A comparison of isometric strength and dynamic lifting capability

Quantifying how functional and structural personal factors influence biomechanical exposures in paramedic lifting tasks

It is unknown how structural (sex, stature, body mass) and functional (strength, flexibility) personal factors influence lifting strategy in paramedic work. We explored whether variance in peak low back forces and kinematic coordination patterns could be explained by structural and functional personal factors in paramedic lifting tasks. Seventy-two participants performed backboard and stretcher lifts. Peak low back forces normalised to body mass, as well as kinematic coordination patterns, were calculated as dependent variables. Being female, stronger, shorter, having higher body mass, and/or having greater lower body range of motion (ROM) were all independently associated with lower normalised low back forces across backboard and stretcher lifting. Females and stronger individuals seemed to define a movement objective to consistently minimise compressive forces, while individuals with greater hip ROM consistently minimised anteroposterior shear forces. The efficacy of improving strength and hip ROM to reduce low back forces in paramedic lifting should be investigated.Practitioner summary: Females, stronger individuals, and individuals with greater hip range of motion consistently exhibited lower normalised low back forces in paramedic lifting. Improving strength and hip range of motion via training is a potential proactive ergonomics approach to reduce peak low back forces in paramedic lifting tasks.

Human Strength as a Predictor of Lifting Capacity

A sampling of lifting capacity prediction models utilizing human strength measurements as input are presented. The models utilize isometric, isokinetic, and/or isoinertial strength measurements. Comparison of such predictive models appears to indicate that lifting capacity is better predicted using dynamic rather than static strength tests.

Applications of biomechanics for prevention of work-related musculoskeletal disorders

This paper summarises applications of biomechanical principles and models in industry to control musculoskeletal disorders of the low back and upper extremity. Applications of 2-D and 3-D biomechanical models to estimate compressive force on the low back, the strength requirements of jobs, application of guidelines for overhead work and application of strain index and threshold limit value to address distal upper extremity musculoskeletal disorders are presented. Several case studies applied in the railroad industry, manufacturing, healthcare and warehousing are presented. Finally, future developments needed for improved biomechanical applications in industry are discussed. The information presented will be of value to practising ergonomists to recognise how biomechanics has played a significant role in identifying causes of musculoskeletal disorders and controlling them in the workplace. In particular, the information presented will help practising ergonomists with how physical stresses can be objectively quantified.

Definition and assessment of specific occupational demands concerning lifting, pushing, and pulling based on a systematic literature search

AIMS

(1) To find a universal strategy for the identification of specific demands of a job or task, focusing on occupations in which there may be an increased risk for health complaints owing to these specific demands. (2) To select reliable and valid tests concerning lifting, pushing, and pulling, which consider the relation between occupational work demands and the assessment of the maximally acceptable load on an individual level.

METHODS

Literature search was performed using Medline (1988 to May 2001), Embase (1966 to May 2001), and NIOSHTIC (1971-98).

RESULTS

No universal strategy was found for the definition of specific occupational demands. Therefore a "three step strategy" was formulated for defining specific occupational demands in a job or a task in order to prevent health complaints on an individual level. Many tests were found in the literature concerning lifting, but only a few concerning pushing and pulling. None of the tests concerning pushing, pulling, or lifting considered the relation between work demands and the assessment of the maximally acceptable load on an individual level. Furthermore, none of the tests met the criteria of reliability and prognostic value for musculoskeletal complaints completely. Only for the prognostic value of relative strength capacity tests concerning pushing pulling and lifting, did there appear to be limited proof for the development of musculoskeletal complaints.

CONCLUSIONS

In general, for the prevention of work related health complaints, it can be suggested that more attention should be paid to: (1) the definition of specific occupational demands; (2) the assessment of specific occupational demands; and (3) the quality of tests for specific occupational demands.

