previous studies have consistently shown reductions in resting blood pressure following isometric training [ 14 ] . however , it is not fully understood whether these reductions are accompanied by changes to the local vasculature or what role exercise intensity has in adaptations to this type of training . most studies have focussed on the changes in resting blood pressure after training at a given intensity , mostly involving arm training at 30% maximum voluntary contraction ( mvc ) [ 1 , 2 , 5 , 6 ] rather than examining the role of exercise intensity in the adaptations to training . two studies have investigated the effects of isometric training intensity on reductions in resting blood pressure , one using handgrip   and the other using double - leg extension . however , it is difficult to interpret the role of training intensity in the study by wiley et al .   because they reported two different training studies , performed at intensities of 30% and 50% mvc , with differing lengths of contractions ( 4    2  min , 4    45  s ) and recovery times ( 3  min , 1  min ) for periods of 8 and 5 weeks , respectively . the investigation by wiles et al .   is the only study to prospectively investigate the effects of different isometric training intensities on the training - induced reductions in resting blood pressure . it appears from the data presented in their study ( figure 3 ) , that most of the reductions in resting blood pressure occurred in the first 4 weeks in their higher - intensity training group ( approximately 20% mvc ) , whereas the changes in the lower - intensity group ( approximately 10% mvc ) took 8 weeks . indeed , in a subsequent study by the same group , higher - intensity double - leg training produced significant reductions in resting blood pressure after 4 weeks . interestingly , blood pressure reductions associated with aerobic training ( dynamic ) are not thought to be dependent on exercise intensity [ 8 , 9 ] . taken together , these studies suggest that isometric training intensity might be important in the adaptations seen on the completion of such training , and in the rate at which they occur . the mechanism by which the isometric training - induced reduction in resting blood pressure is achieved remains unclear . studies using dynamic exercise to elicit reductions in resting blood pressure have reported concurrent adaptations in both local and/or systemic vasculature   that seemingly lowers total peripheral resistance . these changes have been associated with modified sympathetic nerve activity , increased artery lumen diameter and improved endothelium - dependent vasodilatation [ 10 , 12 ] . recently , tinken et al .   also reported elevated nitric oxide ( no ) levels following extended exercise training ( 8 weeks ) which are thought to mediate vascular remodelling [ 13 , 14 ] resulting in an increased lumen diameter [ 11 , 15 ] . however , few studies to date have identified changes in resting vascular dimensions or blood flow after isometric training . previous isometric training studies have shown alterations in central artery compliance [ 2 , 16 ] and improved endothelium - dependent vasodilatation , but attempts have not been made to relate these changes to the intensity of training . therefore , the aims of this study were to determine whether reductions in resting blood pressure are associated with adaptations in the muscle vasculature ( of trained and untrained limbs ) and whether the time course of these adaptations , is training intensity dependent . these individuals did not engage in regular or structured exercise sessions and were classified as having a low level of physical activity following completion of the international physical activity questionnaire . the first training group ( hi ) undertook isometric training at an intensity equivalent to 14% mvc ( 85% of their peak heart rate obtained during an incremental isometric exercise test ; n = 10 ) . the second group ( lo ) trained at an intensity that was equivalent to 8% mvc ( 70% of their peak hr ; n = 10 ) and the final group acted as controls ( con ; n = 10 ) . after receiving institutional ethical approval , each participant received a detailed information sheet explaining the experimental protocol and potential risks involved then signed and completed a consent form and pretest medical questionnaire . all tests were performed in a consistent laboratory environment using an isokinetic dynamometer ( biodex medical systems , inc . the dynamometer was fitted with an extended leg attachment to enable double - leg isometric extensions to be performed . to allow for precise synchronisation of torque , electromyography ( emg ) , heart rate blood pressure , and all data generated by the dynamometer were transferred to the mp150ws biopac data acquisition system using acknowledge ( v4 ) software ( biopac , ca , usa ) via an analogue output . all other data ( emg , heart rate , and blood pressure ) were recorded using the biopac data acquisition unit with a sampling frequency of 2000  hz .   