4 

c** 

*- - . NASA Technical Memorandum 102227 

Muscle Changes With 

Eccentric Exercise: 

Implications on Earth 

and in Space 

Alan R. Hargens, Scott Parazynski, 

Michael Aratow, and Jan Friden 

August 1989 

(NASA-TM-102227) MUSCLE CHANG€S U1Ti-i N39-29016 

ECCENTRIC EXERCISE: IPPLXCATIONS ON EARTH 

AND IN SPACE (NASA, Ames Research Center) 1-f P CYCL 06P Uncl dS 

63/52 0232569 

NASA 

National Aeronautics and 

Space Administration 
- 

NASA Technical Memorandum 102227 

Muscle Changes With 

Eccentric Exercise: 

Implications on Earth 

and in Space 

Alan R. Hargens, Scott Parazynski, and Michael Aratow 

Ames Research Center, Moffett Field, California 

Jan Friden, Umea University Hospital, UmeA, Sweden 

August 1989 

NASA 

National Aeronautics and 

Spa= Administration 

Ames Research Center 

Moffett Field, California 94035 

,*' *, 
Muscle Changes With Eccentric Exercise: 

Implications on Earth and in Space 

Alan R. Hargens, Scott Parazynski, Michael Aratow, and Jan Fridh* 

Space Physiology Laboratory(239- 17), Life Science Division, NASA Ames 

Research Center, Moffett Field, CA 94035, USA and *Department of Hand and 

Reconstructive Surgery, UmeA University Hospital, S-901 85 UmeA, SWEDEN 

ABSTRACT 

In this paper, we review recent investigations of fluid pressure, morphology, 

and enzyme activities of skeletal muscle exercised eccentrically or 

concentrically in normal human subjects. Intramuscular pressures were 

measured before, during, and after submaximal exercise and correlated with 

subjective muscle soreness, fiber size, water content, and blood indices of 

muscle enzymes. High-intensity eccentric exercise is characterized by postexercise pain, elevated intramuscular pressures, and swelling of both type 1 

and 2 fibers as compared to concentric exercise. Thus, long periods of 

unaccustomed, high-level eccentric contraction may cause muscle injury, fiber 

swelling, fluid accumulation, elevated intramuscular pressure, and delayed 

muscle soreness. Training regimens of progressively increasing eccentric 

exercise, however, cause less soreness and are extremely efficacious in 

increasing muscle mass and strength. It is proposed that on Earth, postural 

muscles are uniquely adapted to low levels of prolonged eccentric contraction 

that are absent during weightlessness. The almost complete absence of 

eccentric exercise in space may be an important contributor to muscle atrophy 

and therefore equipment should be designed to integrate eccentric 

contractions into exercise protocols for long-term spaceflight. 

INTRODUCTION 

Eccentric contraction occurs when a given muscle elongates (Le., fibers 

lengthen) while active tension is produced. Conversely, concentric contraction 

is defined as muscle shortening during active tension. Eccentric contractions 

in everyday activities are uniquely related to the presence of a normal 

gravitational field in that the muscle counteracts the force of gravity while it 

is lengthening. For example, walking downstairs, bending, and jumping are 

common activities that include significant amounts of eccentric work (Davies & 
Barnes, 1972). As a consequence of these activities, eccentric contraction 

level of submaximal speed (Abbott et al., 1952; Abbott & Bigland, 1953; 

Asmussen, 1952). Thus, work is being performed on the eccentrically 

contracting muscle by stretching (Abbott et al., 1950). 

I subjects the muscle to higher tensions than concentric contraction at a given 

Muscle stiffness and soreness can occur one or two days following 

unaccustomed exercise. The initial episode of soreness is related to muscle 

fiber damage, edema, and elevated intramuscular pressure (FridCn et al., 

1986; FridCn et al., 1988). Associated with post-eccentric exercise soreness is 

an elevation of muscle enzymes in serum (Armstrong et al., 1983; Friden et al., 

1989; Newham et al., 1983a; Tiidus & Ianuzzo, 1983). Although this delayed 

muscle soreness is noted at the beginning of an eccentric contraction training 

program, the muscle increases its eccentric work capacity and the delayed 

soreness diminishes as an adaptation to prolonged eccentric training (FridCn et 

al., 1983a). 