An isometric predictor for maximum acceptable weight of lift for Chinese men

The aim of this study was to examine the practicality of the modified isometric strength tests to predict the maximum acceptable weight of lift (MAWL) of Chinese men. The modified strength tests allow the participant to pull on the load cell in front of the body and to apply force in a functional free posture. Both the modified and the standard strength data of each participant were used as predictors for the MAWLs. The prediction models were constructed and evaluated under task conditions of two lifting ranges, two box sizes, and three lifting frequencies. To realize the effect of modifications, testing posture was recorded and the joint angles were calculated. A stepwise multiple regression analysis indicated that modified composite strength (MCS), chest circumference, and acromial height accounted for 86% to 91% of the variance. Because the strength of the upper extremity body was also recruited in the test, the weak upper extremity strength of the Chinese participants would therefore be better reflected. Evidence for the existence of a close match between MAWL and MCS values, as well as the task conditions for its existence, suggest that a simple isometric strength measure is a good predictor for the MAWL.

Effects of asymmetric dynamic and isometric liftings on strength/force and rating of perceived exertion

A laboratory study was undertaken to determine the postural and physical characteristics and subjective stress during dynamic lifting of a usual load (10 kg) compared with during isometric lifting. The authors also aimed to clarify the effects of asymmetric lifting on these parameters. The subjects were thirteen male college students. They were asked to lift a box weighing 10 kg. They performed sixteen different lifting tasks from the floor to a height of 71 cm, involving a combination of three independent factors: two lifting modes (isometric lifting and dynamic lifting), four lifting angles in relation to the sagittal plane (sagittal plane, right 45 degree, right 90 degree and left 90 degree planes) and two lifting postures (squat and stoop). For each lifting task, strengths or forces and ground reaction forces were measured. At the end of each task, the authors asked the subjects to rate their perceived exertion (RPE) during lifting at ten sites of the body. Angle factor had a significant effect on isometric strengths and dynamic peak forces. Isometric strengths during the maximum 3 s were highest in lifting in the right 45 degree plane, followed by that in the sagittal plane, while those in the right 90 degree and left 90 degree planes were the lowest. However, peak forces in dynamic lifting were the highest in the lifting in the sagittal plane, followed by that in the right 45 degree plane, while those in the right 90 degree and left 90 degree planes were the lowest. Postural factor had a significant effect on height at peak force, which is higher in squat lifting than in stoop lifting. RPEs for the left arm, the backs and the right whole body in isometric lifting were significantly higher than in dynamic lifting of 10 kg. There were remarkably high RPEs for the ipsilateral thigh to the box in right 90 degree and left 90 degree planes during both isometric and dynamic liftings. Locations of the resultant force consisting of three component forces on the force plate were closer to the foot on the same side as the box in asymmetric lifting. Thus, some similarities and differences were found between isometric lifting and dynamic liftings regarding the indexes of strength used in this experiment. The authors consider that the subjects used the foot nearer to the box as a fulcrum during asymmetric lifting. Dynamic measurement using the 10 kg weight is less stressful than the conventional isometric measurement. It was possible to obtain the height data at peak force and time-based changes in the force and the box location during lifting only through dynamic lifting measurement. The results provide new knowledge about the biomechanical features of dynamic lifting tasks.

Variability of grip strength during isometric contraction

The use of measures of strength variability as a means of determining sincerity of effort is becoming a more common practice, particularly in medico-legal and rehabilitation settings. The stability of such variability measures, however, has not been documented. This research investigated, in two studies, the trial-to-trial variability of grip strength under maximal and submaximal effort conditions. In the first study, 63 subjects were asked to give 100% grip effort, and in the second study 40 subjects from the original group were asked to give 50% grip effort. The Jamar hand dynamometer was used to measure grip strength in both experiments, and a coefficient of variation (CV) was calculated for every three repeat measures at each handle position. Testing was conducted on two separate occasions for both experiments. Although the interoccasion reliability of grip strength was very high, in comparison, the CV was not stable over test occasions, with interoccasion reliability indices close to zero. Factors significantly influencing CV were effort level, with submaximal effort producing larger CVs, and gender, with females having greater strength variability. If the rule is applied that one or more CVs above the 7.5% cut-off value could indicate submaximal effort, then for this sample of subjects giving maximal effort, 97% of females and 64% of males would be misclassified. Applying a single CV classification cut-off value to a mixed sample of subjects appears to unfairly discriminate against the females. Further research into the factors associated with high CV values is essential before the CV can be used with any confidence in a clinical setting as a method for determining sincerity of effort.

