traditionally , aerobic exercise ( ae ) has been considered as the optimal mode for improving factors of cardiorespiratory fitness such as : increases in stroke volume , enzyme activity , mitochondrial protein synthesis , and capillarization ( 8 , 11 , 14 ) .
in contrast , anecdotally , few people would consider aerobic exercise as a means to induce muscle hypertrophy , an adaptation more commonly associated with resistance exercise ( 17 ) .
however , aerobic exercise training has been demonstrated to precipitate skeletal muscular hypertrophy in a variety of populations comparable to the hypertrophy induced by resistance training ( 7 , 12 ) .
it is important to note that the majority of ae investigations on muscle hypertrophy have been conducted using cycling as the mode of exercise .
there is a noticeable gap in research examining the effects of walking and running upon muscle hypertrophy .
a recent review examined the existing body of evidence demonstrating that ae has a far greater influence on hypertrophy than previously thought ( 17 ) .
a progressive 12-week cycling protocol effectively induced hypertrophy of the quadriceps in untrained older women ( 7 , 16 ) and in untrained young and older men ( 9 ) , with growth ranging between 5% and 12% .
similar adaptations were demonstrated in middle - aged and older men following 16 weeks of either moderate ( 18 ) or high - intensity cycling ( 4%10% ) ( 13 ) .
even as little as 6 weeks of high - intensity cycling has resulted in significant hypertrophy in young women ( 7% ) ( 19 ) . in addition , a 12-week comparative study demonstrated analogous hypertrophy of the knee extensor in middle - aged women following either classic resistance training or cycling ( 3.1%3.7%)(12 ) .
in contrast , only one study has examined the impact of a walk / jog protocol on muscle hypertrophy in young and older men ( 23 ) .
the older cohort demonstrated a 9% increase in mid - thigh csa , while the young men did not exhibit growth .
however , it should be noted that the young men , though untrained , had a relatively high fitness level for their age group , which may have limited their adaptive capabilities .
given hypertrophy has been observed in untrained young , middle aged , and older populations utilizing an aerobic cycling protocol , as well as in an older male population through an aerobic walk / jog protocol , it is conceivable that comparable results could be observed in an untrained , young population through an aerobic running program .
an increasingly prevalent model of aerobic exercise is high - intensity interval training ( hiit ) . in recent years
, evidence has demonstrated greater adaptation following hiit than following traditional continuous run training ( ct ) ( 24 , 26 ) in a variety of fitness factors including : vo2max ( 21 ) ; sv ; ejection fraction ( 2 ) ; protein synthesis rates ; and work economy ( 27 ) .
comparable results have been shown across a variety of populations from young , healthy males to cad patients and post - infarction heart failure patients , where both hiit and ct were matched for caloric expenditure ( 14,16 ) .
additionally , the increased stimulus of hiit may be adequate for producing muscle hypertrophy , though this has not been sufficiently examined .
as hiit has been effective in inducing substantial physiological improvements in a variety of populations , we chose to use hiit as our model for a high - intensity running protocol . therefore , it was the purpose of this investigation to examine the influence of a hiit running protocol upon hypertrophy of the vastus lateralis in an untrained , young population .
twelve ( n=12 ) apparently healthy university students ( 19.90.5 yr ; 169.81.9 cm ; 63.82.3 kg ) were recruited for participation in the high - intensity interval training ( hiit ) program .
all subjects were recreationally active , participating in intramurals and other similar physical activities , but had not engaged in any formal endurance or strength training in the previous six months . for muscle size comparison , five additional subjects ( 210.3 yr )
the institutional review board at taylor university approved all procedures , and all subjects provided written informed consent prior to participation . to measure maximal aerobic capacity , hiit subjects performed an incremental treadmill test pre- and post - training intervention ( jas trackmaster , full vision inc , newton , ks ; trueone 2400 , parvo medics , sandy , ut ) . during the test , subjects completed a 4 minute warm - up at 5 kph at 0% grade . subsequently , there were three 2-minute stages of increasing speed based on the subject s comfort level , with the highest speed corresponding to a rate of perceived exertion ( rpe ) of 1315 .
the remainder of each test was conducted at the subject s highest speed , while grade increased by 0.5% every 30 seconds until the subject reached volitional fatigue .
heart rate was monitored throughout the test ( polar ft7f , lake success , ny ) .
maximal heart rate achieved during the test was used to determine intensity throughout the exercise intervention .
