J.P. Paul

In vivo human tendon mechanical properties

1 The aim of the present study was to measure the mechanical properties of human tibialis anterior (TA) tendon in vivo. 2 Measurements were taken in five males at the neutral ankle position and involved: (a) isometric dynamometry upon increasing the voltage of percutaneous electrical stimulation of the TA muscle, (b) real‐time ultrasonography for measurements of the TA tendon origin displacement during contraction and tendon cross‐sectional area, and (c) magnetic resonance imaging for estimation of the TA tendon length and moment arm. 3 From the measured joint moments and estimated moment arms, the values of tendon force were calculated and divided by cross‐sectional area to obtain stress values. The displacements of the TA tendon origin from rest to all contraction intensities were normalized to tendon length to obtain strain values. From the data obtained, the tendon force‐displacement and stress‐strain relationships were determined and the tendon stiffness and Youngs modulus were calculated. 4 Tendon force and stress increased curvilinearly as a function of displacement and strain, respectively. The tendon force and displacement at maximum isometric load were 530 N and 4·1 mm, and the corresponding stress and strain values were 25 MPa and 2·5%, respectively. The tendon stiffness and Youngs modulus at maximum isometric load were 161 N mm−1 and 1·2 GPa, respectively. These results are in agreement with previous reports on in vitro testing of isolated tendons and suggest that under physiological loading the TA tendon operates within the elastic ‘toe’ region.

What is balance

Balance is a term frequently used by health professionals working in a wide variety of clinical specialities. There is no universally accepted definition of human balance, or related terms. This article identifies mechanical definitions of balance and introduces clinical definitions of balance and postural control. Postural control is defined as the act of maintaining, achieving or restoring a state of balance during any posture or activity. Postural control strategies may be either predictive or reactive, and may involve either a fixed-support or a change-in-support response. Clinical tests of balance assess different components of balance ability. Health professionals should select clinical assessments based on a sound knowledge and understanding of the classification of balance and postural control strategies.

Tensile properties of the in vivo human gastrocnemius tendon.

In the present experiment we obtained the tensile properties of the human gastrocnemius tendon, a high-stressed tendon suitable for spring-like action during locomotion. Measurements were taken in vivo in six men. The gastrocnemius tendon elongation during tendon loading-unloading induced by muscle contraction-relaxation was measured using real-time ultrasonography. Tendon forces were calculated from the moment generated during isometric plantarflexion contraction, using tendon moment arm length data obtained in vivo with the tendon travel method. Tendon stiffness data were calculated from the slope of the tendon force-elongation curve, and were then normalized to the tendons original dimensions, obtained from morphometric analysis of sonographs, to estimate the tendon Youngs modulus. Mechanical hysteresis values were obtained from area calculations by numerical integration. The elongation of the tendon increased curvilinearly with the force acting upon it, from 1.7+/-1mm (0.8+/-0.3% strain) at 87.5+/-8.5 N to 11.1+/-3.1mm (4.9+/-1% strain) at 875+/-85 N. The tendon Youngs modulus and mechanical hysteresis were 1.16+/-0.15 GPa and 18+/-3%, respectively. These values fall within the range of values obtained from in vitro experiments and are very similar to the respective values recently obtained from in vivo measurements in the less highly stressed human tibialis anterior tendon (1.2 GPa and 19%), thus indicating that the material properties of tendon are independent of physiological loading and function. Combining the present tendon force-elongation data with previously reported Achilles tendon force data recorded during walking indicates that the gastrocnemius tendon would provide approximately 6% of the total external work produced by the locomotor system. This estimate illustrates the contribution of passive elastic mechanisms on the economy and efficiency of walking. The contributions would be greater in more active exercise such as running.

Ankle joint biomechanics.

