Diminished dynamic performance capacity of back and abdominal muscles in concrete reinforcement workers.

JARVINEN, STAMBEJ, WICKSTROM, G. DimiIllished dyna mk performanoe oapacity of back and <l!bdominal muscles mooncrete retnforcement wOl'kers. Scand. j. work environ. & heCLlth 4 (1978): 5uppl. 1, 39--46.'Dhe dynamic per forman<:e capacttyof back and 'stlradght abdomma!l muscles was <:!linicaJly examined in 295 male ,Finndsh concrete reinforcement woIikens aged 19 oto ~4 yeaJrS and engaged lin heavy physical wonk including prolonged stooptng. In 24 Ofo of ,the men bot'h back and straJight abdominal muscle performance capaoity was reduced, in 18 Ofo only the capadty of the straight abdominal muscles, and in 4 0 10 only the perf<Jrmanoe <Jf ,the back muscle. The prevalence of diminished trunk muscle performance capacity increased rapidly with age. Dimdn,h;lhed back muscle ,performance capacity was associated with a history of sciatica (X 2 = 16.9, P < 0.001) and diminished per formance oapaotty of the stra,ight abdominal muscles wtth a !historY of lumbago (X 2 = '5.9, p < 0.02) after adjustment fur age. 'Diminished back muscle perfol'ffianoe was a,ssodated with backache (X 2 = 16.9, P < 0.001) and sharp paliin. in the back (X 2 = 4.5, p < 0.05) durrng 'an ordinary workday, ,as wa,s a1so drmiinished perfol'm ,anoe of 1iheSitraig'ht abdominal ffiuscl-es (X 2 = 23.8, P < 0.001 'and X2 = 7.'3, P < 0.01, respectiV'ely). No association between length of ,exposure to ,the back loads 'in rein- forcement worikand the preva.lence of dimfinished trunk muscle pedorman,ce capa,c i,ty could be estrubl.ished.

The relattonship between trunk muscle strength and 1ow-back paim is still unclear. Persons with diminished muscle strength have been found to be especially prone to low-back pain. On the other hand, diminished trunk muscle strength 'has been found as 'a residual sign of previous back pain syndromes.
In this ,artide we have presented the prevalence of diminished trunk muscle performance capacity in a group of skilled oonstruction workers. We have also exam- 1 Institute of Occupa,tdonal Health, Helsinki, Fin[ailld. 2 Turku Regional Institute of Occupational Health, Turku, Finland.
ined the relationship between diminished trunk muscle performanoe and a history of 1ow-ba;ck 'Parn syndromes, as well as reported pvesent back symptoms.

MATERIAL AND METHODS
The group studied comprised 2!}5 male Finnish concrete reinforcement workers, aged 19-64 years. A detailed charaderi-:laHon of the group has been given elsewhere (22). An ergonomic analysis of reinfovcement work showed the frequency and duration of the stooping postuves to be extraordinarily common in this occupation    (20). The method used to obtain the history of ,ba,ck diseases and symptoms has been pr·esented by Wkkstrom et aL (22).
The dynamk trunk muscle performance was cliini'cally examined by an experienced and specially trained physiothempist. Ba,ck muscle performance capacity (M. ere,ctor spinae) was examined while the reinforcement worker lay on his stomach with an approximately 10-em high support under his 'stomach to prevent sudden hyperlordosis. The worker kept his arms fixed at his sides. While the examiner held the feet of the worker on the table, he slowly bent his trunk ba,ckward so that the thorax rose 10-15 cm from the table ( fig. 1). Back muscle performance was considered normal if the worker was able to repeat the movement seven times or more, diminished if he r,epeated it only three to six times, and strongly diminished if the repetition was less than three times.
The dynamic performance capacity of the straight abdominal muscles was tested while the concrete reinfor,cement worker lay on his back with his knees flexed. His arms were free and directed towards the knees. The examiner held the man's feet on the table. If the reinforcement worker rose to the sitting po,sition ( fig. 2) smoothly, without jerks, seven times or more, his muscle performance was considered normal; if he r·ose only three to six times, his performance capa,city was classified as diminished; and, if he rose less than three times, it was considered strongly diminished. Jerking movements always led to a classification of strongly diminished muscle performance capacity.
Due to subjoedive discomfort the testing of back muscle performance was omitted from the clinical examination in 13 cases and the test for arbdominal muscle perform-ance in 1 case. The symptom questionnaire was expanded in the beginning of the study, ,and therefore present back symptoms were recorded in detail f.or only the last 223 men examined (22).
In the graphical presentation of the results the observed preval,ences were smoothed by the calculation of 15-year moving age ranges for the prevalence estimates.

RESULTS
The dynamic ba,ck muscle performance capacity was cIassifi,ed as normal in 73 0/0 of the concrete reinforcement workers, moderately diminished in 24 Ofo, and str,ongly diminished in 3 0/0-The dynamic performance ,capacity of the straight abdominal muscles was consider,ed normal in 57 Ofo, moderately diminished in 29 Ofo, and strongly diminished in 14 % of the men.