The purpose of this paper is to review our recent physiologic comparisons of 

eccentric versus concentric exercise and to recommend suitable exercises for 

long-term spaceflight based upon present knowledge in this field. 

MUSCLE INJURY, SWELLING, AND SORENESS ASSOCIATED WITH ECCENTRIC 

EXERCISE 

In an attempt to determine whether delayed muscle soreness is associated 

with swelling and increased intramuscular pressure, the anterior muscles of 

the lower leg of eight volunteers were subjected to repetitive eccentric and 

concentric contractions. By use of the slit-catheter technique (Hargens & 

Mubarak, 1981), tissue fluid pressure in the anterior compartment was 

recorded before, during, immediately after, and two days after exercise. 

Exercise D rotocol The anterior compartments of the lower legs were exercised 

eccentrically and concentrically through the use of an exercise apparatus 

(Brutus 1000 System, Excel) (Fig. 1). The subjects were positioned supine on a 

padded physical therapy table, and a padded strap was taped in place on the 

left foot 10 cm from the medial malleolus. The subject raised a known weight 

(15% of the subject's maximal dorsiflexion torque) with ankle dorsiflexion. At 

the point of full dorsiflexion, the weight was removed, allowing the patient to 

return his foot to a full plantarflexed position without lowering the weight. 

This maneuver was then repeated at a rate of once every 3 seconds for a total 

of 20 minutes (i.e., 400 contractions) and constituted the concentric exercise 

part of the protocol. The eccentric exercise protocol was initiated with the 

right foot being positioned in full dorsiflexion. The same weight was applied 

and the foot was then lowered to a full plantarflexed position. This maneuver 

was also repeated at the same frequency and duration (including 1-minute 

rest periods every 4 minutes) as before. 

2 
c 

Fig. 1. Experimental setup. Subject 

raises by dorsiflexion (concentric 

contraction, left leg, inset) or 

lowers by resisted dorsiflexion 

(eccentric contraction, right leg) a 

given load applied via a pulley 

system to the foot (a). During 

concentric exercise the load is 

lowered by an assistant (b), 

whereas during eccentric exercise 

the load is lifted by an assistant. 

The distance the weight is moved 

(c) is the same for concentric and 

eccentric exercise. (From Fridh et 

al., 1986.) 

Intramuscular Dressu res Using local anesthesia (1 ml of 1% lidocaine) instilled 

superficial to the fascia, a slit catheter was inserted into the muscle belly of 

the tibialis anterior in a cephalad direction and at a 450 angle to the skin 

surface. Placement was at a point 8 cm distal to the level of the tibial 

tuberosity, and the catheter tip was directed 1 cm below the fascia. The 

catheter was secured in place with Steri-Strips. The system was filled with a 

heparinized saline solution, providing a continuous fluid column from the 

catheter tip to the transducer. Resting pressure measurements were always 

performed with the ankle in 100 of plantarflexion. 

On day 0, the tissue fluid pressures were measured at rest and during 2 

minutes of sustained isometric contractions (maximal bilateral dorsiflexion 

without load). Pressures were also continuously recorded before, during, and 

after the concentric and eccentric exercise protocols. Recordings were 

completed when the post-exercise pressures returned to the baseline resting 

levels. On day 2, continuous tissue fluid pressure measurements were taken 

in each leg at rest and during 10 concentric and 10 eccentric contractions 

(against no load as in day 0). Subsequently, intramuscular pressure was 

measured during 2 minutes of isometric contractions (maximal dorsiflexion 

without load). 

