strength and endurance training can lead to many physiological changes within the human 
 body and it aims to improve overall physical performance . training improves performance , in 
 part , due to muscular adaptations in morphology , such as conversion between fiber types , and 
 altered biomechanical properties , tendon stiffness , and increased elasticity of muscles1 . such intrinsic muscular adaptations of 
 mechanical properties are thought to enhance , or impact , muscle performance during muscle 
 contraction2 , 3 . therefore , training is performed in an attempt to enhance the 
 ability of a muscle to generate force , or to control the length change of its fibers in 
 relation to the stretch of its tendon , which depends on the architecture and the 
 physiological properties of its fibers4 . these morphological and biomechanical changes may impose an increased risk of injury on the 
 muscle itself , due to the development of the stiffer muscle tissue necessary to generate 
 increased force . in a study performed to determine the relationship between running economy 
 and muscle stiffness it was postulated that there was a positive relation between muscle 
 stiffness and power production5 . as a 
 muscle increases in strength , it loses flexibility in order to maintain the elasticity to 
 create a forceful contraction . injuries , such as a hamstring strain , are commonly 
 associated with exercise that involves fast movements and sudden acceleration . the 
 hamstrings are recognized as being the most frequently injured structure , and are estimated 
 to account for between 12% and 16% of injuries in professional soccer leagues8 , 9 . hamstring injuries are injuries that often require a long recovery period and are especially 
 prone to recurrent injury10 . the physical fitness component of police cadet training often emphasizes muscle power and 
 endurance training . it 
 is our understanding that there is no literature on the changes in muscle properties 
 elicited by police cadet training . therefore , our study aimed to investigate the effects of 
 this physical exercise on lower extremity muscle properties . by observing the effects of 
 physical exercise on lowell police academy cadets , in lowell , ma in the united states , this 
 study aimed to describe the relationship between overall performance in physical activity , 
 muscle property , and police cadet training the study s hypothesis was that over the 20-week 
 training period , the cadets would show changes in muscle properties and a decline in the 
 flexibility of the hamstring and calf compound . fifty - nine ( 10 females and 49 males ) police cadets from the lowell police academy , class of 
 2013 , were initially recruited for the study . eight males dropped out from the study due to 
 schedule conflicts preventing them from attending all assessment sessions . all subjects 
 prior to the study reported no known neuromuscular disorders , no recent ( within a year ) 
 lower extremity injury or surgery , and no prosthetic joint . subjects were excluded if their 
 range of a straight leg raise ( slr ) was less than 30 , greater than 100 , or the stabilized 
 leg during slr could not maintain at 0 of hip flexion . this study was approved by the institutional review board of university 
 of massachusetts lowell , ma , usa . table 1table 1descriptive characteristics of subjects at the time of recruitmentmale ( n=41)female ( n=10)overall ( n=51)age ( yrs)27 ( 3.8)24.9 ( 3.2)26.6 ( 3.7)height ( cm)177.4 ( 9.3)166.8 ( 7.2)175 ( 9.9)weight ( kg)85.2 ( 15.6)64.5 ( 37.7)81.2 ( 17.2)body fat ( % ) 15.6 ( 6.1)22.8 ( 13.3)17.0 ( 6.7)values are mean ( sd ) shows the demographics of the 51 subjects at the time they were recruited 
 before undergoing the standard cadet training . standard fitness tests and quantitative assessments of muscle properties were conducted 
 pre , during ( 1st and 2nd evaluations ) and at the end ( 3rd evaluation ) of cadet training at 
 five to six weeks interval during the cadet training . standard fitness tests included upper 
 extremity muscle strength and endurance ( bench - press , push - up and pull - up ) , trunk muscle 
 strength and endurance ( sit - up ) , the flexibility of back and leg muscles ( sit and reach 
 test ) , and general power ( timed 1.5 miles run ) . the body size and the body composition were 
 recorded at each of the four sessions . to quantify the biomechanical properties of the hamstrings and calf compound , ajjolos 
 ( seign , llc , north reading , ma , usa ) shown in fig . 