When the findings of diminished back and abdominal muscle performance were compared, a reduction of back muscle capacity only was found in 4 Ofo of the workers, a reduction of abdominal muscle capacity ,only in 18 Ofo,and 'a reduction of both back and 'abdominal musc1e capacity in 24 010. In  indication of an oocupational effect on the prevalence of diminiShed trunk muscle capadty found when the reinforcement workers were compared to another oc'cupational group often working in awkward postures, i.e., computer technicians ( fig. 4).
Of the men wirth diminished b3£k muscle performance ,capacity, 47 010 reported a history of lumbago, ·and 59 010 a history of sci'atica (table 2). The corresponding figures for the men with normal back muscle capacity were 28 010 ·and 36010, respectively. Also after age adjustment the association between diminished b3£k muscle capacity and a history of scioaitica was significant (X 2 = 16.9, P < 0.001).
In the men with diminished performance of the straight abdomina,l muscles a history of lumbago (44 010) and a history of sciatica (52 0 /0) were more common than in the men with normal abdominal muscle capacity (24 010 and 34 P/o,respectively). After age ,adjustment dimiJnished ahdomi. . . nal muscle capadty was stiU associated with ia history of lumbago (X 2 = 5.9, P < 0.02).
The incidence of f.atigue, stiffness, ache, and sharp .pain in the back during an ordinary workday was clarified in 'an in'terview on orthopedic ·symptoms. Table  3 shows reported back symptoms compared to the findings of diminished back and abdominal muscle 'performance ca.pacity. Men with diminished back muscle performance more commonly reported ache (x 2 = 16.9, p < 0.001) and sharp pain (X 2 = 4.5, P < 0.05) in ·the back than lInen with normal muscle performance. The same two back symptoms were also related to diminished performance capacity of the straight a.bdominal muscles (X 2 = 23.8, P < 0.001 and X 2 = 7.3, P < 0.01, respectiv,ely).
Because working in bent-double postures is very common in concrete reinforcement work, the workers were interviewed on ba,ck 'Symptoms as they bent down, while in the bent-d.ouble position, and as they straightened up. The back symptoms in reinforcement workers with and without diminished trunk muscle performance capacity are presented in table 4. Symptoms on bending down were related to the reduction of both back (x 2 = 5.2, P < 0.01) and .abdominal (x 2 = 11.9, P < 0.001) muscle performance capacity.  reduced dynamic performance in back muscles alone, straight abdominal muscles alone, and in back and abdominal muscles simultaneously appears in fig. 3. The data on diminished trunk muscle performance were analyzed for a possible effect of r·einfor:cement work by an ageadjusted comparison of workers with short and long employment times in reinforcement work. No statistically significant results were obtained. Neither was any clear     Flexion is initiated by the abdominal muscles and controlled by the back muscle. When a person leans forward 45°, the load on the erector spina,e muscle is at its height. When bending further forward the burden of the upper part of the body is more and more taken over by passive 'tension so that the back muscle, when foUowedelectromyographically, is nearly silent at 90° (7). The erector spinae muscle endur,es 25 % of the maximal extensi'on strength for 10 to 20 min (13), and 15 % can be maintained "indefinitely" (9,17).
The methods used to measure the maximal strength of the back and abdominal muscles included clinical orthopedic evaluatioon, use of me'chanical devices, and electromyography (4,7,11,15,16,19).
Comparing the results from four differents tests for back muscle strength, Pedersen and Staffeldt (15) found that the results from three tests for maximal muscular strength in 1-3 performances correlated well with each other. The test for endurance time, in principle similar to the tests used by Okada et al. (13), gave different results.
The method we used in our study measured dynamic performance capa,city in a manner similar to Pedersen's and Staffeldt's test III. We chose the dynamic tests for trunk muscle strength determination because they probably measure low-ba,ck disability more accurately than the tests for isometric muscle strength (1,11,18,21). The classification we used is based on clinical experience.
Diminished muscle strength may be due to muscular disease, such as myasthenia, to diseases of the nervous system, such as diabehc polyneuropathy, or to disuse of the muscles. As the examined group consisted of active workers from a strenuous occupation, the possibility of grave muscular or nervous diseases was excluded. Neither were any significant neurological signs not attributable to spinal degeneration found in the clini,cal examination. Due to the demands of reinforcement work the trunk muscles are probably exercised to a consideI'able extent in reinforcement workers. Diminished back muscle performance capa,city is thus supposedly mainly due to peripheral nerve root lesions, but acute, minor trauma or pain causing a reversible reduction of trunk muscle performance capacity may also 44 have been responsible for a part of the findings.