The relation between isometric lifting strength and muscular fitness measures

The purpose of this study was to construct an isometric lift dynamometer and relate isometric lifting strength to dynamic measures of muscular fitness, leg and back strength and muscular power output. Thirty-one male subjects, aged 19-24 years, performed a standing broad jump (for distance), a vertical jump (for flight time), 3 maximal pulls (for peak power) on a rowing ergometer and maximal isometric leg and back extensions on a conventional dynamometer. The results of these tests were correlated with the maximum isometric lifting strength (MILS) obtained on the lift dynamometer using cluster correlation and multiple regression. Significant correlations (p < 0.001) were found between isometric lifting strength and back strength (BS) (r = 0.59), and leg strength (LS) (r = 0.74). A significant correlation was also noted between isometric lifting strength and both standing broad jump (SBJ) (r = 0.58, p < 0.001) and power output (PO) (r = 0.38, (p < 0.05). Multiple regression analysis was used to predict lifting performance from a battery of standard fitness tests. The prediction equation for maximal isometric lifting strength included the terms LS, BS, SBJ and PO (r = 0.80). The relation between isometric lifting strength and other muscular fitness variables suggest that the method used provides an acceptable measure of strength and an indication of the involvement of back and leg musculature in squat lifting.

Effect of computerized instructions on measurement of lift capacity: Safety, reliability, and validity

The evaluation of lift capacity is an important part of most functional capacity evaluations. Several different methods have been developed to evaluate lift capacity in a safe, reliable, and valid manner. Isometric strength testing is one approach which has been demonstrated to be highly reliable. However, questions have been raised about the safety and validity of isometric strength testing as a predictor of "real world" lift capacity. One method to improve safety is to provide real time performance feedback so that the evaluee is able to increase psychophysical input to appropriately gauge his or her effort level. One method to improve reliability is to provide standardized instructions that are well understood by the evaluee. Both of these approaches have been utilized in the ERGOS Work Simulator, a computer-controlled multiple-task evaluation instrument that presents visual and auditory instructions to the evaluee along with "real-time" performance feedback. The safety, reliability, and validity of this computer-automated approach in comparison to an experienced human evaluator was evaluated in this research project. The results of this study demonstrate the efficacy of computerized instructions in an isometric strength testing system to achieve safe, reliable, and valid results. Intra-task variability may be improved by providing pre-test practice trials.

Arm lift strength in work space

This study was conducted to determine arm strength values for isometric and isokinetic efforts around the human trunk. Thirty-eight normal young adults (20 male and 18 female) performed a total of 19 tasks. These consisted of one self-selected optimum posture with upright stance and elbows bent at 90 degrees , designated as standard posture for isometric test. In addition, isometric testing was done sagittally symmetrical 30 degrees and 60 degrees lateral planes at half-, three-quarters- and full-reach distances at knuckle height. The isokinetic tests were done between knuckle height and shoulder height in postures identical to isometric tests. The sequence of these tasks was randomised. The peak strength in standard posture was invariably lower than the peak strength at half-reach in isometric condition in all three planes for both sexes with the exception of one condition among females (60 degrees lateral plane, half-reach isometric). Peak and average arm lift strengths of males were significantly higher than those of females (p < 0.01) and ranged between 44% and 71%. For both sexes isometric strength was significantly higher than isokinetic strength (p < 0.01). The peak and average strengths in the sagittal plane were invariably higher than those of asymmetric postures, with one exception among females. With increasing reach distance the strength declined significantly for all conditions among both genders (p < 0.01). The ANOVA showed that the gender, mode of lifting, postural symmetry and reach of lifting, in addition to affecting the peak and average strength individually (p < 0.01), had significant 2-way and 3-way interactions (p < 0.01). All strength values were inter-correlated (p < 0.01). The regressions predicting peak and average strengths from anthropometric characteristics and sagittal plane strengths accounted for 63% to 89% of all variance and were highly significant (p < 0.01).

