A variable inertial system for measuring the contractile properties of human muscle.

PURPOSE: A flywheel system of variable inertia is described for inferring the mechanical properties of human muscle during a single explosive movement. METHODS: The system consists of a lightweight aluminum disk mounted on a shaft onto which a driving cog is mounted. The inertia of the system can be varied from 0.024 to 0.69 kg.m(2) by attaching semicircular steel plates to the disk. A rotary encoder detects displacement of the wheel with a resolution of 1 degrees. Digital signals from the encoder are collected using an A/D converter interfaced to a PC. The data are then processed for the calculation of torque, velocity, power, work done, and acceleration. The mechanical properties of the muscles employed are inferred from calculations of flywheel displacement, time, and force. In addition, a pretension release mechanism can be incorporated into the system to allow isometric force to be developed before movement. This can increase power generation at the low inertias where the time of contraction is typically less than 200 ms. Seven subjects were test-retested using the device. Measures of both average and peak power were made. RESULTS: When mounted in the apparatus described by Bassey and Short, the maximum values for peak and average power were on average 965 +/- 103 and 448 +/- 47 W, respectively. Upon retesting, these results were found to be reliable (cv = 3.3% and 3.0%, respectively). CONCLUSIONS: The inertial system described has been shown to have validity in reproducibility and provided a suitable method of determining a number of muscle output properties during short-term single exertions. This tool could prove useful in a research or clinical setting and may also prove useful as a training device as it negates the need for a strain gauge or goniometer attachment.

Hydro-resistive measurement of dynamic lifting strength.

A device is described for measuring strength and power outputs of dynamic vertical lifts between heights of 0.4 and 2.2 m. The device is safe, robust, and easily transportable. It consists of a water-filled tube 2 m high and 200 mm internal diameter. The subject pulls vertically on a handle which is connected with flexible wire rope via a series of pulleys to a piston suspended inside the tube. The piston has holes which can be closed with bungs. The drag force is proportional to the square of the velocity. The constant of proportionality can be chosen over a more than 100-fold range and is independent of temperature. Manual force is measured using a strain gauged cantilever over which the rope passes. Rope movement is monitored with a shaft encoder. These devices are sampled synchronously by an interfaced computer. Velocity and power are derived from the measurements of displacement, time and force. The device is highly accurate. Power measurements are not significantly different on two separate days although repetitions on one day show a warming-up effect. This device allows the study of dynamic lifts ranging from slow, high force, quasi-isokinetic lifts to lifts where high velocities and accelerations occur.

The generation of torque in the horizontal plane at the foot-base during laterally directed manual exertions.

Two experiments were carried out to investigate the mechanisms by which both translational forces and a turning moment in the horizontal plane at the foot-base are transmitted by the lower limb during laterally directed manual exertions. The first used a force plate to measure forces and moments exerted by the feet separately or together in order to investigate the contribution of each foot to the moment. The second examined the role of flexors and extensors of the hip, knee and ankle in the transmission of torque. The results suggest that approximately half of the moment is generated by each foot exerting a translational force at a distance from, and in the opposite direction to, the other. The other half is by each foot exerting an individual torque upon the floor. It is suggested that biomechanical models of asymmetric exertion should take these mechanisms into account.

The use of artificial neural networks to identify patients with chronic low-back pain conditions from patterns of sit-to-stand manoeuvres.

OBJECTIVE: To investigate whether artificial neural networks (ANNs) can categorize healthy subjects, chronic low-back-pain (LBP) patients, and subjects pretending to have low-back pain problems, based upon patterns of stand-sit-stand manoeuvres. DESIGN: A non-invasive laboratory study of human subjects. BACKGROUND: Normal strategies for sit-stand manoeuvres are modified in cases of chronic LBP. Subtle changes and many parameters are unsuitable for conventional statistics. METHODS: Fourteen healthy subjects, 10 chronic LBP patients, and 12 subjects pretending to have LBP participated. Forces and centres of pressure at the feet and knees, plus hip and lumbar movements provided inputs into a three-layer feed-forward ANN with sigmoidal transfer functions. The ANN was trained with data from 35 of the 36 subjects, and its ability to classify the left-out subject was tested. This was repeated with each subject omitted from training in turn. RESULTS: The ANN correctly classified 31 of 36 subjects. The subjects were also classified by nine physiotherapists from videos of the manoeuvres. Their success rate was significantly lower that that of the ANN, which is not surprising for an unusual procedure without training. CONCLUSIONS: ANNs should be considered as additional tools in assessment and possible diagnosis of pathological movements. RELEVANCE: The capacity of ANNs to discriminate patterns of human movement offers a technique that may prove to be a useful tool for assessment and diagnosis.