achievement of vo2max was accepted when two of three criteria were met : 1 . )
hr within 10 bpm age - predicted hrmax ; 2 . ) respiratory exchange ratio was higher than 1.10 ; 3 . )
less than 2.0 ml / kg / min change in vo2 observed with each increase in workload .
data was collected every 30 seconds ( trueone 2400 , parvo medics , sandy , ut ) , and the mean of the highest consecutive values was used for analysis .
the maximal aerobic capacity of the ctl group was not assessed , as they were examined for muscle size comparison only . to determine csa of the vastus lateralis ( vl ) before the 10-week exercise program , all subjects reported to the lab following a minimum of 36 hours of rest from physical activity extending beyond daily ambulation ( i.e. walking to class ) ( 20 ) .
ctl subjects followed the same protocol for the post - test , while hiit subjects reported to the lab 36 hours after their final hiit session . upon arrival
, subjects completed 30 minutes of supine rest to account for fluid shifts ( 3 ) before panoramic ultrasound images were captured using a ge logic p3 ultrasound ( fairfield , ct ) .
the length of the femur for each subject was recorded as the distance from the superior border of the patella to the asis ( figure 1 ) , and a line was drawn perpendicular to the femur 1/3 of the distance up from patella ( figure 2 ) .
high - density foam was adhered to the thigh at and above this marker to act as a guide for the sound head ( figures 34 ) , ensuring consistent planes of the muscle were measured pre- and post - intervention .
a set of three images was captured at both pre- and post- tests , for all subjects .
all images were assessed via manual planimetry using image j software ( figure 5 ) .
the same examiner analyzed both pre- and post - intervention images and was blinded to both group and time . for both time points ,
each image was measured five times , with a coefficient of variation less than 1.0% between measurements . to prevent any potential outlier images from influencing our results , we chose to use the median , rather than the mean , of the three image values in the statistical analysis .
training sessions were conducted on average 3 dwk on non - consecutive days , for 10 weeks ; sessions included a 10 minute warm - up period before hiit , and concluded with a 3 minute cool - down at 70% hrmax , as determined by the maximal aerobic capacity test . during each trial
, hiit subjects completed four sets of 4 minutes running at 9095% hrmax followed by 3 minutes active rest at a predetermined speed that would elicit ~70% hrmax , as previously described ( 10 ) . in addition to the hiit protocol , a subset of seven of the original twelve subjects completed an additional 2 dwk of continuous running for 30 min at 7075% hrmax , as part of a larger study .
however , as there were no significant differences in response between groups , all data were combined for analysis .
all sessions were performed on a treadmill ( integrity series , lifefitness , rosemont , il ) at 0% grade .
all exercise sessions were supervised by a member of the research team , and heart rate was monitored ( polar ft7f , lake success , ny ) to ensure desired intensity was attained .
ctl subjects were advised to maintain their current activity level , while abstaining from any formal training for the 10-week intervention period .
the mean pre- and post - intervention measurements of aerobic capacity were analyzed through spss software using a dependent t - test with a significance level of p<0.05 .
mean muscle csa values were analyzed using a one - way anova with significance level set at p<0.05 .
twelve ( n=12 ) apparently healthy university students ( 19.90.5 yr ; 169.81.9 cm ; 63.82.3 kg ) were recruited for participation in the high - intensity interval training ( hiit ) program .
all subjects were recreationally active , participating in intramurals and other similar physical activities , but had not engaged in any formal endurance or strength training in the previous six months . for muscle size comparison , five additional subjects ( 210.3 yr )
the institutional review board at taylor university approved all procedures , and all subjects provided written informed consent prior to participation .
to measure maximal aerobic capacity , hiit subjects performed an incremental treadmill test pre- and post - training intervention ( jas trackmaster , full vision inc , newton , ks ; trueone 2400 , parvo medics , sandy , ut ) . during the test
, subjects completed a 4 minute warm - up at 5 kph at 0% grade .
subsequently , there were three 2-minute stages of increasing speed based on the subject s comfort level , with the highest speed corresponding to a rate of perceived exertion ( rpe ) of 1315 .
the remainder of each test was conducted at the subject s highest speed , while grade increased by 0.5% every 30 seconds until the subject reached volitional fatigue .
heart rate was monitored throughout the test ( polar ft7f , lake success , ny ) .
maximal heart rate achieved during the test was used to determine intensity throughout the exercise intervention .
achievement of vo2max was accepted when two of three criteria were met : 1 . )
hr within 10 bpm age - predicted hrmax ; 2 . ) respiratory exchange ratio was higher than 1.10 ; 3 . )
less than 2.0 ml / kg / min change in vo2 observed with each increase in workload .
