obesity is responsible for a wide range of chronic diseases , such as heart disease , 
 hypertension and certain cancers1 , 2 . obesity is also associated with 
 musculoskeletal conditions ( mcs)3,4,5 , 
 such as pain , stiffness , loss of joint mobility and although these conditions are not fatal , 
 they decrease quality of life and life expectancy by interfering with social exchanges and 
 aggravating obesity . the individual and social costs of mcs are considerable ; the direct and indirect costs have 
 reached $ 214.9 billion in the usa3 , 
 equivalent to three percent of the country s annual gross domestic product ( gdp ) . in europe , mcs have been recognized as having the highest cost burden of all disease categories6 . in australia , the costs of mcs are second 
 only to those of cardiac vascular diseases , the most cost - intensive disease category7 . given these individual and socio - economic 
 impacts , mcs require effective management . muscle weakness and increased fat mass may be primary risk factors of mcs8 . based on the premise that resistance 
 training is desirable to increase muscle mass and strength , we have observed that muscle 
 strengthening through resistance exercise increases physical function , decreases pain , and 
 reduces self - reported disability9,10,11 . exercises with weight bearing and a high joint loads , such as weight lifting and resistance 
 training , may prove detrimental to obese individuals12 . because previous studies have yielded 
 inconsistent results , this study sought to investigate the relationship between obesity and 
 muscle mass and strength to develop measures for mcs in obese male adults . a total of 259 japanese males aged 3064 years were recruited via advertisement placed in 
 local newspapers in ibaraki from 20122015 . the study inclusion criteria were as follows : 1 ) 
 males aged 3064 years ; 2 ) no terminal diseases or history of recent muscle injury , or 
 surgery ; and 3 ) no history of drug or alcohol abuse . this study was conducted in 
 accordance with the guidelines proposed in the declaration of helsinki , and the study 
 protocol was reviewed and approved by the ethics committee of the university of tsukuba , 
 japan . height was measured to the nearest 0.1  cm using a wall - mounted stadiometer ( yg-200 ; yagami , 
 nagoya , japan ) , and body weight was measured to the nearest 0.1  kg using a digital scale 
 with the subject in light clothing and without shoes ( tbf-551 ; tanita , tokyo , japan ) . bmi 
 was calculated as weight ( kg ) divided by height ( m ) squared . body composition was assessed using whole - body dual energy x - ray absorptiometry ( dexa ; qdr 
 4500 , hologic , inc . , hologic software was utilized to estimate the fat , lean , and bone 
 tissue masses ( kg ) . extended analyses were performed to obtain separate fat , lean and bone 
 tissue masses for the arms , legs , and trunk . visser et al.14 reported that lean mass , excluding bone mineral content , is a valid 
 representation of skeletal muscle mass in the extremities . we calculated the appendicular 
 skeletal muscle mass of each participant as the sum of the lean mass of both the upper and 
 the lower extremities , excluding bone mineral content . a height - adjusted index was then 
 calculated by dividing each participant s appendicular skeletal muscle mass in  kg by the 
 square of his or her height in  m ( m)15 . the height - adjusted appendicular skeletal muscle index was defined 
 as the skeletal muscle mass index ( smi ) . the percentage of muscle mass index ( % mmi ) was 
 calculated by dividing each participant s appendicular skeletal muscle mass in  kg by body 