Muscle so reness Two days after the exercise, all subjects reported severe 

soreness and stiffness in the anterior compartment of the eccentrically 

exercised limb. These symptoms were reported to occur throughout the entire 

compartment, with the most painful regions located proximally. Palpation and 

isometric contractions caused marked exacerbation of the pain. Only occasional 

5 
stiffness was perceived from the concentrically exercised muscles. Our 

subjects reported that the preponderance of their symptoms was in the right 

(eccentrically exercised) leg 48 hours after exercise. This correlated with an 

increase in resting pressure and the peak tissue fluid pressure during 

isometric contraction 48 hours later. 

Muscle pain occurring 24-48 hours after unaccustomed exercise is a 

phenomenon familiar to most individuals. Eccentric contractions are 

particularly associated with delayed muscle soreness and stiffness 

(Armstrong, 1984; Asmussen, 1952; FridCn et al., 1983b; Newham et al., 

1983b). Various distributions of symptoms have been reported. The 

musculotendinous junction appears to be the primary site of palpable 

tenderness, whereas the muscle belly is relatively spared (Edwards et al., 

1981; Newham et al., 1983b). We found this to be true with our subjects, 

where the muscle origins and insertions were the principal sites of point 

tenderness. However, delayed muscle soreness, a symptom that was easily 

aggravated when isometrically contracting the previously eccentrically 

exercised compartmen t, was reported to be distributed equally throughout the 

muscle. 

Muscle fibers are oriented most obliquely just proximal to the 

musculotendinous junction, diminishing their ability to withstand the high 

levels of tension reportedly produced within a muscle undergoing eccentric 

contractions. This may help explain the higher degree of muscular discomfort 

seen after performing eccentric exercise (Newham et al., 1983b). Also, muscle 

pain receptors are most concentrated in the regions of tendinous and 

connective tissue (Kumazawa 

account for our finding that 

maximal point tenderness. 

& Mizumura, 1977; Stacey, 1969), which may 

the musculotendinous junctions are the site of 

Fig. 2. Resting tissue fluid 

pressure (mean 2 SE; n=8) in 

anterior compartment of 

lower leg before and after 

concentric (left leg) and 

eccentric (right leg) exercise. 

(From FridCn et al., 1986.) 

Post-exercise pressures 

were significantly higher 

than pre-exercise pressures 

in eccentrically exercised 

legs. 

0 Presrercise, Do) 0 

13 Imncdkte post-exercise, Do) 0 

m DOY 2 

pcD5 

W 

v, 

v, 

a 

3 15 

LEFT RIGHT 

Resting intramuscular pressures The tissue fluid pressures in the right and 

left anterior compartments were equal prior to the exercise regimens (Fig. 2). 

On day 0, the pressure immediately following the eccentric exercise regimen 

was 13.1 2 4.7 mmHg. This was significantly higher than the pre-exercise 

4 
i 

250 

W 

ct 

3 200 v) 

v) 

W 

Fig. 3. Peak intramuscular 

pressure (mean 2 SE; n=8) 

resting pressure of 3.6 0.7 mmHg (p<0.05). The resting pressure before 

exercise in the eccentrically exercised leg was significantly higher on day 2 

compared to day 0 (10.5 2.5 vs. 3.6 0.7 mmHg, P<0.05). This was not the 

case for the concentrically exercised muscles of the left anterior compartmen t, 

where no significant elevation in tissue fluid pressure occurred immediately 

or 48 hours after exercise. 

- 0 Day 0 

I 0 DOY 2 

- it* D<001 

p<m - 

The pre-exercise pressures are well within the normal range of 0-10 mmHg 

reported previously (Mubarak, 198 1). The elevated compartment pressures 

that occurred 48 hours after the eccentric exercise regimen (10.5) are still 

below what is considered a borderline resting pressure (15 mmHg) seen in 

patients with chronic compartment syndrome (Mubarak, 198 1). The 

significant elevation of the immediate post-exercise pressures following the 

eccentric exercise regimen (13.1 mmHg) is far lower than the post-exercise 

pressures (30 mmHg) seen in patients with the reproducible symptoms of 

chronic compartment syndrome. Apparently compartment compliance is 

decreased after eccentric exercise, but not to the extent hypothesized in 

chronic compartment syndrome. No subject experienced "ischemic" pain after 

exercise, as the pressures generated are not sufficient to compromise the 

microvascular circulation. 