The influence of task resistance on the characteristics of maximal one- and two-handed lifting exertions in men and women.

Dynamic lifting strength was assessed at lift velocities ranging from 0 to 1 m.s-1 using a hydrodynamometer in which the viscous resistance to motion could be preset. Nine men and nine women performed one- and two-handed dynamic exertions against low, medium, and high resistances. The subjects were required to lift as forcefully and as fast as possible from a height of 400 mm to just above head height. The instantaneous forces recorded at knee, knuckle, hip, shoulder, elbow, and head heights were compared with those obtained during maximal one- and two-handed isometric lifts performed at the same heights. The position of peak force relative to stature was lower for one-handed lifts (35.9% of stature) than for two-handed lifts (38.4% of stature) (P < 0.005), but was unaffected by lift resistance (P > 0.05) and subject sex (P > 0.05). Lifting forces during two-handed exertions were on average 1.5 times greater than for one-handed exertions (P < 0.0001). However, the difference in the strength of one- and two-handed lifts was highly dependent on hand height (P < 0.0001) and task resistance (P < 0.005), and was greater for the men than for the women (P < 0.001). When static lifting strength was measured at heights relative to stature and normalized by dividing by body weight, there was no significant difference in performance between the men and the women [mean normalized female:male: (f:m) strength ratio = 0.76, P > 0.05]. Under dynamic conditions the sex difference in lifting performance was pronounced (mean normalized f:m strength ratio = 0.68, P < 0.01), especially for power output (mean f:m power.unit body weight-1 ratio = 0.52, P < 0.005). The proportion of generality (r2 x 100) between dynamic and static lifting strength was found to range between 63% and 87%. These data suggest that the dynamic and static tests of lifting strength measured a common, intrinsic ability to produce maximal lifting forces. The finding that sex differences in strength are dependent on whether the exertion is static or dynamic has important implications for biomechanical models of human strength.

The use of neural networks to recognize patterns of human movement: gait patterns.

Artificial neural networks and a statistical method, linear discriminant analysis, were both applied to the recognition of temporal gait parameters associated with altered gait patterns. The duration of the double support and right and left single support phases were measured at seven speeds and three walking conditions. Data from 10 subjects were used to train neural networks, which were then tested using data from 10 other subjects. The overall performance of the networks was at least as high as that of linear discriminant analysis. The relative ease with which neural networks can be set up in a computer, and their discriminatory power, suggests that the technique has a useful role to play in gait analysis. RELEVANCE: The capacity of neural networks to recognize alteration of gait patterns suggests that they might provide an alternative approach for gait assessment. They might be proved to be a useful diagnostic tool.

Relationships between one-handed force exertions in all directions and their associated postures.

Photographs were taken of subjects exerting in specified directions with one hand on the handle of a triaxial force measurement system. The applied forces were recorded and posture analysis was undertaken to investigate relationships between three-dimensional force exertion and posture. The postural stability diagram, which in previous studies has been applied to fore-and-aft exertions, was applied to the vertical plane containing the manual force vector and to the horizontal plane. The vertical plane analysis provided an insight into postures associated with weak and strong exertion. The horizontal plane analysis emphasized the importance of developing torque as well as thrust at the foot base in order to exert laterally directed forces. Exertions involving a right or left component were associated with a horizontal moment at the feet of the order of 50 Nm. This moment is an important factor in the demands made upon the body during asymmetrical exertion, and the mechanisms for achieving it deserve further investigation. RELEVANCE: Exertion is not normally restricted to the sagittal plane. The approach adopted in this paper gives an insight into how body deployment relates to the direction and magnitude of exertion. Biomechanical models of asymmetric exertion should reflect the principles that have emerged.

System for the triaxial measurement of manual force exertions.

A triaxial force measurement system has been developed to allow the measurement of whole-body manual force exertions in the three axes of the Cartesian system. The design of the system precludes the exertion of torques, thus enabling a description of the linear forces that an individual can exert in three dimensions. Forces are measured independently for each hand, allowing the examination of triaxial forces in both one- and two-handed whole-body exertions, and the analysis of the forces in the closed chain of the upper limbs in two-handed exertions. The hardware and computer software that constitute the triaxial force measurement system are described, as are the method of calibration used and the accuracy of the system. Asymmetric postures and exertions are seen as risk factors leading to musculoskeletal disorders. Measurement and descriptions of manual strength involving asymmetric postures and lateral components of exertion will enable risk analysis of such tasks to be undertaken.