A three-dimensional analysis of the human ankle joint is presented to analyse data obtained from gait laboratory tests. The ankle was treated as consisting of two joints, the talocrural (Tc.) and the talocalcaneonavicular (Tcn.), and relevant anatomical dimensions were based upon cadaveric anthropometric data. Seven adult male subjects were studied during the stance phase of normal locomotion. Data was acquired from three orthogonally placed cine cameras and a force platform. Two models were investigated based on force equilibrium; a Mark I model which excluded the posterior tibial and peroneal muscle groups and a Mark II model, which included them. The Mark II model gave the following resultant peak forces expressed as multiples of body weight; Tc. joint force = 3.9; Tcn. joint forces--anterior facet = 2.4, posterior facet = 2.8. The latter model was felt to have good potential in the analytical assessment of ankle pathologies and endoprostheses.

A knee simulating machine for performance evaluation of total knee replacements

A knee simulating machine is required for the design and evaluation of total knee replacements, the kinematics and the long-term wear being aspects of particular importance. There are no generally agreed design criteria, such that existing designs of simulator have a wide variety of input and constraint conditions. In this study, it was postulated that in order to reproduce physiological wear patterns, the correct kinematics is required, on the basis that the wear will be a direct function of the sliding, rolling and tractive rolling conditions at the joint surfaces. In turn, the correct kinematics would only be achieved by the input of physiological forces, by the appropriate constraints on the fixtures holding the components, and by simulating the soft tissue restraints. A knee simulating machine based on these principles was constructed, and used to test the kinematics of a range of contemporary condylar replacement knees. The displacements and rotations varied over a range of almost two times, even with the soft tissue restraints. Without the restraints, the low constraint designs would have dislocated or moved unrealistically. It was concluded that a simulating machine should be based on the input of forces and moments, rather than on displacements and rotations, in order to provide data of kinematics and wear.

A biomechanical analysis of the metacarpo-phalangeal joint

Abstract A three dimensional biomechanical analysis of the metacarpo-phalangeal joint of the index finger is presented. The activities investigated were developing an isometric moment on a fixture representing a water tap and pinching a 45 mm cylinder with index finger and thumb. The external force measurements were made using a six component load transducer in conjunction with displacement measurements using two cine-cameras. Cadaveric studies were made relating the position and orientation of the major load bearing structures. The equilibrium equations were solved to assess the joint loadings taking into account the relevant constraint conditions. Compressive forces up to 190 N were calculated to be acting on the joint surface.

In vivo human tendinous tissue stretch upon maximum muscle force generation

In the present study, we examined the hypothesis that stretch of tendinous tissue in the human tibialis anterior (TA) muscle-tendon unit upon isometric dorsiflexion maximum voluntary contraction (MVC) varies along the entire tendinous component length. Ultrasound-based measurements of the excursions of the TA tendon origin and proximal end of the TA central aponeurosis were taken in the transition from rest to MVC in six men. Subtracting the TA tendon origin excursion from the excursion of the aponeurosis proximal end, the aponeurosis excursion was estimated. Estimation of the aponeurosis proximal region excursion was obtained subtracting the excursion of the insertion point of a central region fascicle on the aponeurosis from the whole aponeurosis excursion. Subtracting tendon excursion from the excursion of the central fascicle insertion point, the aponeurosis distal region excursion was estimated. Strain values were calculated dividing the excursions obtained by the original resting lengths. All excursions and lengths were measured in the mid-longitudinal axis of the TA muscle-tendon unit at the neutral anatomical ankle position. Tendon excursion and strain were 0.5+/-0. 08 cm (mean+/-SE) and 3.1+/-0.2%, respectively. Aponeurosis excursion and strain were 1.1+/-0.15 cm and 6.5+/-0.6%, respectively. Aponeurosis distal region excursion and strain were 0.3+/-0.05 cm and 3.5+/-0.3%, respectively. Aponeurosis proximal region excursion and strain were 0.8+/-0.12 cm and 9.2+/-1%, respectively. Aponeurosis excursion and strain were larger by 110-120% (P<0.05) compared with tendon. Aponeurosis proximal region excursion and strain were larger by 165-170% (P<0.05) compared with aponeurosis distal region. These findings are in line with results from in vitro animal material testing and have important implications for theoretical models of muscle function.