Peak pressu res during ecce ntric a nd co ncentric exercise Figure 3 

demonstrates that the maximum pressures generated during the eccentric 

exercise regimen were significantly greater than those generated during 

concentric exercise on both day 0 (236 & 12.3 vs. 157 & 16.0 mmHg, 

respectively, P< 0.001) and day 2 (194 19.9 vs. 151 +. 18.4 mmHg, 

respectively, Pe0.05). 

legs. (From FridCn et al., 

1986.) 

0 I 

LEFT 

I T 

RIGHT 

The muscle tensions developed with eccentric contractions are higher 

(Asmussen, 1952), and a linear relationship exists between muscle tension and 

intramuscular pressure during isometric exercise (Sejersted et al., 1984). We 

assume that not only the muscle, but also the fascia, is strained during 

eccentric contraction. Stretching the fascia would thereby cause a narrowing 

5 
of the cylindrical anterior compartment. This can further reduce compliance, 

raise the intramuscular pressure, and explain the greater tension per load 

during eccentric contractions. Intramuscular pressure elevations have been 

reported to be associated with extremes of passive ankle plantarflexion and 

dorsiflexion with a fully extended knee, causing significant stretch of the 

anterior compartment (Gershuni et al., 1984). 

250 - 0 Re-exercise. Day 0 

- Post-exercise, Day 0 T 

Fig. 4. Peak intramuscular 

pressure (mean & SE; n=8) 

generated during sustained 

isometric contraction performed before and after 

concentric (left leg) and 

eccentric (right leg) exercise. 

(From Fridtn et al., 1986.) 

150 

100 

50 

I aLEFT RIGHT 

* 

Peak messure - during isometric contractions The pre-exercise peak isometric 

contraction pressures were equivalent for the right and left anterior 

compartments (Fig. 4). After exercise on day 0, the peak isometric contraction 

pressures were significantly higher on the right (eccentrically exercised) side 

compared with the left (concentrically exercised) side (P<O.OOl). This 

difference persisted on day 2 (P<O.OOl). Peak isometric contraction pressure 

was significantly greater 48 hours post-exercise compared with pre-exercise 

in the eccentrically exercised compartment. The concentrically exercised leg 

showed no significant difference between these two pressures. There was a 

rapid decline in generated pressures in the eccentrically exercised 

compartment whereas the concentrically exercised compartment showed a 

less rapid falloff in its pressure profiles during the isometric contractions. 

This was also seen after exercise on days 0 and 2. Although generating similar 

pressures during isometric muscle contractions before concentric or eccentric 

exercise protocols of comparable workload, there is a marked disparity 

between the compartment pressures produced for maximal effort 

dorsiflexions after these protocols. It has been well documented that eccentric 

1 

6 
exercise produces much more muscle fiber damage than concentric exercise of 

equal power (Fridkn et al., 1983b). We found that 2 days after eccentric 

exercise, the muscles produce a significantly higher compartment pressure 

during an isometric contraction despite the well-documented degradation of 

these muscles and their impaired ability to generate forces (Fridkn et al., 

1986). Therefore, we believe that the increased tissue fluid pressure is due to 

the swelling of the compartment and a decrease of its compliance. 

15 r 

MUSCLE FIBER SIZES - 0 flPI 1 the left (concentrically 

(eccentrically exercised) 

tibialis anterior muscles. 