 1fig . 1.experimental setup for evaluating the biomechanical properties of the hamstring and 
 the calf compound . a device containing a pressure plate and an inflatable ball 
 connected to an electric pump was connected to a computer to record the tension 
 changes during passive stretching . dead 
 man switch so that the experimenter could stop ball inflation by taking a finger off 
 the switch . was used to evaluate subjects within the same week of the standard fitness test . subject assumed the pre - stretch position , and he / she was positioned supine on the mat . a 
 coin flip determined which lower extremity would be measured first for each subject . once 
 the subject was in proper alignment with the device and self - reportedly comfortable , the 
 subject was instructed to raise the randomly selected leg and place it ( ankle or calf ) upon 
 the inflatable ball . in most cases , the subject s initial positioning resulted in the raised 
 leg having a slr of approximately 70 or more depending on each individual s flexibility 
 ( fig . once the test position was reached , 
 the subject was requested to relax and to allow the device to passively apply a gentle 
 stretch to the lower extremity . the device s passive applicator was gradually inflated to 
 0.3 psi , over approximately 5 seconds , applying a gentle stretch to the subject s hamstring 
 musculature . at the end of the inflation , the pressure placed on the ball over a 60 second after 60 seconds , the device gradually released the applied passive 
 stretch by deflating the ball . once the subject was back in the initial ( pre - stretch ) 
 position , he / she was asked to assume a similar position with the opposite lower extremity , 
 and the procedure was repeated . experimental setup for evaluating the biomechanical properties of the hamstring and 
 the calf compound . a device containing a pressure plate and an inflatable ball 
 connected to an electric pump was connected to a computer to record the tension 
 changes during passive stretching . dead 
 man switch so that the experimenter could stop ball inflation by taking a finger off 
 the switch . the recorded pressure data were plotted against time over the 60-second period for each 
 lower extremity ( fig . 2.an example of the pressure data measured by the device during the test in two 
 consecutive sessions ( before and after the police cadet training ) . the decay curve 
 indicates the stretched muscle became more relaxed over the one minute of stretching 
 resulting in less pressure being applied to the inflated ball . the initial stretch 
 force indicates the resistance caused by the stretched leg . ) . the slope 
 of the fitted curve was defined as the relaxation coefficient ( relx ) in this study . a 
 smaller relx ( more negative number ) implies a more compliant , i.e. less stiff , passive 
 element of the soft tissues . the highest point of the stretching curve was defined as the 
 initial stretch force ( isf ) . an example of the pressure data measured by the device during the test in two 
 consecutive sessions ( before and after the police cadet training ) . the decay curve 
 indicates the stretched muscle became more relaxed over the one minute of stretching 
 resulting in less pressure being applied to the inflated ball . repeated measures anova was used in this study to compare measures ( the relaxation 
 coefficient ( relx ) , initial stretched force ( isf ) , sit - and - reach ( sr ) distance , sit - up and 
 1.5 miles run time ) and body size / composition changes among the four longitudinal sessions . bonferroni correction was then used as post - hoc test to examine the main 
 effects . all analyses were performed using ibm spss statistics 20 ( ibm , chicago , il , usa ) 
 with a significance level of 0.0125 . table 2table 2.body size and compositionheight ( cm)weight ( kg)body fat ( % ) mean ( sd)mean ( sd)mean ( sd)pre175.4 ( 9.4)82.1 ( 16.2)17.0 ( 6.3)1st eval175.8 ( 8.9)82.0 ( 16.1)16.0 ( 5.8)*2nd eval175.8 ( 8.9)80.1 ( 15.2)*^16.3 ( 5.6)3rd eval175.9 ( 9.5)79.8 ( 14.4)*^16.5 ( 5.6)values are mean ( sd ) . * : p<0.0125 compared to pre session , ^ : 
 p<0.0125 compared to second session demonstrates the body size and the body composition of all subjects over the 
 time course of the study . no significant height changes were seen while statistically 
 significant weight losses were observed at the second and third evaluations compared to the 
 pre and first evaluation sessions ( p<0.001 ) . body fat decreased significantly after the 
 first five weeks of the cadet training ( p<0.001 ) but increased at the second ( p=0.009 ) 
 evaluation session . values are mean ( sd ) . * : p<0.0125 compared to pre session , ^ : 
 p<0.0125 compared to second session table 3table 3.time course changes in general fitness of the police cadetssit- up1.5 mile runsit - and - reachpre36.1 ( 8.0)710.6 ( 67.3)17.3 ( 2.4)1st eval41.0 ( 7.4)*653.8 ( 63.9)*18.4 ( 2.2)*2nd eval43.5 ( 7.1)*^641.5 ( 81.7)*19.1 ( 2.3)*^3rd eval45.0 ( 6.0)*^#641.2 ( 84.2)*19.9 ( 2.1)*^#values are mean ( sd ) . * p<0.0125 compared to pre session , ^ : 
 p<0.0125 compared to post session , # : p<0.05 compared to the 
 previous session shows the changes of general fitness after the cadet training . the general 
 fitness improved as expected . the time spent in running 1.5 miles significantly reduced at 
 about the five weeks into the cadet training ( p<0.001 ) and the effect was maintained to 
 the end of training . group data demonstrated that sr and sit - ups improved significantly at 
 all three evaluation sessions ( p<0.001 ) . * p<0.0125 compared to pre session , ^ : 
 p<0.0125 compared to post session , # : p<0.05 compared to the 
 previous session table 4table 4.biomechanical properties of the hamstring and calf compoundisf ( dominant)isf ( non - dominant)relx ( dominant)relx ( non - dominant)pre10.73 ( 2.94)10.03 ( 2.46)0.0079 ( 0.0118)0.0075 ( 0.0135)1st eval9.63 ( 2.76)9.66 ( 2.67)0.0061 ( 0.01054)0.0055 ( 0.01322)2nd eval9.43 ( 2.55)*9.53 ( 2.62)0.0079 ( 0.01162)0.0101 ( 0.0119)3rd eval9.23 ( 2.61)*8.96 ( 2.31)*0.0035 ( 0.02329)0.0039 ( 0.1314)#values are mean ( sd ) . * : p<0.0125 compared to pre session , # : 
