauts trained and flew jointly with 12 russian cosmonauts over a total period of 977 days (the average stay was 140 days) of spaceflight, which occurred during the period from march 1995 to june 1998. the major contribution of the joint u.s./russian effort on the mir space station relevant to the current risk topic was the first use of mri to investigate volume changes in the skeletal muscles of astronauts and cosmonauts exposed to long-duration spaceflight. this began with the first joint mission, mir-18, and continued until the final mir-25 mission. the data indicated that loss of muscle volume, particularly in the legs and back, was greater than with short-duration spaceflight but not as great as the data from short-duration flight might have predicted. a comparison between volume losses in the selected muscle groups in short-duration spaceflight on the space shuttle, long-duration (119 d) bed rest, and a (115 d) shuttle-mir mission demonstrates the relative time course of the losses (figure 6-9). figure 6-9. percent change in selected muscle groups during short (8 d; n = 8) and long (115 d; n = 3) spaceflight (mir 18) compared to long-duration bed rest (119 d). data from references and the shuttle/mir final report. there is good correlation between long-duration bed rest and spaceflight of similar duration except that losses in the back muscles are much less with bed rest. this likely reflects use of these muscles during bed rest to adjust body position and to reduce the potential for vascular compression and tissue injury. during spaceflight the back muscles are apparently less used because they do not have to support the upright body against earth gravity and are not used with great force to make positional adjustments of the body as they are during the recumbency of bed rest. international space station (iss) the international space station's (iss) first crew (expedition 1) arrived in october 2000; since then there have been 15 additional increments. the data presented here were collected during the first 11 of the iss expeditions. the complexities and shortcomings of collecting scientific data from a laboratory orbiting more than 300 miles above the earth and completing 18 orbits per day at a speed of more than 17,000 mph with discontinuous voice and data communications, combined with the