Average values are 23 

and 27% higher on the 

right side for the type 1 

and type 2 fibers, respectively. Fi ber-type 

proportions did not 

differ significantly be- 

, exercised) and right a TlW 2 

0- - 

The accumulation of interstitial or intracellular fluid can be explained as a 

response to the myofibrillar disintegration that follows exercise with extreme 

tension demands (Fridkn et al., 1983b). This may promote formation of 

protein components accompanied by the release of protein-bound ions, 

possibly increasing the osmotic pressure within the affected fiber. A pilot 

study utilizing measurements of calf circumference and muscle compartment 

ultrasound evaluations 2 days after eccentric and concentric calf exercise 

revealed increased muscle volume as a result of eccentric exercise (Fridkn, 

unpublished observations). No associated pressure elevations were recorded 

in the more compliant superficial posterior compartment. We conclude that 

the higher pressures seen during rest and contraction in the eccentrically 

exercised muscles must be in part attributed to the decreased compliance of 

the compartment as a result of fluid accumulation. 

Muscle biopsies were obtained from the tibialis anterior of all the subjects 

under local anesthesia at 48 hours after exercise. Portions were frozen for wet 

and dry weight, and the remainder was processed for fiber typing (FridCn et 

al., 1988). Fiber sizes were greater in the eccentrically exercised muscle as 

compared to those concentrically exercised (Fig. 5). Furthermore, water 

content of the eccentric muscle was significantly higher than that for the 

concentric muscle (82.67 2 0.49% vs. 79.67 2 0.99%, respectively, Pc0.05). 

7 
I MUSCLE ENZYME ACTIVITIES WITH ECCENTRIC EXERCISE 

Before the onset of the three exercise protocols and 48 hours later, 5 ml blood 

was drawn via antecubital venipuncture for assaying muscle enzymes. Serum 

enzyme analyses were completed within 2 hours after blood sampling. Serum 

glutamic oxaloacetic transaminase (SGOT) and lactic dehydrogenase (LDH) 

were measured on a Technicon SMAC System continuous flow analyzer. 

Serum creatine kinase (CK) was measured on a Cobas Bio centrifugal analyzer. 

We selected 48 hours post-exercise for blood sampling in an attempt to 

determine whether the serum levels of muscle enzymes may reliably reflect 

soreness . 

250 

1 ' 200 

c 

2 

F 150- > 

0 

w 

a 

f 100- N 

z 

w 

50 

0. 

After eccentric contractions, serum CK increased by 36% from 148.8 IU (k17.6) 

to 201.0 IU (229.9) (Pc0.05) (Fig. 6). The CK levels did not change as a result of 

either isometric or concentric contractions. Similarly, the SGOT levels increased 

significantly after the eccentric exercise. The increase was 17%. 24.0 IU/L 

(- +1.4) versus 29.0 IU/L (k1.25) (P<0.05). The LDH levels were not significantly 

L 

- 

1 

- 

changed at 48 hours after 

Fig. 6. Post-exercise 

serum activity levels: 

CK, CRL, pre-exercise 

controls; ISO, isometric 

contractions; CON, concentric contractions; ECC, 

eccentric contractions. 

(From FridCn et al., 

1989.) 

any of the exercise regimens. 

300 r 

CRL 

'i 

N.S. 

IS0 CON ECC 

It is evident that high-tension exercise imposed on the relatively small 

anterior compartment is sufficient to increase the activities of muscle enzymes 

in the blood. Numerous studies have demonstrated that exertion of untrained 

muscles also brings about soreness and enzyme release (Newham et al., 

1983a). The release of enzymes indicating muscle damage from skeletal 

muscles after exercise may occur as a result of muscle hypoxia, ischemia, 

necrosis, or other conditions that cause a change in fiber membrane 

allowed them to perform all exercises adequately in the protocol. But because 

the extensor muscles of the lower leg are generally not subjected to any 

I permeability (Spitler et al., 1984). Our subjects had fitness levels which 

8 
+ 

significant lengthening contractions during "normal" exercise, it is not 

surprising that a sensation of soreness developed. 

Serum CK has been reported to pass its peak 8-24 hours post-exercise (Tiidus 

& Ianuzzo, 1983), yet at 48 hours, it was increased most (as a percentage of 

the activity) of all the enzymes assayed in this study. These elevated levels 

probably indicate fiber injury (Armstrong et al., 1983). Previous studies 

reporting CK elevations have primarily focused on the intensity or duration of 

the exercise rather than the mode of contraction. Armstrong & co-workers 

(1 983) reported increased CK activity after eccentric-biased treadmill exercise. 