for the purposes of isometric training , emg was measured from the vastus lateralis ( vl ) of both legs during double - leg extension using active bipolar surface electrodes ( mp - a2 biopac , ca , usa ) with a 25  mm interelectrode distance . the electrodes were placed approximately one - third of the muscle length above the patella and orientated parallel to the direction of the underlying muscle fibres . the emg signal was amplified ( gain = 300 , input impedance = 10  g ) by the active electrodes and directed to a high - level transducer ( hlt100c biopac , ca , usa ) . the emg was further filtered using 20  hz high - pass and 500  hz low - pass filters , smoothed with a 250  ms moving average and was root mean squared . the root mean squared values from both limbs were then averaged to give a single root mean squared emg value .   blood pressure ( systolic , diastolic , and mean arterial pressure ) was also continually measured using a noninvasive blood pressure ( nibp ) monitor ( nibp100a , biopac , ca , usa ) . the mean pressure values of the pressure waveform are displayed on a monitor via a sensor positioned percutaneously across the radial artery at the distal edge of the radius with the reading being updated approximately every 10 seconds . the use of this type of blood - pressure monitoring has been assessed for accuracy and reliability of measurement   and has been reported to be an acceptable tool for measuring blood pressure . coefficients of variation , expressed as a percentage of the mean , ranged from systolic 2.2% to diastolic and 3.0% indicating a high reliability . resting blood pressure measures were recorded for 60 seconds following 15 minutes of rest , with the participants in the supine position . heart rate was recorded continuously through a single - lead electrocardiogram using pregelled disposable electrodes ( el501 , biopac , ca , usa ) . an ultrasound video imaging system ( logiq book xp , general electric , bedford , uk ) in the pulse wave doppler mode , equipped with a 11  mm diameter linear transducer probe ( 8l - rs , general electric ) operating at an imaging frequency of 8  mhz with a variable doppler frequency of 410  mhz was used to measure two - dimensional ( 2d ) femoral and brachial artery diameter and mean arterial velocity simultaneously , at rest . all continuous mean blood velocity measures throughout the investigation were taken with a doppler transducer probe insonation angle of 60. when recording femoral measures , the probe was positioned over the common femoral artery distal to the inguinal ligament and above the bifurcation into the superficial and profunda femoral branch . for the brachial artery , the probe was placed over the brachial artery approximately 9  cm proximal to the medial epicondyle . the ultrasound gates were adjusted to the width of the artery to ensure complete insonation of the artery . arterial diameter was determined by a perpendicular measure from the lumen - intima interface of the rear wall to the lumen - intima interface of the far wall of the vessel . vascular diameter and velocity measure were determined as the mean of three measures collected at 20  s intervals during the 1 minute data collection period and analysed by the same investigator [ 19 , 20 ] . the mean arterial diameter and mean blood velocity measures were processed by the ultrasound software to generate a mean blood flow value . vascular conductance was calculated from mean blood flow and mean arterial pressure measures . the day - to - day reliability of these measures was determined prior to the start of the study . the cv for the two trials was ; 2.42 ( femoral ) and 3.06 ( brachial ) mean artery diameter , 3.19 ( femoral ) and 3.45 ( brachial ) for mean blood velocity and 2.76 ( femoral ) and 3.73 ( brachial ) for mean blood flow . isometric training was performed in the same way as detailed previously [ 4 , 21 ] . the training was performed on 3 days per week for a period of 8 weeks , with training sessions at least 24 hours apart , using target emg values that equated to either 85% ( hi ) or 70% ( lo ) of the peak heart rate ( hrpeak ) achieved during the final exercise intensity of the incremental test . each training session comprised 4  2-minute bouts interspaced with two - minute rest periods . during all training sessions emg , heart rate , blood pressure , and torque the target emg values were recalculated following a repeat of the incremental isometric exercise test at the end of week four . this allowed for adjustments to the intensity as training progressed and took account of any adaptations which may have occurred over the initial four weeks of training . following the initial familiarisation session and prior to the first incremental isometric exercise test , participants resting heart rate and blood pressure measures were recorded . also , brachial and femoral artery blood flow , blood velocity , artery diameter , and venous compliance were measured and recorded . these measures were subsequently recorded at the end of week 4 and after - training . it has been reported   that the magnitude of the blood pressure reductions are related to the initial resting values . therefore , analysis of