data was collected every 30 seconds ( trueone 2400 , parvo medics , sandy , ut ) , and the mean of the highest consecutive values was used for analysis .
the maximal aerobic capacity of the ctl group was not assessed , as they were examined for muscle size comparison only . to determine csa of the vastus lateralis ( vl ) before the 10-week exercise program , all subjects reported to the lab following a minimum of 36 hours of rest from physical activity extending beyond daily ambulation ( i.e. walking to class ) ( 20 ) .
ctl subjects followed the same protocol for the post - test , while hiit subjects reported to the lab 36 hours after their final hiit session . upon arrival
, subjects completed 30 minutes of supine rest to account for fluid shifts ( 3 ) before panoramic ultrasound images were captured using a ge logic p3 ultrasound ( fairfield , ct ) .
the length of the femur for each subject was recorded as the distance from the superior border of the patella to the asis ( figure 1 ) , and a line was drawn perpendicular to the femur 1/3 of the distance up from patella ( figure 2 ) .
high - density foam was adhered to the thigh at and above this marker to act as a guide for the sound head ( figures 34 ) , ensuring consistent planes of the muscle were measured pre- and post - intervention .
a set of three images was captured at both pre- and post- tests , for all subjects .
all images were assessed via manual planimetry using image j software ( figure 5 ) .
the same examiner analyzed both pre- and post - intervention images and was blinded to both group and time . for both time points ,
each image was measured five times , with a coefficient of variation less than 1.0% between measurements . to prevent any potential outlier images from influencing our results
, we chose to use the median , rather than the mean , of the three image values in the statistical analysis .
training sessions were conducted on average 3 dwk on non - consecutive days , for 10 weeks ; sessions included a 10 minute warm - up period before hiit , and concluded with a 3 minute cool - down at 70% hrmax , as determined by the maximal aerobic capacity test . during each trial
, hiit subjects completed four sets of 4 minutes running at 9095% hrmax followed by 3 minutes active rest at a predetermined speed that would elicit ~70% hrmax , as previously described ( 10 ) . in addition to the hiit protocol , a subset of seven of the original twelve subjects completed an additional 2 dwk of continuous running for 30 min at 7075% hrmax , as part of a larger study .
however , as there were no significant differences in response between groups , all data were combined for analysis .
all sessions were performed on a treadmill ( integrity series , lifefitness , rosemont , il ) at 0% grade .
all exercise sessions were supervised by a member of the research team , and heart rate was monitored ( polar ft7f , lake success , ny ) to ensure desired intensity was attained .
ctl subjects were advised to maintain their current activity level , while abstaining from any formal training for the 10-week intervention period .
the mean pre- and post - intervention measurements of aerobic capacity were analyzed through spss software using a dependent t - test with a significance level of p<0.05 .
mean muscle csa values were analyzed using a one - way anova with significance level set at p<0.05 .
subjects completed 24.50.6 training sessions each , averaging 92% hrmax during intervals and 72.4% during recovery .
aerobic capacity , assessed as maximal oxygen uptake ( vo2max ) , increased significantly following the high - intensity interval protocol .
subjects exhibited an increase in both relative ( 5.22.2% ) and absolute ( 6.02.3% ) vo2max .
absolute changes in vo2max were 2.41.1 ml / kg / min and 0.20.1 l / min , respectively .
vl csa significantly increased by an average of 10.62.7% in the hiit group following the intervention ( figure 6 ) .
absolute muscle csa increased from 16.11.1 cm to 17.61.0 cm ( p<0.05 , =0.34 ) .
while evidence continues to accrue regarding the positive impacts of aerobic exercise upon muscle hypertrophy , the majority of studies have utilized cycling as their mode of intervention . there is a significant gap in the research examining the affects of a running protocol upon muscular hypertrophy . to address this , we examined hypertrophy of the vastus lateralis following a 10-week hiit running protocol in untrained college students .
additionally , subjects exhibited a 6% increase in vo2max , a value in accordance with those of comparable studies ( 6 , 10 , 15 , 22 ) .
adaptation in csa of the vastus lateralis was congruent with that demonstrated through cycling protocols of comparable duration and intensity ( 9 , 18 , 19 ) . to our knowledge , this is the first study to examine run training induced hypertrophy in an untrained , young population including both males and females , though similar adaptation has been exhibited by an older , male population following a high - intensity walk / jog protocol ( 23 ) .