Those CK increments, however, were caused by fiber damage occurring in 

several muscles in the four legs of a rat, whereas our results concentrate on 

significant changes caused by eccentric exercise of a single compartment. 

Thus, we conclude that the mechanical stress on the ankle dorsiflexors during 

the eccentric exercise regimen exceeded their normal daily stress loads. 

SGOT elevations have been reported in association with high-intensity physical 

exercise among well-trained athletes (Metivier et al., 1980), although other 

studies have failed to demonstrate any relationship between dynamic exercise 

and SGOT levels (Tiidus & Ianuzzo, 1983). Here we present a clear correlation 

to the eccentric mode of exercise. It is likely that this represents the delayed 

effect of subcellular injuries to the contractile material subjected to 

unaccustomed tension demands and/or possible disruption of sarcolemma. 

Both these mechanisms can be involved in enzyme efflux into the blood 

(Armstrong et al., 1983). An increase in SGOT has also been attributed to its 

increased synthesis during times of hypoxia (Highman & Altland, 1963). 

No significant elevations of LDH were found 48 hours after either of the 

regimens. This is in accordance with the findings of Schwane & co-workers 

(1983). The specific time course of LDH serum levels after muscle injury (Le., 

peak level occurring 8-16 hours post-exercise) may be a plausible explanation 

for the absence of increased LDH activity, at least following the high-tension 

eccentric protocol. 

We assume that the low compliance of the lower leg anterior compartment 

(Hargens et al., 1978) makes the muscles more susceptible to strain-induced 

damage, swelling, and pressure elevation, which further increases the 

likelihood of structural injury. Based on these findings, we conclude that postexercise serum levels of muscle enzymes at the time of maximal soreness are 

qualitatively reliable markers of muscle damage caused by high muscle 

tension. Whether the serum levels of muscle enzymes quantitatively reflect 

the degree of structural injury is presently unknown. 

MUSCLE MORPHOLOGY WITH ECCENTRIC EXERCISE 

Light microscopy The first light microscopic evidence of post-eccentric 

exercise changes was obtained in toluidine blue stained longitudinal sections 

9 
(FridCn et al., 1981). These sections displayed a distorted sarcomeric band 

I pattern. The degenerative changes were focal (involving a few sarcomeres and 

myofibrils) and were extensively distributed. 

Using immunofluorescence microscopy, disorganized intermediate filament 

architecture was revealed in sore muscle following eccentric exercise (FridCn 

et al., 1984). The specimens displayed frequent transverse fluorescent 

striations between 2-disks, not observed in normal muscle samples. These 

changes may reflect mechanical disturbance of the intermediate filament 

system caused by the high, damaging tensions developed during eccentric 

contraction or distension of the cytoskeleton caused by edema. An increased 

intracellular pressure due to the release of osmotic components caused by the 

disruption of Z-disks may distribute forces unevenly on the cytoskeleton 

attachments. A second possible explanation for the cytoskeletal abnormalities 

is that degeneration is a secondary response to the extensive myofibrillar 

lesions. 

Increased lysosomal enzyme activity has been demonstrated in several animal 

studies in relation to strenuous physical exercise. Microscopically, this activity 

is indirectly detected by lipofuscin granules which are regarded as the 

indigestible residue of protein degradation. Increased density of lipofuscin 

generally relates to ischemic fiber injuries, but also occurs after a single bout 

of eccentric exercise (Fridh et al., 1984). This is interpreted as a sign of 

acutely increased protein degradation following exercise with tensions 

exceeding a certain critical level. 