covariance ( ancova ) was used to determine if there was a significant difference in the mid and after - training blood pressure compared to the baseline measures , using the baseline values as the covariate . microsoft graphpad prism software , which uses ancova to compare the slopes and intercepts of the linear regression lines , was used to analyse the data from the incremental isometric exercise test . an alpha level of 0.05 was accepted as being significantly significant . baseline data indicated that there were no significant pre - training differences between groups ( hi versus lo versus con ) for maximum voluntary contraction ( mvc ) , body mass , resting heart rate ( rhr ) , resting systolic ( rsbp ) , diastolic ( rdbp ) , mean arterial blood pressure ( rmap ) , femoral artery blood flow ( fabf ) , femoral mean blood velocity ( fmbv ) , femoral mean artery diameter ( fmad ) , and femoral venous compliance ( fvc ; p > 0.05 in all cases ) . there was a significant reduction in rmap and rsbp after training in the hi group ( from 98.3    5.5 to 93.6    6.6  mmhg for map ; p < 0.01 ; from 138.7    7.0 to 127.9    8.0  mmhg , for sbp ; p < 0.01 ; ancova ) . however , there were no significant changes in rmap or rsbp following the eight - week training intervention in either the control or the lo groups ( all p > 0.05 ; ancova ) . no significant changes in rdbp were evident in any group after training ( all p > 0.05 ; ancova ) . individual analysis of all three groups using ancova revealed no significant difference in the slopes ( p > 0.05 ) for all blood pressure measures emg relationships . there was also no significant difference in the elevation ( intercept ) of all blood pressure regression lines ( p > 0.05 ) for the con and lo groups . however , there was a significant difference in the elevation of the rsbp regression line ( p < 0.01 ) for the hi group . a significant correlation was identified between the initial rsbp values and the training - induced changes in rsbp ( r = 0.79 , p < 0.01 ; figure 1 ) . there was also a correlation evident ( see figure 2 ) between fmad and rsbp changes ( r = 0.79 , p < 0.05 ) after training intervention . a significant decrease in rhr was found in the hi group ( from 71.0  7.7 to 66.2  5.4  b.min ; p < 0.05 ) . however , these reductions were not observed in the lo or the control groups ( p > 0.05 in both cases ) . fabf increased significantly after training in the hi group ( from 215.9  39.2 to 298.0  50.4  mlmin , p < 0.01 ) . this appeared to have occurred via significant increases in fmbv from 7.2  1.1 to 7.9  1.0  cms ( p < 0.01 ) and fmad from 8.1  0.9 to 9.1  1.1  mm ( p < 0.01 ) . these changes appeared to have occurred mostly between 4 and 8 weeks of training ( see table 3 ) . there were no significant changes in these same variables in either the lo or the control groups ( all p > 0.05 ) . fvc also significantly increased compared to baseline values ( from 2.19  0.5 to 3.18  0.5  u ; p < 0.01 ) . there were no significant changes in brachial artery measures ( artery blood flow , mean blood velocity , and mean artery diameter ) following training ( all p > 0.05 ) . < 0.05 ) in mvc at the midtraining point ( end of week 4 ) in the hi group . however , there were no further increases in mvc between the mid- and posttest ( p > 0.05 ) . neither the lo ( 8.6  32.6  nm , p > 0.05 ) nor the control group ( 2.3  5.1  nm , p > 0.05 ) showed a significant change in their mvc at the end of the 8-week training period . the results of this study showed that training - induced reductions in resting blood pressure and concomitant vascular adaptations are dependent on isometric training intensity . the vascular adaptations comprised of increased resting femoral artery diameter , blood flow , blood velocity , and vascular conductance . these vascular changes only occurred in the higher - intensity training group and appeared to have occurred mostly between 4 and 8 weeks of training . these adaptations were not apparent in the vasculature of the untrained limbs ( brachial artery ) showing that the vascular adaptations were not only intensity - dependent , but were also localised and only became evident in the latter stages of the training intervention . there are several factors associated with isometric training intensity that could be responsible for the intensity - dependent changes seen in the present study . the intensity of isometric exercise is known to influence the magnitude of central command , and therefore cardiovascular centre irradiation [ 1 , 23 ] . therefore , as intensity increases , there is a greater cardiac output , exercising systolic , and mean arterial blood pressure . because much of the effects of any type of exercise are known to be mediated via autonomic control , isometric exercise intensity , therefore , also influences the contribution of the sympathetic autonomic nervous system to the responses to such intense exercise . isometric exercise intensity is also known to affect the extent to which blood flow into the exercising muscle is occluded . for example , although there have been conflicting reports , it appears that as isometric exercise intensity rises above 1015% mvc , the degree to which blood flow is occluded increases in proportion to the force generated within the muscle [ 25 , 26 ] . it has also been suggested that the intensity - dependent accumulation of exercising metabolites , although much lower than during dynamic exercise , plays an important role in metaboreceptor stimulation and thereby , could cause an intensity - dependent pressor response , during isometric exercise . it is possible that the combination of powerful pressor response and high level of occlusion during higher - intensity isometric exercise is responsible for a marked effect on blood flow , and particularly on vascular shear rates , which might explain our localised vascular adaptations ( this will be discussed in more detail , later in this section ) . it would follow that , at intensities where the pressor response is weak and the occlusion is incomplete , the stimulus for local vascular adaptations is too weak to see significant changes in resting blood pressure within 8 weeks .   the data reported in figure 2 indicate a significant correlation between the decrease in rsbp and increases in fmad . it is possible that these changes could have at least partly been caused by altered resting vascular tone , via modulation of sympathetic tone , especially since rhr was also reduced after training . although both     and -adrenergic receptors are involved in control of resting vascular tone , there have been recent studies that have demonstrated reduced vascular vasoconstrictor responsiveness to noradrenaline after training in animals . this was explained as a training - induced attenuation of noradrenaline - mediated vasoconstriction , which is thought to occur through an upregulation of the endothelial -receptor - nitric oxide synthase signalling pathway . however , similar findings have previously only been reported after resistance training in humans   and have been reported alongside increases in plasma concentrations of noradrenaline , not decreases , as shown in animals .   however , a more likely explanation is that at least part of the stimulus for such changes arises from changes in bioavailability and/or bioactivity of no resulting from shear stresses during exercise [ 3 , 5 , 14 ] . as mentioned earlier , the augmented pressor response and the higher level of occlusion experienced during the higher - intensity training could have exposed the vasculature to especially high shear stresses , thereby , enhancing the release of no . not only would this stimulate vascular adaptations such as an increased lumen diameter [ 11 , 15 ] , but it also appears that the timecourse of these adaptations could explain some of our findings . tinken et al .   suggested that there might be a biphasic pattern to the shear stress - induced adaptations in the local vasculature , comprising a no - mediated increase in vascular function initially ( 24 weeks ) and following longer - term training ( 8 weeks ) , the vasculature is remodelled to normalise shear stress [ 13 , 14 ] . this would correspond with the vascular adaptations seen in our study which mostly occurred between 4 and 8 weeks of bilateral - leg isometric training . however , presently these explanations are somewhat speculative , not least because there is little previous evidence for such shear stress - induced adaptations in the femoral artery , which is thought to have reduced responsiveness to shear stress stimuli , compared to other vessels . to the authors ' knowledge , no previous isometric training study has reported reduced resting heart rate following this type of training . the mechanisms responsible for the attenuated resting heart rate observed in the hi group are not easily explained . a combination of factors may be responsible , including changes to central command associated with reduced sympathetic nerve activity and a decrease in muscle mechano-/metabo - receptor afferent activity . indeed , participants in the hi group also demonstrated significant improvements in their mvc at the midpoint of training . research suggests that trained muscles ( possibly evident in the hi group at week 4 ) are more fatigue - resistant and muscle fibres may be recruited more synchronously . therefore , participants may experience reduced accumulation of metabolites [ 27 , 32 ] , thereby reducing the afferent metabo - receptor response [ 27 , 33 ] .   however , whilst this could explain a reduced exercising heart rate , it does not explain the reduced heart rate at rest . some previous researchers have reported changes in heart rate variability after isometric training , a marker for autonomic adaptations , which may better explain the heart rate data . heart rate variability ( and blood pressure variability ) analyses showed increased vagal activity in that study . other researchers have reported no change in mean sympathetic nerve activity   after such training ( although their reductions in resting blood pressure were modest ) . however , more - recent work by millar et al .   