decrease of muscle strength and muscle mass is caused by aging and decrease of physical 
 activity1 , 2 .
the american college of sports medicine3 and the american heart association4 recommend that in resistance training , training should be 
 done two to three times per week to prevent a decrease in muscle mass and muscle strength . 
 to obtain augmentation of muscular strength and muscular hypertrophy ,
it has been 
 reported1,2,3,4,5 that a training intensity of 
 more than 60 to 70% of maximum strength is required
. however , high intensity exercise 
 exposes the elderly to high risk , because it imposes a high burden on articular and 
 cardiovascular functions1 , 6 , 7 .
accordingly , the 
 elderly often perform low intensity exercises for safety reasons1 , even though it is not anticipated to increase muscle 
 strength and mass .
therefore , we consider that it is necessary to develop a safe and 
 effective low intensity training program for the elderly . an eccentric contraction has been shown to produce greater augmentation of muscular 
 strength and muscular hypertrophy than a concentric contraction after resistance 
 training8 .
furthermore , an eccentric 
 contraction exercise tends to exert less stress on the cardiovascular system9 .
when eccentric contractions are used , 
 lower - intensity training can be more effective than concentric contractions at augmenting 
 muscular strength and eliciting muscular hypertrophy .
we recently observed that an aerobic 
 intensity exercise of 50% lactate threshold10 using an eccentric contraction increased soleus muscle mass in 
 rats11 .
we employed a method of 
 eccentric exercise using downhill running , since , from the point of view of soleus muscle 
 activity , downhill running can be considered a low intensity exercise12 . in our previous study11
, the rats performed an endurance exercise for a long period , but 
 elderly persons would most likely find it difficult to perform an endurance exercise for 
 such a long time .
clinically , elderly and cardiovascular disease patients perform interval 
 exercises when they are indicated not to perform endurance exercises13,14,15 .
interval exercise involves repeated short to long bouts 
 of rather high intensity exercise interspersed with recovery periods ( light exercise or 
 rest)16 .
the methods of interval 
 exercise have been developed mainly in the field of sports medicine , but interval exercise 
 may also be suitable for the training of the elderly , because it has rest periods between 
 exercise periods . therefore , the primary aim of this study was therefore to compare the 
 effect of 8 weeks of endurance exercise using an eccentric contraction versus aerobic 
 interval exercise using an eccentric contraction on the muscle strength and mass of rats . 

our hypothesis was that an aerobic interval exercise would increase muscle strength and mass 
 to the same degree as endurance exercise .
twenty - one female wistar rats ( 10 weeks of age , 223.5  5.7  g ) were used in this study .
the 
 animals were housed in a temperature - controlled room at 23  c on a 12-hour light - dark cycle , 
 and were given free access to standard rat food and water .
the protocols of this study were 
 approved by the animal experiment committee of the prefectural university of hiroshima 
 ( no.12ma009 ) .
the rats were randomly divided into three groups of seven rats each : the control group , the 
 aerobic endurance exercise group , and the aerobic interval exercise group .
armstrong et al.17 studied eccentric contractions in rats performing downhill running , 
 a widely used method of eccentric contraction training for rat skeletal muscles17,18,19 . in light of this ,
in the present study , 
 the aerobic endurance exercise and aerobic interval exercise group ran on a treadmill with a 
 16 incline at 16 m / min .
the aerobic interval exercise group ran using an interval protocol for a total of 
 90 minutes : eighteen 5-minute bouts separated by 2 minutes rest . a treadmill made for rats 
 and mice ( exer-3/6 , columbus co. , ltd . ,
the aerobic endurance 
 exercise group and the aerobic interval exercise group performed treadmill exercises once 
 every three days for 60 days ( a total of twenty sessions ) . the aerobic endurance exercise 
 load for the eccentric exercise used in our previous study11 of muscle hypertrophy induction was used .
on the 60th day after the start of the experiment , all of the rats body weights were 
 measured , and then they were anesthetized with pentobarbital ( 50  mg / kg , ip ) . the left soleus 
 muscle was excised .
the excised soleus muscles were quick - frozen and cooled by dry ice in acetone .
the right 
 soleus muscle was excised along with the bony origin part , and mounted on a force recording 
 device ( mltf-500/st , ad instruments co. , ltd .
the muscle was placed vertically in a 
 chamber filled with ringer solution which was then oxygenated with a mixture of 95% 
 o2 and 5% co2 , and the muscle was stretched to 110% of its resting 
 length .
the muscle was stimulated with a 1  msec square wave delivered by two parallel 
 platinum electrodes using an electric stimulator ( sen-3401 , nihon kohden co. , ltd .
tetanus tension was elicited for a 1  msec 
 duration , 100  hz , 1 sec train and with 130% peak twitch tension voltage .
peak twitch tension 
 and tetanus tension data were recorded using analysis software ( labchart 7 japanease , ad 
 instruments co. , ltd .