Electron microscoDv At the ultrastructural level, the most prominent finding 

following eccentric exercise was disorganization of the contractile material, 

particularly the myofibrillar Z-disks. The degree of alteration varied 

considerably from minor streaming of the 2-disks to total disruption of many 

sarcomeres. While in some cases only one single 2-disk of one myofibril was 

affected, there was evidence for involvement of several sarcomeres and 

myofibrils in others. The myofilamentous material in sarcomeres adjacent to 

the affected Z-disks was either supercontracted or disorganized and out of 

pattern, even in areas where neither streaming nor broadening was seen. Loss 

of thick myofilaments was apparent. Mitochondria were seldom seen in or 

close to the abnormal area. Transverse sections through A-bands confirmed 

the disturbance arrangement of filaments. A correlation between the degree of 

structural deviation and the time from eccentric exercise to the point of biopsy 

has been described (Fridh et al., 1983b). It is of interest to note that the 

ultrastructurally defined type 2 fibers (fast-twitch) seem to be more 

susceptible to damage than the type 1 fibers; a 3:l damage ratio was seen 

three days post-exercise. The high tension demands during eccentric exercise 

may preferentially recruit type 2 fibers. On the other hand, type 2 fibers are 

designed primarily to contribute to high tension development, so this 

interpretation may be unlikely. A more probable explanation is the fact that 

I register. The 2-disks showed frequent gaps in their normally regular lattice 

I 

10 
the type 1 fibers with their broader Z-disks have mechanically firmer 

connections between their contractile units. Reparative processes occur as 

early as three days post exercise, as evidenced by frequent polyribosomemyofilament complexes in affected regions. This finding is regarded as a sign 

of intense filament formation. 

IMPLICATIONS FOR EXERCISE IN SPACE 

The postural musculoskeletal system must continually work against the force 

of gravity in normal daily activities. Whether walking or in a stance position, 

large muscle groups are contracting to prevent the collapse of the skeletal 

framework. In the near-weightlessness of spaceflight, however, 

musculoskeletal loading and muscle tonic activity is altered substantially 

(Lestienne & Gurfinkel, 1988); there is a large reduction in the magnitude and 

frequency of musculoskeletal loading, resulting in widespread physiological 

adaptations to this new environment. Postural muscles and bones no longer 

resist gravity, nor are they the primary effectors of self-propulsion around the 

spacecraft. Therefore, in microgravity, contractions with lengthening of the 

postural muscles are rare. Eccentric contraction is a common phenomenon in a 

gravity-dependent environment, present in every loading cycle of normal 1-G 

activities, but infrequent during spaceflight. 

The lower limbs undergo significant atrophy during lengthy missions, 

particularly in the slow twitch soleus muscle (Thornton & Rummel, 1977). 

Furthermore, neuromuscular coordination is impaired after long-duration 

spaceflight: Soviet cosmonauts' patellar tendon reflexes are reported to be 

significantly slowed postflight (Cherepakhin & Pervushin, 1970; Kakurin et al., 

1971), and U.S. astronauts often subjectively complain of transient postflight 

instability. Exercise countermeasures have long been favored to prevent these 

adverse physiological adaptations, but strenuous exercise regimens of up to 

four hours per day have not proven totally successful in the Soviet space 

program (Atkov, personal communication, 1989). While reversible 

musculoskeletal changes occur with even short spaceflights, extendedduration missions could conceivably cause irreversible damage to muscle 

strength. 

Lestienne and Gurfinkel (1 988) performed biomechanical studies aboard the 

Space Shuttle, revealing a slouching posture and markedly attenuated tonic 

soleus muscle activity. The lower body remained basically inactive, and 

moving about the cabin was primarily accomplished by the upper body 

musculature. In fact, the only lower limb muscle used with any frequency 

was the tibialis anterior muscle, an ankle dorsiflexor, for perching in 

footstraps. In contrast, a normal loading cycle experienced by the lower limb 

musculature during normal gait (Morrison, 1970; Winter, 1982) incorporates 

both concentric and eccentric components of several major muscle groups. Not 

only are other muscle groups involved at much higher power in 1-G, but the 

postural muscles are actively absorbing energy during eccentric contraction. 

