muscle weakness and atrophy often affect activities of daily living , disease prevention and 
 disease outcome1 . to obtain hypertrophy of 
 skeletal muscle , it has been reported that it is necessary to use a high intensity 
 exercise2 .
however , high intensity 
 exercise exposes the elderly to high risk3 , because it imposes a high burden on articular and cardiovascular 
 functions .
accordingly , the elderly often perform low intensity exercise for safety reasons 
 even though it is not expected to induce hypertrophy of the skeletal muscle
. therefore , we 
 consider that it is necessary to develop a safe and effective low intensity training program 
 for the elderly .
skeletal muscle contraction is classified into three types4 according to the relationship between the muscle torque and the load 
 torque .
the three types are isometric contraction , concentric contraction , and eccentric 
 contraction . in an isometric contraction , the muscle torque is equal to the load torque , and 
 the muscle length does not change . in a concentric contraction , the muscle torque is greater 
 than the load torque , and the muscle length shortens . in an eccentric contraction ,
the load 
 torque is greater than the muscle torque , and the muscle length lengthens .
an eccentric contraction has been shown to produce greater muscle hypertrophy than a 
 concentric contraction after resistance training5 .
in addition to greater hypertrophy , eccentric contraction training 
 tends to exert a lower stress on the cardiovascular system6 .
when eccentric contractions are used , lower - intensity training can 
 be better used than when concentric contractions are used to produce muscle hypertrophy .
downhill running is widely 
 used as a method of eccentric contraction training for rat skeletal muscles7,8,9 .
the purpose of this study was to examine whether a low intensity exercise using an 
 eccentric contraction would result in skeletal muscle hypertrophy in rats . in this 
 experiment ,
this speed corresponds to 
 about 50 to 60% of the lactate threshold in rats10 .
eighteen female wistar rats ( 10 weeks of age , 223.9  2.2  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 six rats each : the control group , the 
 level group , and the downhill group .
the two exercise groups ran on 
 the treadmill at 16 m / min for 90 minutes , once every three days for a total of twenty 
 sessions . a treadmill made for rats and mice ( exer-3/6 , columbus co. , ltd . )
was used . in 
 general , the resistance training period required for muscle hypertrophy is about eight 
 weeks11,12,13 .
the experimental groups performed treadmill exercise once 
 every three days for 60 days . on the 60th day after the start of the experiment ,
the 
 relative weight ratio of the muscle was calculated as the ratio of the wet weight of the 
 soleus muscle divided by the body weight at the end of the experiment .
the excised soleus 
 muscles were quick - frozen and cooled by dry ice in acetone .
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 ,
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 al14 .
the body weight , the muscle wet weight , the relative weight ratio of the muscle , and the 
 cross - section minor axis of the muscle were analyzed using one - way analysis of variance 
 ( anova ) .
when a significant difference was present , a multiple comparison test was performed 
 using tukey s test .
the analyses were performed using the spss version 20.0 ( spss inc . ) . 

body weight at the beginning of the experiment was 223.81.9  g in the control group , 
 223.82.9  g in the level group , and 224.22.1  g in the downhill group .
body weight at the 
 end of the experiment was 321.58.0  g in the control group , 333.89.0  g in the level group , 
 and 327.33.3  g in the downhill 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 weight , relative weight ratio , and fiber cross - section minor 
 axes are shown in table 1table 1.muscle wet weights , relative weight ratios and fiber cross - section minor axes 
 ( meansem)groupmuscle wet weight(mg)relative weight ratio(mg / g body weight)fiber cross - sectionminor axis ( m)control group146.3  6.10.46  0.0250.0  0.25level group157.9  4.60.47  0.0149.6  0.24downhill group 178.4  4.4 * 0.55  0.01 * 55.9  0.24 * * p < 0.01 , significant difference the control group .  
p < 0.01 , 
 significant difference the level group .. the muscle wet weights and the relative weight ratios of the downhill group 
 were significantly larger than those of the control and level groups ( p<0.01 ) .
there were 
 no differences in the muscle wet weights and the relative weight ratios between the control 
 group and level group .
the muscle fiber cross - section minor axes of the downhill group were 
 significantly longer than those of the control and level groups ( p<0.01 ) .
there was no 
 difference in the muscle fiber cross - section minor axes between the control group and level 
 group .