 weight , and multiplying the result by 100% . to evaluate muscle strength , we measured handgrip strength for the upper extremities and 
 knee extensor strength for the lower extremities . handgrip strength has been widely adopted 
 for the measurement of muscle strength because it is easy to measure . knee extensor strength 
 measurement is particularly important16 
 because it evaluates the femoral muscle at the most common site of musculoskeletal 
 conditions . 5401 ; takei 
 scientific instruments , tokyo , japan ) in each hand alternately with maximum effort while 
 lowering the arm naturally to the side of the body17 . the measurement was performed twice for each hand , and the best 
 result was recorded as the handgrip strength for each hand . handgrip strength was expressed 
 as absolute , body weight - normalized , and arm muscle mass ( amm)-normalized values . isometric 
 and isokinetic knee extensor strength were both measured using a biodex system 3 dynamometer 
 ( biodex medical systems , shirley , ny , usa ) . the participants were seated in the biodex 
 system 3 dynamometer and tightly secured with chest , pelvis and thigh straps with the back 
 supported and the hips flexed at 120. the knee s axis of rotation and the biodex system 3 
 dynamometer were precisely aligned before each test using visual inspection . the knee was 
 extended to 60 for the isometric assessment because this angle provides the quadriceps with 
 close - to - optimum muscle length for the production of maximal force13 . the protocol of the isometric assessment consisted of 3 
 maximal extension efforts , each lasting 3 seconds with intervening 15-second rests . the 
 isokinetic assessment comprised 3 maximal extensions at an angular velocity of 60/s , as is 
 widely used to evaluate isokinetic muscle strength18 . the highest muscular force output at any moment during the 
 assessment was defined as the peak torque and is reported in absolute terms ( nm ) and 
 normalized to body weight , represented as the body - weight - normalized ( nm / kg ) peak torque . the amount of work accomplished for an entire assessment was defined as the total work and 
 is reported as the absolute value ( w ) , whereas the average of total work divided by time was 
 defined as the average power , which is also reported as the absolute value ( j ) . the 
 measurements were performed on both legs , and the average strength of both legs was 
 calculated to determine lower - extremity muscle strength16 . the results are expressed as absolute , body weight - normalized , and 
 leg muscle mass ( lmm)-normalized values . we used a jump test to discriminate between the physical performances of obese and 
 non - obese individuals because it directly reflects the participants lower extremity muscle 
 strength and body weight . the test was performed as follows : the participants placed their 
 feet on the circular board of the dynamometer ( jump - md , t.k.k . 5106 , takei scientific 
 instruments , tokyo , japan ) with the dynamometer around their waist . the participants leaped 
 vertically as high as possible using knee countermovement and landing on the circular board 
 of the dynamometer . the measurement values are expressed as the means  standard deviations ( sds ) or standard 
 errors ( ses ) . one - way analysis of variance ( anova ) was used to analyze the differences 
 between the groups . one - way analysis of covariance ( ancova ) , with age as a covariate , was 
 utilized to test for the significance of mean body composition and muscle strength among the 
 four groups . the bonferroni post hoc test was used when the ancova results exhibited 
 significant differences ( p < 0.05 ) . the jonckheere - terpstra test was used to assess the 