Methodology for long-term wear testing of total knee replacements.

This article begins to address the validation requirements of wear testing on total knee replacements in a knee simulator. The knee simulator has four stations. The axial force is variable but reaches a maximum of 2.3 kN. Physiologic anteroposterior shear force and rotational torques are supplied to the knee. The forces and displacements are timed to coincide with those of a typical gait cycle. Kinematics of the simulator are dependent on the type of knee being tested. Tests of designs with well known clinical histories were done to 10 million cycles. The relative amounts and types of wear shown by the designs were similar to that found in their clinical histories. Wear tracks on more conforming designs were larger, and the penetration into the plastic appeared to be less. This did not necessarily mean that wear, as measured by loss of material, was reduced on conforming designs. Delamination of the plastic was achieved only after aging the tibial components. Wear particles isolated from the lubricating fluid were similar in size and shape to those isolated from in vivo specimens. However, the relative amounts of wear particle shapes were different depending on the design. At the start of the tests, all of the flexibly mounted tibial components showed more motion than after 5 million cycles, indicating that the surface of the plastic became more conforming. This study showed that knee wear similar to wear observed in vivo can be reproduced in the laboratory. The parameters and methods elucidated in this introductory study should form the basis for use in preclinical wear tests of total knee replacements.

Hysteresis measurements in intact human tendon

Mechanical hysteresis in tendons has traditionally been quantified from tensile testing of isolated specimens. Limitations associated with tendon displacement measurement and clamping, and uncertainties as to whether in vitro material represents intact tendon function necessitate measuring hysteresis under in vivo conditions. In the present study such measurements were taken in the human tibialis anterior (TA) tendon. Having the foot fixed on a dynamometer footplate, the displacement of the TA tendon during stimulation and relaxation of the TA muscle was recorded by means of ultrasonography in six men. Combining moment data corresponding to 0, 20, 40, 60, 80 and 100% of maximum voltage moment and the respective tendon-displacement data, a hysteresis loop was obtained between the load-displacement curves during contraction and relaxation. Measurement of the hysteresis loop area yielded a value of 19%. This value agrees with results from in vitro tensile tests of low-stress tendons, suitable for tensile force transmission and joint displacement control. In fact, the human TA tendon has such functional characteristics. The methodology presented allows design of longitudinal and cross-sectional experimental protocols, and in vivo assessment of tendon function and propensity to overheat.

A methodology for studying the effects of various types of prosthetic feet on the biomechanics of trans-femoral amputee gait

This paper reports on a methodology developed for studying the effects of various types of prosthetic feet on the gait of trans-femoral amputees. It is shown that an analysis in three planes of motion of not only the prosthetic, but also the sound limb provides important information on the performance of prosthetic feet. Two male trans-femoral amputees were tested with four different prosthetic feet; the Springlite II, Carbon Copy III, Seattle LightFoot and the Multiflex foot. A detailed analysis of the results of one amputee and a summary of the most important results of a second subject is presented. The tests were carried out at normal (1.16 m s(-1)) and fast (1.56 m s(-1)) walking speeds. Three dimensional gait analysis was carried out to derive the time curves of the joint angles, intersegmental moments and power at the ankle, knee and hip joints at both the prosthetic and sound sides. A higher first peak of the ground reaction force at the sound side with the Seattle LightFoot compared to that with the Springlite II, may be the result of the lower late stance dorsiflexion angle with the former. Compared to the other two feet, the Carbon Copy III and the Springlite II showed higher prosthetic dorsiflexing moments and positive power at late stance, which could assist in the push-off. The 3D intersegmental loads at the ankle and knee can be used as a guide for design and for compilation of standards for testing of lower limb prostheses incorporating flexible feet.