The age dependence of muscle strength has been investigated by Asmussen and age 20-29 years to 60-69 years, while abdominal muscle strength showed only a 10 % reduction ov,er the same age period (2). The prevalence of diminished trunk muscle performance capacity among concrete reinforcement workers can be com-paI'ed to findings among computer technicians, investigated by idenUoal methods in another cross-sectional study (12). The computer technici.ans (104 Finnish males, aged 20-54 years) often worked in awk-waI1d postures especially demanding for the back and the knees. The age-adjusted prevalence of diminished back muscle capacity was 1.7 times higher among reinforcement workers, but the corresponding chi-squar,e value of 3.1 was not statis-Uoally significant. The age-adjusted prevalence of the diminished capacity of the straight abdominal muscles was equally high in both groups ( fig. 4).
Over the years cnnsiderable evidence has been presented in favor of an associatinn between diminished trunk muscle strength and the occurrence of low-back pain, but the extent to which diminished trunk muscle strength may cnntribute to the nccurrence of low-back pain and the degree to which episodes of low-back pain leads toa reduction of trunk muscle strength are questions still open for discussion and further study.
Kraus and Weber 1 (10) found that the reduction of trunk muscle strength and flexi'bili ty belnw a certain level was assodated with a high expectancy of back pain. Poulsen and J0rgensen (16) showed that maximal isometric strength should exceed two-thirds of a person's body weight to avoid fatigue and pain in the ba(:k from work in a forward-leaning position. Chaffin and Herrin (5) also f{Jund that workers with good "torso lifting strength," mainly -a measur-e of back muscle strength, had signifkantly less lowback pain than wea'ker workers. On the other hand, Hirsch (8) oonsidered muscular persons rto be as susc-eptible to low-back pain -as asthenics, 'and no ev~dence for an association betwe,en diminished trunk muscle strength and low-back pain has been found in two thorough investigations (11,14).
Chronic low-back pain is frequently accompanied by generalized weakness of the trunk muscle's (1). Up to 80 Ofo of patients with low-back pain may be classifi'ed as deficient in trunk muscle strength (10). A large prospectiv,e study (19) has shown that 'abdominal muscle weakness, found at the time of the low-back pain attack, often disappears with the remission of the episode of pain, but that it sometimes is left as a residual sign of previous low-back pain. Persons without experience of lowbaoek pain had weakened abdominal muscles less often (12 Ofo) than persons with a previous spell of low-back pain unaccompanied by an incapadty to work (26 0/0) and persons previously incapacitated because of low-back pain (50 Ofo). Nachemson and Lindh (11), excluding cases of sciatica from their study, found no difference in the extensor or flexor str,ength of the trunk ibetwe'en persons without lowback pain and thDse incapacitated with back pain f,or less than a month. In persons incapadta'ted by low~ba,ck pain for more than a month they r-ecorded a reduction of maximal isometric tension in the back 'and abdominal muscles; this condi-Hon was 'considered probably due to inactivity because of pain or fear of pain.
In the present study on concrete reinfor-oement workers the performance capacity of both back 'and straight abdominal muscles were found to be related. to previous spells of both lumbago and sciatica. As the prevalences of diminished trunk muscle capacity and of a reported history of low-back pain syndromes both increase with age, the associations between these variables were ,analyzed 'after age adjustment. The 'a-ssoda'tion between diminished back muscle capacity and a history of sciatica was found to be clear, while the relati'on between diminished abdominal muscle capacity and a history of lumbago was weaker.
When specific back symptoms were compared to ,a reduction of trunk muscle performanc-e, no relationship was found fDr the milder symptoms of "fatigue" and "stiffness, " while "a,che" and "sharp pain" were associated with a reduction of both back and abdominal muscle performance capacity. Thus our assumption that the sensation of "fatigue" arises in the musculoligam-entous structures from excessive demand on muscular contraction was not supported by our data.
There is a definite assodati'on between diminished strunk muscle strength and lowback pain. Th~s relation can be of three types: 1. Diminished trunk muscle strength oauses low-back pain. P.ossible mechanisms: (a) pain arises in the muscular tissue itself or in stretched ligaments when demand on muscular contraction exceeds performance; (b) pain arises in the functional spinal unit (vertebral disc-nerve root-facet joint) from excessive mobility due to weakness of stabilizing trunk musdes or from abnormal patterns of ffiDvement in ,assodation with degenerative changes -and weak muscles.
2. Low-back pain causes diminished trunk muscular strength. Possible mechanism: pain causes relative immobilization of the back structures -and leads to diminished trunk muscle strength from bck of use.
3. Diminished trunk muscular strength and low-'back pain are both effects of a common cause. Possible mechanism: degeneration of in tervertebral discs and/or apophyseal joints causes damage to both the motor and the sensory roots of the spin-a1 nerves.
In our opinion none of the these relationships can be ruled out at the present level of -knowledge. Primarily we would eonsider the relation to start from degenerative spinal changes causing lesions of both the sensory and the motor nerve roots. As the back muscle is innervated from the corresponding spinal level, r<~duc-tion of back muscle strength may be due to lumbar disc degeneration, but the mechanism hehind the reduction of abdominal muscle strength is l€ss clear. Diminished trunk muscle strength may in turn contribute to further low-back morbidity as discussed by Alston et al. (1).