has shown improved neurocardiac regulation immediately after a single bout of isometric handgrip exercise . it is possible that the differences in the magnitude of the reduction in resting blood pressure may be due , in part , to the initial resting levels . indeed the data presented in figure 1 identified a significant correlation between these measures . the majority of training studies that have noted reductions in resting blood pressure following isometric exercise training in older individuals have used participants who were either hypertensive [ 5 , 6 ] or high normotensive [ 2 , 36 ] . interestingly , the majority of the observed blood pressure changes in these and the present training study tend to be greater than those seen when using normotensive adults . results of training studies recruiting predominantly younger normotensive subjects report reductions in systolic blood pressure of approximately 4 - 5  mmhg [ 7 , 34 ] . the only other study to investigate the blood pressure responses to isometric bilateral - leg extension over an eight - week period reported systolic blood pressure reductions of 5  mmhg , approximately half of that observed in this investigation .   as hypertension is associated with endothelial dysfunction , it is possible that the mechanisms responsible for the observed reductions in resting blood pressure in participants with elevated blood pressure may differ to those seen in normotensive adults . previous research using hypertensive adults [ 5 , 36 ] have reported elevations in endothelial - dependent vasodilation of the trained limb following isometric exercise training . they noted that shear stress mediated increased bioactivity and/or bioavailability of no could be responsible for the training induced improvements in endothelial function . furthermore , as these studies reported no changes in endothelial - independent vasodilation , it was suggested that these changes were influenced by functional rather than structural adaptations . however , as the elevations in endothelial - dependent vasodilation were only evident in the trained limb rather than systemically , and blood pressure is primarily mediated by resistance vessels , it is unlikely that the changes in femoral structure would be fully responsible for the improvements in after training resting blood pressure in hypertensive individuals . although , it is possible that the elevations in the no - dilator system may be involved in the systemic reduction in resting blood pressure .   to date , 40  yrs [ 1 , 4 , 34 ] or older adults 60  yrs [ 2 , 5 , 37 ] , when investigating blood pressure adaptation and its potential causes , with no research specifically focusing on middle - aged adults ( 4560  yrs ) . ageing is suggested to be associated with increased physiological stress during submaximal exercise , a reduced capacity to perform exercise   and elevated levels of resting heart rate and blood pressure . these changes have been explained by age - related functional decline of bodily systems including the cardiovascular and musculoskeletal systems . more specifically , research has reported a loss of large artery compliance and elevated sympathetic vasoconstriction [ 41 , 42 ] , between the ages of 3060 , with a more pronounced functional decline thereafter . all of these physiological changes could theoretically influence the responses and adaptations of individuals in this age group to isometric training . the findings of the present study showed that training - induced reductions in resting blood pressure were associated with changes in the local vasculature , which were dependent on isometric training intensity . these changes occurred mostly in the later stages of training ( between 4 and 8 weeks ) and were not evident in untrained limbs .
the purpose of this study was to establish whether changes in resting blood pressure and the vasculature of trained and untrained limbs are dependent on training intensity , following isometric - leg training . 
 thirty middle - aged males undertook an 8 week training programme ( 4  2  min bilateral - leg isometric contractions 3 times per week ) . 
 two groups trained at either high ( hi ; 14%mvc ) or low ( lo ; 8%mvc ) intensity a third group ( con ) acted as controls . 
 all parameters were measured at baseline , 4-weeks and post - training . 
 resting sbp ( 10.8  7.9  mmhg ) , map ( 4.7  6.8  mmhg ) and hr ( 4.8  5.9 
 bmin1 ) fell significantly in the hi group post - training with concomitant significant increases in resting femoral mean artery diameter ( fmad ; 1.0  0.4  mm ) , femoral mean blood velocity ( fmbv ; 0.68  0.83  cms1 ) , resting femoral artery blood flow ( fabf ; 82.06  31.92  mlmin1 ) and resting femoral vascular conductance ( fvc , 45% ) . 
 no significant changes occurred in any brachial artery measure nor in any parameters measured in the lo or con groups . 
 these findings show that training - induced reductions in resting blood pressure after isometric - leg training in healthy middle - aged men are associated with concomitant adaptations in the local vasculature , that appear to be dependent on training intensity and take place in the later stages of training .