the frozen muscles were sliced into 10-m - thick sections using a cryostat microtome and 
 stained with hematoxylin and eosin ( he staining ) .
for histological analysis , all sections 
 were observed under a light microscope to exclude abnormal findings .
in addition , all the 
 sections were photographed using a digital camera for microscopy .
the section images were 
 transferred to a computer , and the cross - section minor axes of the muscles were measured 
 using image analysis software ( nih image j ) , randomly targeting 300 fibers in each muscle 
 sample .
the cross - section minor axes of the muscles were measured according to the method 
 reported by brooke et al22 . the body weight , muscle wet weight , cross - section minor axis of the muscle and the twitch 
 tension were analyzed using one - way analysis of variance ( anova ) .
when a significant 
 difference was present , a multiple comparison test was performed using the tukey test .
the 
 tetanus tension was analyzed using the kruskal - wallis test , and scheffe s post - hoc test was 
 then performed .
mean body weights at the beginning of the experiment were 224.14.3  g in the control group , 
 222.95.8  g in the aerobic endurance exercise group , and 223.47.5  g in the aerobic interval 
 exercise group .
mean body weights at the end of the experiment were 322.918.2  g in the 
 control group , 326.47.8  g in the aerobic endurance exercise group , and 331.911.3  g in the 
 aerobic interval exercise group .
there were no differences among the body weights of the 
 three groups at the beginning and at the end of the experiment .
the results for the muscle wet weights and fiber cross - section minor axes are shown in 
 table 1table 1.results of muscle wet weights and fiber cross - section minor axesgroupmuscle wet weight(mg)fiber cross - sectionminor axis ( m)control group146.7  13.650.0  10.6aerobic endurance exercise group177.4  10.055.5  10.3aerobic interval exercise group168.1  10.654.9  10.4p < 0.01 .
the muscle wet weights of the aerobic endurance exercise and aerobic interval 
 exercise groups were significantly larger than those of the control group ( aerobic endurance 
 exercise , p<0.0001 ; aerobic interval exercise , p=0.007 ) .
there was no significant 
 difference between the muscle wet weights of the aerobic endurance exercise and aerobic 
 interval exercise groups .
the muscle fiber cross - section minor axes of the aerobic endurance 
 exercise and aerobic interval exercise groups were significantly wider than those of the 
 control group ( both , p<0.0001 ) .
there was no significant difference between the muscle 
 fiber cross - section minor axes of the aerobic endurance exercise and aerobic interval 
 exercise groups .
histologically , no abnormal findings , such as fiber necrosis or 
 inflammatory cell infiltration , were noted in any of the muscles of the three groups ( fig .
1.histology of the soleus muscle ( he staining)bar=50 m.control ( a ) , aerobic endurance exercise ( b ) , aerobic interval exercise ( c ) ) .
meansd histology of the soleus muscle ( he staining ) control ( a ) , aerobic endurance exercise ( b ) , aerobic interval exercise ( c ) the results for twitch tension and tetanus tension are shown in table 2table 2.results of twitch tension and tetanus tensiongrouptwitch tension ( g)tetanus tension ( g)control group11.4  1.8675.0 ( 62.981.5)aerobic endurance exercise group13.4  1.9082.8 ( 70.885.3)aerobic interval exercise group14.1  1.0381.4 ( 78.489.7)p < 0.05 .
the twitch tension of the aerobic interval exercise group was significantly 
 larger than that of the control group ( p=0.016 ) .
the tetanus tension results of the aerobic 
 endurance exercise and aerobic interval exercise groups were significantly larger than that 
 of the control group ( aerobic endurance exercise , p=0.036 ; aerobic interval exercise , 
 p=0.016 ) .
there was no significant difference between the tetanus tension results of the 
 aerobic endurance exercise and aerobic interval exercise groups .
the current study investigated whether an aerobic interval exercise using an eccentric 
 contraction in downhill running would increase soleus muscle strength and mass to the same 
 degree as an aerobic endurance exercise .
the muscle wet weights and fiber cross - section 
 minor axes of the aerobic endurance exercise group were significantly larger than those of 
 the control group .
the aerobic endurance exercise group performed the exercise at the same 
 load as in our previous study11 and ran 
 continuously on a treadmill with a 16incline at 16 m / min for 90 minutes .
the results of 
 this study are similar to those of our previous study11 , reconfirming that an aerobic endurance exercise using an eccentric 
 contraction in downhill running induces hypertrophy of the soleus muscle .
the tetanus 
 tension of the aerobic endurance exercise group was significantly greater than that of the 
 control group .
therefore , the aerobic endurance exercise using an eccentric contraction in 
 downhill running induced muscle hypertrophy , and , moreover , it also resulted in increased 
 muscle strength .
the muscle wet weights and fiber cross - section minor axes of the aerobic interval exercise 
 group were significantly larger than those of the control group .