 trends among the values in the four groups . the trend test was 2-tailed , with a significance 
 level of p < 0.05 . the participants were 
 classified into groups a ( non - obese , n = 60 ) , b ( obesity class i , n = 142 ) , c ( obesity class 
 ii , n = 47 ) and d ( obesity class iii , n = 10 ) based on japanese obesity guidelines19 . the anthropometric characteristics and 
 trends among the 4 groups are presented in table 
 1table 1.anthropometric characteristics and trends among the four groupsa . non obesity ( 95% ci)b . obesity class iii ( 95% ci)post hocstandardized(n = 60)(n = 142)(n = 47)(n = 10)statisticage , year50.99.3(48.5 , 53.3)50.39.0(48.8 , 51.8)47.78.6(45.2 , 50.2)44.08.5(37.9 , 50.1)ns2.43height , cm171.45.7(169.9 , 172.9)171.56.1(170.5 , 172.5)171.65.3(170.1 , 173.2)172.86.4(168.2 , 177.3)ns0.36body weight , kg69.06.1(67.4 , 70.6)80.76.9(79.6 , 81.8)94.17.7(91.8 , 96.4)119.819.3(106.0 , 133.6)a < b < c < d13.95bmi , kg / m23.51.3(23.1 , 23.8)27.41.4(27.2 , 27.7)31.91.4(31.5 , 32.3)40.04.9(36.5 , 43.5)a < b < c < d16.29values are means  sd . significant group differences were found among the four groups 
 ( * p < 0.05 ) . significant trend was found among the four groups ( p < 
 0.05 , p < 0.01 ) . jonckheere - terpstra test was used to 
 assess the trend among the four groups . the anova results demonstrated that body weight and bmi differed 
 significantly among the 4 groups . a progressive trend of decreasing age from group a to 
 group d was observed ( standardized statistic = 2.43 , p < 0.05 ) . significant group differences were found among the four groups 
 ( * p < 0.05 ) . significant trend was found among the four groups ( p < 
 0.05 , p < 0.01 ) . jonckheere - terpstra test was used to 
 assess the trend among the four groups . ns : not significant ; bmi : body mass index table 2table 2.characteristics and trends of body composition among the four groupsa . obesity class iii ( 95% ci)post hocstandardized(n = 60)(n = 142)(n = 47)(n = 10)statisticwhole body lean mass , kg56.60.8(55.1 , 58.0)61.70.5(60.7 , 62.6)68.90.9(67.2 , 70.6)76.81.9(73.1 , 80.4)a < b < c < d10.40whole body fat mass , kg13.30.5(12.3 , 14.3)19.60.3(18.9 , 20.2)24.50.6(23.4 , 25.7)34.91.3(32.4 , 37.4)a < b < c < d13.05% whole body fat , % 18.90.5(18.0 , 19.9)23.90.3(23.3 , 24.5)26.00.5(25.0 , 27.1)30.31.2(28.0 , 32.6)a < b < c , d9.78amm , kg5.90.1(5.7 , 6.1)6.40.1(6.3 , 6.6)7.30.1(7.0 , 7.5)7.70.3(7.2 , 8.2)a < b < c , d8.34lmm , kg17.50.3(16.8 , 18.1)18.70.2(18.3 , 19.2)20.80.4(20.0 , 21.6)23.10.9(21.4 , 24.8)a < b < c , d7.16smi , kg / m7.90.1(7.7 , 8.2)8.50.1(8.4 , 8.7)9.60.1(9.3 , 9.8)10.50.3(9.9 , 11.1)a < b < c < d9.05% mmi , % 33.70.4(32.9 , 34.5)31.10.3(30.6 , 31.7)30.00.5(29.1 , 30.9)26.41.0(24.4 , 28.4)a > b , c > d 7.03values are means  se . significant group differences were found among the four groups 
 ( * p < 0.05 ) . significant trend was found among the four groups ( p < 
 0.05 , p < 0.01 ) . the kruskal wallis permutation test 
 result was consistent with that of the ancova analysis . the 
 jonckheere - terpstra test was used to assess the trend among the four groups . % whole 
 body fat : percentage of whole body fat ; amm : arm muscle mass ; lmm : leg muscle mass ; 
 sm : skeletal muscle index ; % mmi : percentage of muscle mass index presents the body composition results of the 4 groups . to avoid the influence 
 of age on the results , ancova was performed with age as a covariate . the post hoc tests 
 demonstrated significant differences among the 4 groups for all of the variables . groups a , 
 b , c and d ranked in descending order for all variables except % mmi . the trend test also 
 demonstrated increases from group a to group d for all variables except % mmi ( standardized 