Protocol for the omnidirectional assessment of manual strength.

This study presents a protocol for assessment of manual strength over all directions in three dimensions. The reliability and validity of the protocol was tested on four male subjects. They performed one-handed maximal exertions overthe entire sphere of exertion at hand heights of 1.0 and 1.75 m and at a horizontal foot distance of 0.5 m from the centre of force application. Testing sessions consisted of five 2 min data collection periods interspersed with rest pauses of the subject's own chosen duration. At least four testing sessions performed on separate days were required for a subject to fulfil the criteria for completion of the sphere of exertion. The results showed the methodology to be reliable. Repeat trials at the 1.0 m hand height revealed a high reliability coefficient for the three-dimensional forces (r = 0.96). The protocol was also suitable for assessment of three-dimensional manual strength at the higher hand height of 1.75 m. The distinctive patterns of forces over the sphere of exertion observed at the 1.0 and 1.75 m heights illustrated the importance of hand height on manual strength in three dimensions. The methodology presented in this paper provides a good basis on which to develop future clinical or scientific studies investigating human strength capabilities over all directions.

Short-stepping gait in severe heart failure.

BACKGROUND: Patients with severe chronic heart failure seem to take shorter steps than healthy controls when walking on a treadmill and when walking freely along a corridor. In healthy individuals the pattern of walking affects the oxygen cost of exercise, and so this observation might be relevant to the limitation of exercise in heart failure. METHOD: Length of stride was analysed as stride/stature index in 15 controls, 10 patients with moderate heart failure, 10 patients with severe heart failure, and 10 patients with angina, walking at a constant speed/stature index. RESULTS: The stride/stature index was 0.64 in the controls in patients with New York Heart Association (NYHA) class II heart failure, and in patients with angina. It was 0.49 in patients with NYHA class III heart failure. In the patients with heart failure the stride/stature index correlated with exercise capacity determined as peak oxygen consumption VO2max (R = +0.62, p < 0.005). When healthy controls walked in time to a metronome adjusted to decrease their stride/stature index to approximately that seen in severe heart failure steady-state oxygen consumption increased by a mean of 15%. CONCLUSIONS: The length of stride is reduced in severe heart failure, and when healthy controls adopt this gait the oxygen cost of walking is increased. A short-stepping gait may contribute to the limitation of exercise capacity in heart failure.

Posture, performance and discomfort in pregnancy.

This study examined changes in posture, discomfort and performance during various stages of pregnancy. Twelve subjects performed a typing task in the second and third trimesters of pregnancy. Posture was measured using goniometry and comfort was assessed using body-mapping techniques. The subjects showed a marked increase in their angles of trunk inclination and a general increase in discomfort between the second and third trimesters of their pregnancies. All body parts were found to be more uncomfortable in the third trimester and the largest changes occurred in the low back and lower legs. Advancing pregnancy had no effect on typing performance.

Performance of physical tasks in pregnancy.

Two hundred women in the third trimester of a first pregnancy were interviewed, and 50 of them were interviewed again at 4 months post partum. They were asked whether they experienced greater current difficulty in performing 46 common tasks than they did before pregnancy and were asked to grade their responses. Additional information concerning specific factors affecting each activity, such as discomfort and fatigue, was obtained, together with data concerning socioeconomic status and stress perception. The data were analysed to identify the activities whose performance was most and least affected by pregnancy, indicating that the pregnant condition affects perceived difficulty of performance in many activities. Although some activities are not amenable to ergonomic intervention, the study highlighted some in which equipment design and selection could improve the match between a task and the pregnant.

The measurement and prediction of isometric lifting strength in symmetrical and asymmetrical postures.