conducted a 27-week study wherein they examined the impact of a high - intensity walk / jog intervention upon hypertrophy of the quadriceps in both young and older men . the protocol consisted of 45 min .
the older cohort exhibited a 9% increase in mid - thigh csa , a value comparable to that induced through cycling protocols and the current study ; however , the young men did not experience any adaptation . upon closer examination , we discovered that these young men , though untrained , would have been categorized in the 85 percentile of fitness for their age group , according to current acsm standards in relation to vo2max ( 1 ) .
in contrast , the older cohort in the schwartz study fell into the 25 percentile , or  poor  fitness category .
further investigation revealed that subjects examined in the existing body of cycling interventions would have fallen into or below the  below average  fitness category ( 5  40 percentiles ) , according to their initial vo2max values ( 7 , 9 , 16 , 18 , 19 ) .
average  fitness category . as the young cohort of the schwartz study were the only subjects assessed to surpass the 50 percentile of fitness before intervention , as well as the only subjects that did not experience hypertrophy
, we suspect the high initial fitness of the young men inhibited their adaptive capabilities , thereby preventing muscle growth , despite the intensity of the intervention .
the results of this study are consistent with the existing body of evidence in regards to muscle hypertrophy .
additionally , although we did not assess cellular changes , mechanisms associated with muscle growth and aerobic exercise have been reported .
specifically , aerobic exercise has been demonstrated to inhibit myostatin within the cell ( 16 ) , thereby allowing for heightened activity of the akt - mtor pathway , which results in muscle protein synthesis .
simultaneously , the repeated muscle contraction of aerobic exercise augments the functioning of pgc-1 ( 4 ) , leading to a decrease in muscle protein breakdown through the inhibition of foxo3a , a catalyst for muscular catabolism .
both of these aerobically induced adaptations lead to a positive muscle protein balance within the cell , and therefore to hypertrophy of the fiber .
aerobic exercise has been characterized by inducing significant cardiorespiratory adaptation across a wide variety of populations .
furthermore , it has been demonstrated to be effective in producing significant muscular hypertrophy following various aerobic cycling and now running protocols in both young and older populations ( 23 ) .
these findings suggest that beginners could utilize high - intensity running as an effective method of achieving concurrent increases in vo2max and muscle csa .
given the often - cited reason of lack of time for exercise , our findings may offer motivation for novices looking to increase aerobic fitness as well as quadriceps muscle size .
in addition , as skeletal muscle is a primary site of glucose disposal within the body , we speculate that the increase in muscle size may contribute to the improved whole body glucose disposal rates associated with ae , though further investigations are necessary ( 5 , 25 ) .
our findings support evidence suggesting aerobic exercise to be an effective mode of activity for the promotion of cardiorespiratory fitness and increase in whole muscle size of the quadriceps .
 aerobic cycling has been repeatedly shown to induce hypertrophy in skeletal muscle across a variety of populations , while there has been a lack of investigation into the impact of running upon hypertrophy . 
 an increasingly popular model of aerobic exercise is high - intensity interval training ( hiit ) ; in addition to its positive impact upon cardiovascular health , hiit may be sufficient for inducing significant muscular hypertrophy . 
 therefore , the purpose of this investigation was to examine the influence of a high - intensity interval running protocol upon hypertrophy of the vastus lateralis in an untrained , young population . 
 twelve recreationally active university students ( male : 2 ; female : 10 ; 19.90.5 yr . ; 169.81.9 cm ; 63.82.3 kg ; vo2max : 42.11.6 mlkg1min1 ) completed 24.50.6 sessions of high - intensity interval run training over 10 weeks . 
 the protocol consisted of four sets of 4 minutes running at 9095% hrmax followed by 3 minutes active rest at 70% hrmax . 
 relative and absolute aerobic capacity increased 5.22.2% and 6.02.3% respectively as a result of the intervention ( p < 0.05 ) . 
 cross - sectional area ( csa ) of the vastus lateralis was measured via panoramic ultrasound imaging pre- and post - intervention . 
 following the protocol , csa of the intervention group was 10.62.7% greater ( p < 0.05 ) , while that of the control group did not change . 
 this is the first data to demonstrate hypertrophy of the vastus lateralis in a young population following a running protocol . 
 these data support the existing body of evidence suggesting aerobic exercise to be an effective mode of improving cardiorespiratory fitness as well as increasing whole muscle size of the quadriceps . 
INTRODUCTION
METHODS
Participants
Protocol
Statistical Analysis
RESULTS
DISCUSSION