 statistic = 10.40 , 13.35 , 9.78 , 8.34 , 7.16 , 9.05 , respectively ; p < 0.01 for all ) . the % 
 mmi trended in the opposite direction for all the other variables ( standardized statistic = 
 7.03 ; p < 0.01 ) . these results indicate that both body fat and muscle mass increase in 
 parallel with obesity . significant group differences were found among the four groups 
 ( * p < 0.05 ) . significant trend was found among the four groups ( p < 
 0.05 , p < 0.01 ) . the kruskal wallis permutation test 
 result was consistent with that of the ancova analysis . the 
 jonckheere - terpstra test was used to assess the trend among the four groups . % whole 
 body fat : percentage of whole body fat ; amm : arm muscle mass ; lmm : leg muscle mass ; 
 sm : skeletal muscle index ; % mmi : percentage of muscle mass index table 3table 3.characteristics and trends of muscle strength and physical performance among the 
 four groupsa . non obesity ( 95% ci)b . obesity class i ( 95% ci)c . obesity class iii ( 95% ci)post hocss(n = 60)(n = 142)(n = 47)(n = 10)hgs , kg*43.490.88(41.77 , 45.22)42.140.57(41.02 , 43.26)45.690.99(43.73 , 47.65)44.182.16(39.93 , 48.43)b < c1.45hgs / bw , kg0.630.01(0.61 , 0.65)0.520.01(0.51 , 0.54)0.490.01(0.47 , 0.51)0.380.03(0.33 , 0.43)a > b , c > d8.82hgs / amm , kg7.420.12(7.20 , 7.65)6.590.08(6.44 , 6.74)6.311.13(6.05 , 6.57)5.890.29(5.33 , 6.46)a > b , c , d6.66imt60 ptq , nm182.354.91(172.68 , 191.01)194.783.19(188.50 , 201.06)226.515.56(215.56 , 237.46)235.6712.09(211.86 , 259.49)a , b < c , d6.41imt60 ptq / bw , nm / kg2.630.06(2.52 , 2.75)2.410.04(2.34 , 2.49)2.420.07(2.29 , 2.54)1.970.14(1.70 , 2.25)a > b , c , d3.08imt60 ptq / lmm , nm / kg10.590.28(10.03 , 11.15)10.510.19(10.15 , 10.87)10.970.32(10.34 , 11.60)10.430.70(9.06 , 11.81)ns1.13ikt60 ptq , nm156.234.08(148.20 , 164.26)169.122.65(163.91 , 174.34)192.304.62(182.21 , 201.40)203.3110.05(183.52 , 223.09)a , b < c , d6.30ikt60 ptq / bw , nm / kg*2.260.05(2.16 , 2.35)2.100.03(2.03 , 2.16)2.040.05(1.94 , 2.15)1.720.12(1.48 , 1.95)a > c , a > b > d3.15ikt60 ptq / lmm , nm / kg9.040.25(8.56 , 9.53)9.130.16(8.82 , 9.45)9.340.28(8.80 , 9.89)9.000.60(7.81 , 10.19)ns1.08ikt60 tw , j440.2211.77(417.04 , 463.39)472.447.64(457.39 , 487.49)535.2813.33(509.03 , 561.53)592.1828.99(535.08 , 649.27)a , b < c , d6.18ikt60 tw / bw , j / kg6.360.14(6.08 , 6.64)5.860.09(5.68 , 6.04)5.700.16(5.39 , 6.01)5.020.35(4.34 , 5.71)a > b , c , d3.23ikt60 tw / lmm , j / kg25.470.66(24.16 , 26.77)25.450.43(24.60 , 26.30)26.060.75(24.57 , 27.54)26.161.64(22.94 , 29.38)ns1.02ikt60 ap , w97.472.81(91.94 , 103.01)103.531.82(99.93 , 107.78)117.263.18(110.99 , 123.53)130.576.92(116.94 , 144.20)a , b < c , d5.32ikt60 ap / bw , w / kg*1.410.03(1.34 , 1.48)1.280.02(1.24 , 1.33)1.250.04(1.17 , 1.32)1.100.08(0.94 , 1.27)a > b , c , d3.09ikt60 ap / lmm , w / kg 5.650.16(5.33 , 5.97)5.590.11(5.38 , 5.80)5.690.19(5.33 , 6.05)5.780.40(4.99 , 6.67)ns0.75jump*44.790.90(43.01 , 46.57)42.840.59(41.68 , 43.99)41.511.02(39.50 , 43.53)32.202.23(27.81 , 36.59)a , b , c > d2.33values are means  se . significant group differences were found among the four groups 
 ( * p < 0.05 ) . significant trend was found among the four groups ( p < 
 0.05 , p < 0.01 ) . the kruskal wallis permutation test 
 result was consistent with that of the ancova analysis . the 
 jonckheere - terpstra test was used to assess the trend among the four groups . ns : not 
 significant ; hgs : handgrip strength ; bw : body weight ; amm : arm muscle mass ; lmm : leg 
 muscle mass ; imt60 ptq : isometric60 peak torque ; ikt60 ptq : isokinetic60 peak torque ; 
 ikt60 tw : isokinetic60 total work ; ikt60 ap : isokinetic60 average power ; ss : 