Static lifting strengths of nine men and nine women were measured at six heights from just above the floor to just above the head, at two horizontal reaches from the mid-ankles (equal to the elbow to grip and acromium to grip distances), in the sagittal plane and also at 45 degrees and 90 degrees to the right for two-handed exertions and at 45 degrees and 90 degrees to each side for one-handed exertions, making a total of 96 postures. A second and different group of 18 subjects (nine men and nine women) were studied in 20 two-handed and 40 one-handed postures intermediate to those of the first group. A third group of 16 subjects (eight men and eight women), with six drawn from the other groups, were used to determine maximum possible reach (at which lifting strength is zero) at the same heights and planes as those for the first group. When strength was expressed as a fraction of body weight and height and reach were expressed as fractions of stature, predictive equations of static lifting strength were obtained which were gender free. The predictive equations may be used to generate isodyne contours for an individual in any chosen planes. Individuals exist whose strengths are consistently greater or less than the prediction. The possibility of identifying such persons in a process of worker selection is discussed.

Human strength capabilities during one-handed maximum voluntary exertions in the fore and aft plane.

Maximal static strengths were determined for one-handed exertions in all directions in the fore and aft plane. Data from 12 males and 10 females (mean age 30.7 yrs, standard deviation (SD) = 8.9 yrs, n = 22) were obtained with handle heights of 1.0 and 1.75 m. Twelve of the subjects also performed two-handed exertions at the same handle heights. The ratio of mean strengths of females to that of males ranged from 0.50 to 0.83 (for absolute forces) and from 0.63 to 1.00 for forces normalized to body weight. The ratios of one-handed to two-handed strengths ranged from 0.64 to 1.04. Two-handed strengths commonly exceeded one-handed strengths at the lower handle height, but showed fewer significant strength differences (p less than 0.05) according to direction at 1.75 m. Both female/male and one-handed/two-handed strength ratios were found to be dependent on direction of exertion and handle height. The observed strength dependencies upon number of hands (one or two-handed), direction of exertion, handle height and sex are discussed. The strength data have implications for use in biomechanical models and task analysis.

Demands on the back during maximal exertion.

Michigan-type computerized biomechanical models have the potential to predict hazard levels of exertion in many activities besides lifting. With suitable refinement, they could provide guidelines for non-lifting activities in a shorter term than can be expected from epidemiological and psychophysical studies. This paper examines some of the assumptions in the models, particularly as regards muscle action at joints. Observed features of whole symmetrical exertions are described which could be used for comparison with and validation of the simpler models before extending their use to production in the real-life conditions of asymmetrical exertions.

A description of normal relaxed standing postures.

A technique for the measurement of relaxed standing postures, called the Posturegram, is described. The method is inexpensive, non-invasive and is suited for clinical use. Recording is done by photography. The present study examines features of standing that were exhibited by 42 normal subjects, based upon the disposition of landmarks in the coronal and sagittal planes. Degrees of assymetry that would merit clinical attention were tentatively identified. Repeat observations, 1 week apart, of the disposition of landmarks were strongly correlated, although the lowest correlations applied to the region of the pelvis and lower trunk. Many highly significant correlations were found between the various tilts and deviations, suggesting that the analysis of postures will finally depend upon considering variables in groups rather than singly.

Force, speed and power output of the human upper limb during horizontal pulls.

The purpose of the experiment was to examine how force, speed and power output of horizontal pulling with the upper limb was affected by the height of pull. Fourteen seated male subjects made horizontal pulls with maximal effort at eye, shoulder and elbow level from their positions of full reach when the trunk and shoulder girdle were rigidly constrained. Dynamic pulls were performed against a water-filled viscous dynamometer in which the resistance, proportional to the square of the velocity, could be varied. The height of pull had no significant effect on either static or dynamic performance. A force-velocity-position surface is presented which describes the conditions at the handle during the pulls. It confirms the importance of degree of reach upon the dynamic performance, and over a greater range of velocities than has been studied previously. A simple model shows that the similarity of performance at eye, shoulder and elbow heights is remarkable because they occur under very different biomechanical circumstances. The total work done in a complete pull increases with resistance. Peak power output is obtained against the same resistance (50 kg m-1) that was reported for elbow flexion and standing pulls.

Pressures under normal feet in standing and walking as measured by foil pedobarography.

An economical method of recording peak pressures under the feet while standing or walking is described for clinical application. Normal values at 10 defined locations are presented. Correlation matrices indicate functional links between regions of support which influence patterns of deviation from the norms.

The influence of posture on power output generated in single pulling movements.

The power generated in single pulling movements in seated and standing postures was measured by means of novel dynamometers. The power available is strongly dependent upon the resistance to movement, as well as the posture. Some of the results have been applied to an assessment of the matching of outboard motors to their users, illustrating an ergonomic value of the methodology.