Skylab Reduced muscle mass, strength and performance in space

figure 6-2. changes in upper , lower limb volumes obtained circumference measurements of 3-cm segments in 3 crewmembers skylab 4. because of higher exercise volume in skylab 4 crewmembers, loss of muscle volume less observed in crewmembers skylab 2 , 3. reference.

upper , lower limb volumes obtained on 3 crewmembers of skylab 4 shown in figure 6-2. fluid shifts contributed largest changes lower limb volumes, loss of leg tissue mass evident, particularly in commander. shown in graphs, significant loss of leg volume occurs within first few days of microgravity exposure while changes in upper limbs less remarkable. upon return earth, of loss of leg volume corrected , there short over-correction or overshoot. once fluid shift resolves, true loss of muscle mass remaining in legs revealed more returns baseline or preflight level (see figure 6-2, leg during recovery on right side of graph 3 crewmembers).

in skylab 4 commander, loss in leg volume appears 300 cc. (figure 6-2, topmost graph). because complement of exercise equipment mission largest (consisting of cycle ergometer, passive treadmill, , mini gym , modified commercial devices provided capability low-load resistive exercises) losses in muscle mass , strength less in previous 2 missions of shorter duration.

during skylab program, exercises , exercise devices added incrementally , testing expanded each mission. produced different exercise environment each flight in reality, there 3 separate related orbital experiments, each n=3. results each mission affected next.

preflight , postflight evaluation of muscle strength performed on right arm , leg of each crewmember 3 skylab orbital missions means of cybex isokinetic dynamometer. protocol completed on each crewmember included thorough warm-up, , 10 maximum-effort full flexions , extensions of arm @ elbow , of hip , knee @ angular rate of 45° per second. isokinetic leg strength 3 missions, body weights , leg volumes, presented in figure 6-3.

figure 6-3. average changes in body weight, isokinetic leg strength, , leg volume of crewmembers on 3 skylab missions. bicycle ergometer used on skylab 2, mk , mk ii “mini gym” exercisers added skylab 3, , passive “treadmill” flown on skylab 4. average work load per day on cycle ergometer provided mission comparison. reference.

on skylab 2, bicycle ergometer available in-flight exercise, testing performed 18 days before launch , 5 days after landing. while realized these times temporally remote flight, best achieved due schedule constraints. time day 5 muscle testing completed, recovery in function had occurred; however, marked decrement still remained. decrement in leg extensor strength 25%; arms suffered less exhibited marked losses (data not shown). commander s arm extensors showed no loss, since used these muscles in hand-pedaling bicycle, being skylab crewmember adopt mode of arm exercise. illustrated fundamental point in muscle conditioning: maintain strength of muscle, must stressed or near level @ have function. leg extensor muscles, important in standing , providing propulsive forces during walking, capable of generating forces of hundreds of pounds, while arm extensor forces measured in tens of pounds. forces developed in pedaling bicycle ergometer typically tens of pounds , incapable of maintaining leg strength. bicycle ergometer proved excellent machine aerobic exercise , cardiovascular conditioning, not capable of developing either type or level of forces needed maintain strength walking under 1g.

immediately after skylab 2, work started on devices provide adequate exercise arms, trunk, , legs. commercial device, termed mini gym , modified extensively , designated mk-i . exercises benefited arms , trunk achievable device. while forces transmitted legs greater cycle ergometer, still limited inadequate level, since level not exceed maximum strength of arms, represents fraction of leg strength.

a second device, designated mk-ii , consisted of pair of handles between 5 extension springs attached, allowing development of maximum forces of 25 pounds per foot. these 2 devices flown on skylab 3, , in-flight nutrition support , exercise time , food increased. crew performed many repetitions per day of favorite maneuvers on mk-i , lesser extent, on mk-ii. also, average amount of work done on bicycle ergometer more doubled on skylab 3, crewmembers participating actively.

it perceived skylb life scientists device allowed walking , running under forces equivalent earth gravity provide more strenuous exercise. after completion of skylab 2, work begun on treadmill skylab 4. mission preparation progressed, launch weight of skylab 4 escalated final design of treadmill constrained weight limitations. final weight device mere 3.5 pounds. passive device (figure 6-4) consisted of teflon-coated aluminum walking surface attached skylab iso-grid floor. 4 rubber bungee cords provided equivalent weight of 80 kilograms (175 lbs) , attached shoulder , waist harness worn crewmembers during use. angling bungee cords user pulled forward, equivalent slippery hill created. high loads placed on leg muscles, calf, , fatigue rapid device not used significant aerobic work because of bungee/harness design. absolutely necessary wear socks , no shoes provide low-friction interface teflon surface.

figure 6-4. first u.s. “treadmill” utilized during spaceflight passive device used on skylab 4 mission of 84 d duration. high loading (175 lbs) via bungee cords provided more of resistive rather aerobic modality. consisted of teflon-coated aluminum plate attached skylab isogrid floor. exercising crewmember wore waist , shoulder harness attached iso-grid floor surrounding treadmill plate means of 4 bungee cords. socks had worn provide low-friction interface between plantar surface of feet , teflon-coated treadmill plate. reference.

on skylab 4, crew used bicycle ergometer @ same rate on skylab 3, mk-i , mk-ii mini gym exercisers. in addition, typically performed 10 minutes per day of walking, jumping , jogging on treadmill. food intake had again been increased.

upon return earth , before muscle testing, apparent skylab 4 crewmembers in physical condition. able stand , walk long periods without apparent difficulty on day after landing (r+1), in contrast crewmembers earlier 2 missions. results of strength testing confirmed surprisingly small loss in leg strength after 3 months of microgravity exposure (figure 6-3). in fact, knee extensor strength increased on pre-flight level (figure 6-13).