A comparison of isometric strength and dynamic lifting capacity in men with work-related low back injuries

Job classification, return to work status, and job placement are determined by the results of lifting capacity tests. Lifting capacity is often assessed by maximum static or dynamic physical exertions. The purpose of this study was to compare maximum isometric lifting strength with maximum dynamic lifting capacity in subjects with work-related low back injury. Twenty-seven men performed five dynamic lifting tasks and two isometric strength tests. The mean Pearson product-moment correlation between maximum isometric lifting strength measurements and maximum dynamic lifting capacity measurements was .51 with a range of .30-.73. The results indicate low to moderate associations between isometric strength measurements and lifting capacity suggesting that estimates of functional lifting capacity should not be based on static measurements alone. Dynamic lift tests are often a better simulation of the task being assessed and may be more appropriate for a back-injured population.

The reliability and validity of two new tests of maximum lifting capacity

This study investigated the test-retest reliability and validity of the isokinetic and gravity/inertia modes of the Lido Lift (TM) and the Progressive Lifting Capacity II test (PLC II). Maximum lifting capacity tests were performed in the isokinetic and gravity/inertia modes on the Lido Lift and with the PLC II with 29 healthy male subjects. Pearson product-moment correlations were calculated to determine the test-retest reliabilities and to compare PLC II test values with isokinetic and gravity/inertia test values. The correlations for the isokinetic, gravity inertia, and the PLC II werer=.90, .82, and .91, respectively. The correlations between the isokinetic and gravity/inertia tests and the PLC II werer=.64 and .74 during the test andr=.72 and .81 during the retest. The results of this study support the reliability of both Lido Lift (TM) tests and of the PLC II, and the validity of the isokinetic and gravity/inertia tests when compared to the PLC II.

Lifting ability and leg strength

The purpose of this study was to determine the relationship between peak torques of lower extremity muscle groups and lifting ability with a floor-to-knuckle leg lift in a healthy person. Twenty-four healthy, young subjects between the ages of 20 and 40 participated in the study. Each subject was tested for 1) isokinetic peak torque of the knee extensors and flexors and hip extensors and flexors on a Cybex II dynamometer, 2) isometric lifting ability using a strain gauge dynamometer, and 3) maximum lifting ability using a box and free weights. The subjects used a squat technique and maintained a neutral spine position with a lordotic curve during the lifts. No significant correlations were found with the data analysis for the entire group. For 15 male subjects, the regression equations revealed a statistically significant correlation between isokinetic knee extensor peak torque (180 degrees /sec) and isokinetic hip extensor peak torque (30 degrees /sec) with both maximum lifting ability and isometric lifting ability. The regression equations were MAXIMUM LIFT = 46.1 + 0.92Q + 0.10H (F = 4.07, p < 0.05) and ISOMETRIC LIFT = 16.6 + 23Q + 1.05H (F = 6.32, p < 0.025) (Q: isokinetic knee extensor peak torque at 180 degrees /sec; H: isokinetic hip extensor peak torque at 30 degrees /sec). No statistically significant correlations were found for the nine females. These results indicate that in a healthy, young population, isokinetic knee extensor and hip extensor peak torque may be predictors of floor-to-knuckle leg lifting ability in males, but not in females. J Orthop Sports Phys Ther 1991;14(1):24-30.

The relationship between isometric strength of Cantonese males and the US NIOSH guide for manual lifting

A laboratory study was undertaken to assess the relationship between the maximal voluntary isometric strength (MVIS) of a group of Hong Kong Cantonese males and recommended limiting values for lifting proposed in the Work Practices Guide for Manual Lifting (NIOSH, 1981). Forty-one male subjects were required to apply a 'free-style' vertical 'lift' force to a horizontal bar located 150 mm from the ground. Forces were applied with the horizontal location of the midpoint of the ankles at 200, 400, 600 and 800 mm from the bar. The results were compared with the Maximal Permissible Limit (MPL) and the Action Limit (AL) values proposed in the NIOSH guide. The results showed that the mean MVIS of the Hong Kong subjects lay between the MPL and AL when the load was 200 mm and 400 mm from the ankles, but was significantly below the AL at 600 mm. At 800 mm more than half of the subjects were unable to apply any measurable positive vertical force. The compressive force at the L5/S1 disc of four subjects was estimated using a static biomechanical model. The relationship between the disc pressure and the force exerted was largely in accordance with NIOSH guide values. The results are discussed in terms of differences in subject population and lifting technique. Comments on the applicability of the NIOSH standards internationally, are also presented.