there were no differences 
 in the muscle wet weights , fiber cross - section minor axes , and tetanus tension results 
 between the aerobic endurance exercise group and the aerobic interval exercise group .
the 
 results of this study show that an aerobic interval exercise using an eccentric contraction 
 in downhill running increased soleus muscle strength and mass to the same degree as an 
 aerobic endurance exercise .
muscle blood flow decreases when a muscle contracts , and expressions of growth hormone and 
 insulin - like growth factor-1 increase23 , 24 , conditions which are favorable for 
 protein synthesis in the muscle . in an eccentric contraction ,
the activated muscle is forcibly lengthened , and the 
 mechanical stress in the muscle created by an eccentric contraction is greater than that of 
 a concentric contraction8 , 17 , 23 , 24 .
armstrong et al.17 and schewane et al.25 examined whether the rats skeletal muscle injury and plasma 
 creatine phosphokinase of rats were increased by running on a treadmill at 16 m / min for 90 
 minutes with a change in incline for one session .
both studies reported that inflammatory 
 muscle cell and plasma creatine phosphokinase increased after 48 to 72 hours when the rats 
 ran downhill . in our study , no abnormal findings , such as fiber necrosis or inflammatory 
 cell infiltration , were noted in the muscles of the endurance exercise group , which 
 performed downhill running for 20 sessions using the exercise load reported by armstrong et 
 al17 .
we postulated that soleus muscle 
 damage might occur in the initial stage of downhill running , as armstrong et al.17 and schewane et al.25 reported .
however , it seems that excessive soleus muscle 
 fiber damage did not occur , because the rats did not stop running , even though some slight 
 damage that did not interfere with the rats performance may have occurred .
moreover , the 
 eccentric contraction training effect is the result of an increase in the number of 
 sarcomeres in the muscle fibers , which allows the muscle fibers to operate at longer 
 lengths26 , 27 .
the present study showed that augmentation of muscular strength and muscular hypertrophy 
 occurred in the rat soleus muscle through muscle fiber adaptations elicited by 20 sessions 
 of eccentric contraction exercise .
paschalis et al.28 showed that an equal volume of high - intensity and low - intensity 
 eccentric exercise increased plasma creatine kinase after exercise .
in addition , 
 high - intensity eccentric exercise resulted in a greater decline of peak torque than 
 low - intensity eccentric exercise .
these findings suggest that a low - intensity eccentric 
 exercise may be suitable for performing long - term exercises for augmentation of muscular 
 strength and muscular hypertrophy .
interval exercise effectively increases cardiovascular function , metabolic function , muscle 
 strength and physical performance13,14,15,16 , 29 . in general , the protocol of interval exercise involves repeated 
 short bouts of rather high - intensity exercise interspersed with recovery periods .
however , 
 interval exercise exposes the elderly to high risk , because it imposes a high burden on 
 articular and cardiovascular functions by repetition of high - intensity exercise .
the running 
 speed of our study corresponded to about 50% of the lactate threshold10 , which could be described as a very low intensity 
 exercise .
we consider that an aerobic interval exercise using an eccentric contraction is 
 effective at increasing muscle strength and mass by the same degree as an aerobic endurance 
 exercise , even though rest periods are inserted between the exercise periods .
in addition , 
 our findings suggest that an aerobic interval exercise using an eccentric contraction may be 
 suitable and safe for the treatment of the elderly , because low intensity exercise most 
 likely exerts less stress on cardiovascular function and articular structures . in this study ,
if this method were used for human subjects , there is a possibility that falls and injury 
 would occur when running downhill on treadmill . in future studies
, we will attempt to 
 determine effective training methods for increasing muscle strength and mass via aerobic 
 interval exercise using eccentric contraction .
 [ purpose ] the purpose of this study was to examine whether an aerobic interval exercise 
 using an eccentric contraction would result in skeletal muscular hypertrophy and 
 augmentation of muscular strength in rats . 
 [ subjects and methods ] twenty - one female wistar 
 rats were used in this study . 
 the rats were randomly divided into three groups . 
 the 
 control group performed no exercise . 
 the aerobic endurance exercise group ran for 90  min . 
 
 the aerobic interval exercise group ran for a total of 90 minutes in 5 minute bouts 
 separated by 2 minute rest periods . 
 the exercise groups ran on a downhill treadmill 
 incline , once every three days , for a total of twenty sessions . 
 [ results ] the muscle wet 
 weights , the muscle fiber cross - section minor axes , and the tetanus tension results of the 
 aerobic endurance and aerobic interval exercise groups were significantly larger than 
 those of the control group . [ conclusion ] these results indicate that aerobic interval 
 exercise may be an effective method of inducing hypertrophy and augmenting muscular 
 strength in skeletal muscle . 
INTRODUCTION
SUBJECTS AND METHODS
RESULTS
DISCUSSION