 p<0.0125 compared to the previous session shows the biomechanical properties of back leg soft tissues on each side . the 
 isf of the dominant leg decreased after the training ( p=0.03 ) and was maintained at the 
 follow - ups while there was no significant change of relx . the isf of the non - dominant side 
 did not change after the training , but there was a significant difference between pre and 
 2nd follow - up ( p=0.007 ) . a significant difference was found in relx of the non - dominant side 
 between the first and 2nd follow - up ( p=0.025 ) . values are mean ( sd ) . * : p<0.0125 compared to pre session , # : 
 p<0.0125 compared to the previous session as expected , an improvement in general fitness was observed during and at the end of the 
 cadet training . there was no significantly negative change in the flexibility - related 
 measures ( sit - and - reach , isf , relx ) of the subjects between the pre and first evaluation 
 sessions . the isf decreases observed at the 2nd and 3rd evaluation sessions indicate that 
 back leg musculature became less stiff and required less force to be moved to the extreme 
 position . however the changes in isf were not symmetrical which indicates the importance of 
 being able to recognize the differences between two legs . although the changes in the sr 
 test and isf were contrary to our hypothesis , the relx of the both legs showed an increasing 
 trend ( less compliant ) but it was not statistically significant except at the end of the 
 cadet training . the usage of sr is controversial , but it was recommended for estimating the extensibility 
 of the hamstring when other forms of flexibility tests are impractical11 . the criterion - related validity of the sr test might be 
 impacted by the hamstring extensibility specifically among subjects with reduced hamstring 
 extensibility12 . moreover , the sr test 
 alone does not discriminate the potential differences between two legs ( and there were 
 asymmetrical changes in isf ) or the contributors to muscle properties . for example , a stress relaxation curve was 
 used to estimate muscle extensibility post - exercise in rabbits13 : a second order polynomial was fitted to the relaxation 
 curve over the period of 300 seconds after the stress was applied . in our study we a chose 60-seconds 
 recording window and a linear regression model , which is appropriate because most tension 
 decay occurs in the first 60 seconds after the application of a stretching force13 . sr and isf demonstrated changes opposite 
 to relx , but it is possible that the changes in relx were confounded by back muscle 
 extensibility , abdominal muscle strength , body and head positions , etc . our data shows there 
 were increases in both sit - up numbers and sr distance . therefore , the possibility of an 
 increase in sr distance as a result of an enhanced abdominal muscle performance can not be 
 ruled out . measuring the straight leg raise with more sensitive sensors would allow us to 
 better study the compliance of hamstrings . though the odds of developing a hamstring injury may seem intimidating , animal evidence 
 shows that techniques such as passive stretching tend to reduce skeletal muscle 
 injuries14 . this might be one of the 
 reasons that although the standard police cadet training emphasizes muscle power and 
 endurance training , with warm - up and cool - down performed in the training , it seemed to not 
 increase the risk of hamstring injuries . in our data , larger variances in relx of both legs that might be due to the different activities they performed or reduced 
 physical exercise changes in muscle properties . however , it is one of the limitations of 
 this study , that since the activities during the cadet training could not be monitored or 
 restricted , it is not possible to draw a causal conclusion . the non - dominant legs became less compliant at the end of the cadet training ; 
 however no injury was reported at the follow - up , it might be due to the activities that 
 police cadets conduct daily are not the sudden and fast motions made by athletes such as 
 soccer players . nonetheless our data show that muscle compliance changes due to the training 
 involving and that the lower extremities should be taken into account when designing a 
 training protocol . a future study of a longer training period ( more than 20 weeks ) is needed 
 to see if the muscles involved become less compliant .
[ purpose ] this study was performed to examine the relationship between physical 
 performance and muscle properties during police cadet training . 
 the study s hypothesis was 
 that improved physical performance brought about by training , would in turn cause a 
 reduction in muscle flexibility . 
 [ subjects and methods ] fifty - nine police cadets were 
 included in this study . 
 standard fitness tests and quantitative assessments of muscular 
 biomechanical properties were conducted before , during and after the 20-week cadet 
 training . [ results ] 
 general fitness had improved at the end of the police cadet training . 
 there was no significant decrease in muscle flexibility as measured by the sit - and - reach 
 test . 
 however , muscle compliance of the non - dominant leg measured by the relaxation 
 coefficient had decreased at the end of the police cadet training . 
 [ conclusion ] the 
 increased sit - and - reach distance could be due in part to strengthening of the abdominal 
 muscles . on the other hand , the biomechanical test , which was specific to muscle 
 extensibility , showed a reduction in the relaxation coefficient of the non - dominant leg . 
 our data suggests that changes in muscle compliance as a result of lower extremity 
 training should be considered . 
 this data may be useful in the design of a training 
 protocol that prevents the potential injuries caused by reduced muscle flexibility .