Isokinetic pull-up strength profiles of men and women in different working posture

This paper presents isokinetic (dynamic) strength profiles of males and females, engaged in infrequent force exertion, in 15 different working postures. The postures investigated ranged from kneeling to standing and involved both one-handed and two-handed exertions. The results indicated that both males and females exert maximum pull-up force with two hands in the stooped posture. Least force was exerted with the preferred hand at reach distance, in the sagittal plane (arm fully extended), in the sitting posture. As expected, large differences existed in the force exertion capability of males and females.

Human strengths and occupational safety

This paper presents a review of relatively recent North American literature concerning human strengths. It briefly covers basic strength definitions, measurement and prediction of strengths, and the role of strengths in employee screening and job matching procedures. In addition to discussing variations in strength with body posture and duration of exertion, a brief reflection on torque exertion capabilities of individuals with common hand tools is also included.

The 'Liftest' strength test-an accurate method of dynamic strength assessment?

A technique for the assessment of individual dynamic strength and lifting capacity, known as 'Liftest', has been devised and tested in the USA by Kroemer. We sought to confirm, or otherwise, his estimates of the reliability of the test within sessions, and the applicability of the test in a heavy industry (coal mining). The coefficient of variation for maximal overhead lifts from a height of 24 cm by 19 healthy miners was 2·6% (c.f. Kroemer' s value of 3·2% for males), with a mean load of 48·1 kg. A psychophysically acceptable overhead lift, at a frequency of one a minute for a working day, was less reliable. All subjects, except one, were capable of loads in excess of Kroemer's recommended safe maximum for lifts to knuckle height. There was no problem with systematic errors, i.e. fatigue or a learning effect. We concluded that 'Liftest' was a reliable means of dynamic strength assessment for the overhead-reach lift.

Resistive training and selected effects

This article has briefly discussed the various modes and methods of resistive training, and their relative values and effectiveness in achieving desired goals. Evidence has been presented indicating that resistive training can enhance both "physical and emotional fitness as well as reduce some cardiovascular disease risk factors." This may allow a greater satisfaction and enjoyment in daily activities as well as improve athletic performance.

Model for static lifting: relationship of loads on the spine and the knee

An experimental study based on a trigonometric, anthropometric model, was conducted on thirty-five healthy subjects to determine the relationship between knee and back forces during symmetric sagittal plane lifting. Total joint reaction forces for the knee and the back, along with their compressive and shear components, were calculated for each subject, as a function of the knee, back and ankle angles. The shear component was significantly higher in females than in males; the compressive component was significantly higher in males. Strong inverse correlations were found between the back and the knee forces on all subjects. Strong correlations were also found between subject anthropometry and minimum and maximum joint reaction forces. The magnitudes of both back and knee joint reaction forces should be considered in recommending the lift type and position, based upon individual worker anthropometry, and size and weight of the load to be lifted.

Biomechanical stresses as related to motion trajectory of lifting

A laboratory study was conducted to assess the effects of lift angle on biomechanical stresses to the musculoskeletal system, and in particular, on the lower back. Male subjects were required to lift four different tote boxes from the floor to a height of 0.81 m using two different lifting techniques. Maximum acceptable weights were determined using a psychophysical method. The motion trajectory of lift was studied by means of stroboscopic photography. Moments and forces at various body joints were computed using a biomechanical strength model.

Maximum acceptable weights were 16% (range = 9 to 23%) greater for the free-style lifting technique as compared with the squat method of lifting. There were large differences between the maximum acceptable weights and the "maximum permissible limits" recommended by NIOSH. The results showed that the subjects pulled the load toward the body when lifting psychophysically determined maximum weights. The lift angle increased with an increase in box width. The lift angle was greater for the free-style lifting method as compared with the squat lifting technique for lifting compact loads. Lifting at an angle resulted in a decrease in moment at the elbow, shoulder, L5IS1 disc, and hips, and an increase in moment at the knees and ankles. The estimated compressive force on the lumbar spine was, on the average, 11% lower for lifting at an angle as compared with a straight vertical lift.