 standardized statistic presents the muscle strength and physical performance results of the 4 groups . multiple comparisons demonstrated significant differences among the 4 groups for all of the 
 variables except lmm normalized strength . although there was no trend in the absolute values 
 of handgrip strength among groups a to d , the handgrip strength values normalized for body 
 weight and amm exhibited a statistically significant trend ( standardized statistic = 8.82 
 and 6.66 , respectively ; p < 0.01 for both ) . the leg muscle strength absolute values did 
 not differ significantly between groups a and b or between groups c and d but were 
 significantly higher in groups c and d than in groups a and b. the trend test indicated that 
 the absolute muscle strength value for each leg increased progressively from group a to 
 group d ( standardized statistic = 6.41 , 6.30 , 6.18 , 5.32 , respectively ; p < 0.01 for 
 all ) . when leg muscle strength was normalized for body weight , there were no significant 
 differences among groups b , c and d aside from ikt60 ap / body weight , although their values 
 were significantly lower than those of group a. although the ikt60 ap / body weight values of 
 groups a , b , c , and d were ranked in ascending order , no significant differences were 
 observed between groups b and c or between groups c and d. ikt60 ap / body weight was 
 significantly higher in group a than in groups b , c and d. when analyzed for trend , the 
 decrease in the body - weight - normalized leg muscle strength from group a to group d was 
 statistically significant ( standardized statistic = 3.08 , 3.15 , 3.23 , 3.09 , 
 respectively ; p < 0.01 for all ) . the jump test results decreased in value from group a 
 through group d ; the trend was significant ( standardized statistic = 2.16 ; p < 0.05 ) . the mean values of groups a , b and c did not differ significantly , although each was 
 significantly higher than the value of group d. these results indicate that leg muscle 
 strength correlates positively with increasing obesity , but the increases in leg muscle 
 strength that accompany increases in obesity do not improve body weight - normalized leg 
 muscle strength and physical performance . significant group differences were found among the four groups 
 ( * p < 0.05 ) . significant trend was found among the four groups ( p < 
 0.05 , p < 0.01 ) . the kruskal wallis permutation test 
 result was consistent with that of the ancova analysis . the 
 jonckheere - terpstra test was used to assess the trend among the four groups . ns : not 
 significant ; hgs : handgrip strength ; bw : body weight ; amm : arm muscle mass ; lmm : leg 
 muscle mass ; imt60 ptq : isometric60 peak torque ; ikt60 ptq : isokinetic60 peak torque ; 
 ikt60 tw : isokinetic60 total work ; ikt60 ap : isokinetic60 average power ; ss : 
 standardized statistic leg 
 muscle strength correlates positively with obesity ; however , increases in leg muscle 
 strength without weight loss do not improve body weight - normalized leg muscle strength and 
 physical performance . cross - sectional and interventional research has noted positive 
 correlations between the loss of muscle mass and strength and mcs , and between the 
 amelioration of mcs and muscle strengthening with resistance training9,10,11 . however , the findings of the present study indicate that 
 an increase in muscle strength requires an increase in muscle mass , which in turn increases 
 body fat . body weight is a major determinant of muscle mass , and lean mass correlates positively with 
 body fat20 , 21 . in the present study , we observed that smi tended to increase as 
 obesity increased , and that % whole - body fat increased as whole - body lean mass increased . 