11 
Eccentric muscular contraction is clearly an attractive spaceflight 

countermeasure for several reasons. Owing to the lack of gravity and normal 

loading of the postural musculature in space, exercise regimens that 

incorporate eccentric contraction will better simulate and maintain 1 -G 

patterns of motor neuron firing during spaceflight. High-intensity eccentric 

resistance exercise is extremely effective in maintaining or building muscle 

mass (Fridkn et al., 1983a). Added benefits include improved reflex times 

(Tipton & Karpovich, 1966; Francis & Tipton, 1969) and +Gz tolerance (Jacobs 

et al., 1987), which is important during reentry. While heavy resistance 

exercise equipment is not currently available for spaceflight, several 

investigators advocate its use inflight (Convertino, 1987; Parazynski et al., 

1989; Whalen et al., 1988). 

With this in mind, a self-contained, variable-resistance exercise device has 

been designed that could exercise the astronaut through the full range of 

motion of a high stepping gait with significantly higher force levels than 

experienced during normal 1-G gait (Parazynski et al., 1989). Other lower and 

upper extremity exercises built within the system should allow maintenance 

of musculoskeletal integrity during long-duration spaceflight. 

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Acknowledgement: This work was supported by NASA, the Veterans 

Administration, USPHS/NIH grant AM-25501, the Swedish Medical Research 

Council, the Tore Nilsson Foundation, and the Swedish Sports Research Council. 

14 
NASA 

1. Report No. 

NASA TM- 102227 

Report Documentation Page 

2. Government Accession No. 

21. No. of Pages 

16 

7. Author(s) 

Alan R. Hargens, Scott Parazynski, Michael Aratow, and 

Jan FridCn* 

*UmeA Univ. Hospital; UmeA, Sweden 

22. Price 

A02 

~ ~~~~ 

9. Performing Organization Name and Address 

Ames Research Center 

Moffett Field, CA 94035 

19. Security Classif. (of this report) 

Unclassified 

12. Sponsoring Agency Name and Address 

20. Security Classif. (of thi 

Unclassified 

National Aeronautics and Space Administration 

Washington, DC 20546-0001 

3. Recipient's Catalog No. 

5. Report Date 

August 1989 

6. Performing Organization Code 

8. Performing Organization Report No. 

A-89224 

10. Work Unit No. 

1 1. Contract or Grant No. 

13. Type of Report and Period Covered 

Technical Memorandum 

14. Sponsoring Agency Code 

15. Supplementary Notes 

Point of Contact: Alan R. Hargens, Ames Research Center, MS 239-17, Moffett Field, CA 94035 

(415) 694-5746 or FI'S 464-5746 

(Presented at Int. Soc. for Myochemistry mtg.) 

16. Abstract 

In this paper, we review recent investigations of fluid pressure, morphology, and enzyme activities 

of skeletal muscle exercised eccentrically or concentrically in normal human subjects. Intramuscular 

pressures were measured before, during, and after submaximal exercise and correlated with subjective 

muscle soreness, fiber size, water content, and blood indices of muscle enzymes. High-intensity 

eccentric exercise is characterized by post-exercise pain, elevated intramuscular pressures, and swelling of both type l and 2 fibers as compared to concentric exercise. Thus, long periods of unaccustomed, high-level eccentric contraction may cause muscle injury, fiber swelling, fluid accumulation, 

elevated intramuscular pressure, and delayed muscle soreness. Training regimens of progressively 

increasing eccentric exercise, however, cause less soreness and are extremely efficacious in increasing 

muscle mass and strength. It is proposed that on Earth, postural muscles are uniquely adapted to low 

levels of prolonged eccentric contraction that are absent during weightlessness. $,The? almost complete 

absence of eccentric exercise in space may be an important contributor to muscle atrophy and therefore equipment should be designed to integrate eccentric contractions into exercise protocols for longterm spaceflight. 

Weightlessness, Microgravity, 

Exercise space, Muscle, 

Eccentric 

17. Key Words (Suggested by Author(s)) 18. Distribution Statement 

Unclassified-Unlimited 

Subject Category - 52 

I 

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