histologically , no abnormal findings , such as fiber necrosis and inflammatory cell 
 infiltration , were noted in the muscles of any of the groups ( fig .
bar=50 m.(a ) ; control , ( b ) ; level , 
 ( c ) ; downhill ) .
bar=50 m.(a ) ; control , ( b ) ; level , 
 ( c ) ; downhill
as shown in table 1 , the muscle wet weights and 
 relative weight ratios were not significantly different between the level and control 
 groups . in this experiment ,
the running speed corresponded to about 50 to 60% of the lactate 
 threshold in the rats10 , which could be 
 described as a very low intensity exercise .
low intensity exercise effectively increases 
 endurance capacity , but it does not effectively induce hypertrophy of the skeletal 
 muscle15 . therefore , the level group 
 did not show a gain in hypertrophy of the soleus muscle .
in contrast , the muscle wet 
 weights , relative weight ratios , and muscle fiber cross - section minor axes of the downhill 
 group were significantly larger than those of the control and level groups .
these results 
 indicate that a low intensity exercise using an eccentric contraction in downhill running 
 may be an effective method of inducing hypertrophy of skeletal muscle .
the muscle blood flow decreases when a muscle contracts , and expressions of growth hormone 
 and insulin - like growth factor-1 increase16 , 17 .
previous reports5,6,7 , 
 16 , 17 have shown that , compared to a concentric contraction , an eccentric 
 contraction elicits a different response to the mechanical stress and metabolic response 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 contraction5,6,7 , 16 , 17 . for this reason , an 
 eccentric contraction exercise produces greater muscle hypertrophy than a concentric 
 contraction exercise5 .
however , a 
 high - intensity eccentric contraction is also known to cause an increase in muscle fiber 
 damage , induce delayed - onset muscle soreness , and cause a temporary decrease in muscular 
 strength18 . in the current study , no 
 abnormal findings , such as fiber necrosis or inflammatory cell infiltration ,
were noted in 
 the muscles of the level and downhill groups . the treadmill speed of our experiment , 16 
 m / min , provided a low - intensity exercise .
therefore , we consider that the mechanical stress 
 experienced by the downhill group was sufficient to induce muscle hypertrophy without any 
 over - stressing that might have produced muscle fiber damage .
it has been reported that in resistance training , training should be done two or three 
 times per week2 , 19 .
thus , we used this training frequency our exercise frequency in 
 this experiment2 , 19 . the results of exercising at this frequency and low intensity 
 indicate that a low intensity exercise using an eccentric contraction is effective at 
 increasing muscle mass .
in addition , our findings suggest that a low intensity exercise 
 using an eccentric contraction may be suitable for the treatment of the elderly , because low 
 intensity exercise exerts less stress on cardiovascular function and articular 
 structures9 , 20 .
the mechanism of muscle hypertrophy induction through low intensity eccentric exercise is 
 not clearly understood , and investigation using biochemical analysis is needed in future 
 studies .
furthermore , an exercise protocol used for animals can not always be directly 
 applied to humans .
re - examination of the exercise protocol with regard to intensity and 
 frequency is required before clinical application .
 [ purpose ] the purpose of this study was to examine whether a low intensity exercise using 
 an eccentric contraction would result in skeletal muscle hypertrophy in rats . 
 [ subjects 
 and methods ] eighteen female wistar rats were used in this study . 
 the rats were randomly 
 divided into three groups . 
 the control group performed no exercise . 
 the level group ran on 
 a treadmill on a 0 incline . 
 the downhill group ran on a treadmill on a 16 incline . 
 the 
 two exercise groups ran on a treadmill at 16 m / min for 90 minutes , once every three days 
 for a total of twenty sessions . 
 [ results ] the muscle wet weights , the relative weight 
 ratios , and the muscle fiber cross - section minor axes of the downhill group were 
 significantly larger than those of the control and level groups . 
 there were no differences 
 in the muscle wet weights , the relative weight ratios , and the muscle fiber cross - section 
 minor axes between the control group and the level group . 
 [ conclusion ] the stimulation 
 from the low intensity eccentric contraction may have produced enough mechanical stress to 
 induce muscle hypertrophy without the over - stressing that might have produced muscle fiber 
 damage . 
 these results indicate that this technique may be an effective method of inducing 
 hypertrophy in skeletal muscle . 
INTRODUCTION
SUBJECTS AND METHODS
RESULTS
DISCUSSION