 these findings are consistent with the observation that heavier adult males have more muscle 
 mass , and since muscle strength correlates positively with muscle mass , heavier adult males 
 generate more muscle strength . maffiuletti et al.22 reported that adult 
 males with a bmi > 40  kg / m had 18% more lean mass and created significantly 
 more leg muscle power and maximum muscle strength ( 1620% ) than adult males with a bmi < 
 25  kg / m . a possible 
 explanation for this finding is that the heavier body weights of obese adults serve as a 
 training stimulus to gain muscle mass and , consequently , to increase muscle strength23 , 24 . because the lower extremities muscles must maintain and support all 
 of the body s weight , these muscles are stimulated more strongly than those of the upper 
 extremities . this condition likely accounts for the absence of significant obesity - related 
 increases in handgrip strength despite the observed increases in every absolute value of leg 
 muscle strength . body weight - normalized leg muscle strength is significantly lower in obese men than in 
 non - obese men22 . as the trend test 
 indicated , the increase in whole - body fat mass ( standardized statistic = 13.05 ) with 
 increasing obesity was greater than the increase in whole - body lean mass ( standardized 
 statistic = 10.40 ) . excessive increases in body fat are a physical burden on the 
 musculoskeletal system ; as obesity develops , the amount of whole - body fat increases faster 
 than whole - body lean mass does . consequently , mcs may result from low body weight - normalized 
 leg muscle strength25 . syed and 
 davis26 reported that low body 
 weight - normalized muscle strength results in earlier leg muscle fatigue and increases the 
 loading rate on the knee joints . we also observed a significant decrease in the jump test 
 results as obesity increased . these findings suggest that obese men must improve their body 
 weight - normalized muscle strength to treat mcs . however , increasing muscle mass and strength 
 without reducing body fat mass may not significantly improve body weight - normalized leg 
 muscle strength . therefore , losing body weight may be more appropriate than increasing 
 muscle mass and strength for the treatment of mcs . this was the first study in which japanese obesity guidelines were used to study the 
 relationship between obesity and muscle mass and strength in adult males . although this 
 cross - sectional study characterized the basis of the relationship between obesity and muscle 
 mass and strength , scientific evidence of the benefits of body weight loss was limited . consequently , two - pronged investigation comparing the effects of weight loss with that of 
 resistance exercise in cohort and long - term follow - up surveys is necessary . in conclusion , . muscle mass and 
 strength correlate positively with obesity . in the presence of obesity , increases in lmm and 
 strength it 
 may be easier to treat mcs with weight loss than with increased muscle mass and strength . to 
 confirm the conclusion of the present study , a prospective cohort study and follow - up study 
 should be conducted in the future .
to prevent or remedy musculoskeletal conditions , the relationship between obesity and the 
 characteristics of muscle mass and strength need to be clarified . 
 [ subjects and methods ] a 
 total of 259 japanese males aged 3064 years were classified into 4 groups according to 
 the japanese obesity criteria . 
 body composition was evaluated , and handgrip strength and 
 knee extensor strength were measured for the upper and lower extremities , respectively . 
 physical performance was evaluated with a jump test . [ results ] obesity was positively 
 correlated with skeletal muscle mass index , percentage of whole - body fat , and leg muscle 
 strength and negatively correlated with the percentage of muscle mass index , body 
 weight - normalized handgrip strength , and knee extensor strength , and the jump test 
 results . 
 [ conclusion ] weight loss may be a better approach than increasing muscle mass and 
 strength to improve musculoskeletal conditions in obese adult males .