Resultant Joint Force Research Articles

Lower Extremity Biomechanics Differ in Prepubescent and Postpubescent Female Athletes during Stride Jump Landings

Epidemiological evidence suggests the incidence of injury in female athletes is greater after the onset of puberty and that landing from a jump is a common mechanism of knee injury. This investigation compared lower extremity joint kinematics and joint resultant forces and moments during three types of stride jump (stride jump followed by a static landing; a ballistic vertical jump; and a ballistic lateral jump) between pre- and postpubescent recreational athletes to provide some insight into the increased incidence of injury. Sixteen recreationally active postpubescent women (ages 18–25 years) and 16 recreationally active prepubescent girls (ages 8–11 years) participated in this study. High speed 3D videography and force plate data were used to record each jumper’s performance of the stride jumps, and an inverse dynamic procedure was used to estimate lower extremity joint resultant forces and moments and power. These dependent variables were submitted to a 2 × 3 (Maturation Level × Landing Sequence) MANOVA with repeated measures on the last factor. The findings indicated that postpubescents landed with the knee more extended (4.4°) and had greater extension moments (approximately 30% greater hip and knee extension moments) and powers (40% greater knee power). Further, the post-pubescent athletes had greater knee anterior/posterior forces as well as medio-lateral resultant forces. The differences found between the two groups suggest there may be anatomical and physiological changes with puberty that lead to differences in strength or neuromuscular control which influence the dynamic restraint system in these recreational athletes. A combination of these factors likely plays a role in the increased risk of injury in postpubescent females.

Proximal Tibia Anterior Shear Force in Stop-Jump Tasks

Introduction: Anterior cruciate ligament (ACL) injuries are common in sports. The majority of ACL injuries involve no or little direct physical contact between athletes when injuries occur. Female athletes injure their ACLs at four to eight times the rate of male athletes depending on the sport and level of training. Recent studies found that ACL injuries frequently occur in stop-jump related tasks performed in sports such as soccer, basketball, and volleyball. Athletes in these sports frequently perform stop-jump tasks and have an increased incidence of lower extremity injury, including ACL injuries in comparison to other sports. The purposes of this study were (1) to compare selected knee kinetics and kinematics between male and female recreational athletes in three stop-jump tasks: forward stop-jump task, vertical stop-jump task, and backward stop-jump task; and (2) to determine the relationship between selected knee kinetics and kinematics that may contribute to the risk for non-contact ACL injuries. Methods: Seventeen male and Seventeen female healthy recreational athletes volunteered to be subjects for this study. Two Bertec 4060A force plates (Bertec Corporation, Worthington, OH) were used to collect ground reaction forces and moments at a sampling rate of 540 Hz. Four video cameras were used to record each subject's performances during the experiment at a frame rate of 180 frames/second with setup for the Direct Linear Transformation procedure. Each subject performed five successful trials for each task. Three-dimensional coordinates of reflective markers on body landmarks and ground reaction force data were collected in each trial. Lower extremity joint centers were estimated from the coordinates of reflective markers. Joint resultant forces and moments at the knee on the tibia were estimated using an inverse dynamic model. The estimated knee joint resultant force and moment vectors were translated to the tibia reference frames. The peak proximal tibia anterior shear force during landing of each task was identified. The knee flexion-extension moment, valgus-varus moment, and knee flexion angle at the peak proximal tibia anterior shear force were also determined. Two-way analyses of variance with mixed design were conducted to compare the selected knee kinetics and kinematics between genders and tasks. A multiple regression analysis was conducted to determine the relationship between peak proximal tibia anterior shear force and other selected knee kinetics and kinematics. Results: Female subjects had significantly greater peak proximal tibia anterior shear force, knee extension moment, knee valgus moment, and smaller knee flexion angle than male subjects (p = 0.00). Task did not significantly affect the knee flexion-extension and valgus-varus moments at the peak proximal tibia anterior shear force. Task also significantly affects the knee flexion angle at the peak proximal tibia anterior shear force (p = 0.00). Peak proximal tibia anterior shear force was significantly correlated to the knee flexion-extension moment and knee valgus-varus moment with an overall regression determinant of r2 = 0.52 (p = 0.00). Discussion: The results of this study provide significant information related to the risk factors and possible prevention of non-contact ACL injuries. Proximal tibia anterior shear force has been identified as a major contributor to the anterior tibia translation, a motion resisted by the ACL. An increased proximal tibia anterior shear force is likely to increase the strain on the ACL and increase the risk of ACL injuries. The altered lower extremity neuromuscular motor controls in risky tasks are likely to be an important contributor to the increased risk for non-contact ACL injuries in women. An increased knee extension moment can be considered as an indication of an increased quadriceps force or decreased hamstring force. Knee varus-valgus moment can also be considered as the rotation effect generated by the muscle contractions across the knee joint. The significant correlation of these two knee kinetic measures to the peak proximal tibia anterior shear force indicates that the increased peak proximal tibia anterior shear force in female subjects is likely due to altered lower extremity motor controls. Women's altered lower extremity neuromuscular motor controls may bring them close to those awkward body positions or uncoordinated motions and increase their risk for non-contact ACL injuries. Technical training to correct altered lower extremity neuromuscular motor controls may assists in preventing non-contact ACL injuries by keeping athletes away from awkward body positions and uncoordinated motions.

Kinematics and Kinetics in Pole Vaulting

Kinematics and Kinetics in Pole Vaulting

Letter to the Editor

Dear Sir: In the article by Drs. Gagey and Hue, 1 a two-dimensional mechanical model of the upper end of the humerus was described, with three active portions of the deltoid muscle and a fixed axis of rotation. The weight of the arm is included. There are fundamental errors in their model: (1) The force denoted as R in Figure 3A–B is not the force the humeral head exerts on the glenoid; (2) The free body chosen for the model is wrong because force F of the deltoid acts on the humerus three times. If the humerus is cut free below the insertion of the deltoid, two of the forces F on the left side of the model cancel out; and (3) Equilibrium conditions were established for vertical components of the forces, those for horizontal components and moments are missing. To make up for the lack of equations, Gagey and Hue used the assumption that the deltoid force is three to four times the weight of the arm. To go beyond mere criticism we have used a free body diagram of the upper end of the humerus. This model also assumes that the glenohumeral joint can support the vertical components of the resultant force, fixed axis. A simple computer program was written to test the effect of various parameters on the forces of the deltoid and the resultant joint force (Available at: http://www.biomechanik.de). From our calculations it appears that, in the position of initial abduction that Gagey and Hue considered, the upward directed component of the deltoid force is too small to compensate the downward pull of the weight of the arm. Both models have the same critical limitation: they neglect the structures that prevent downward movement of the hanging arm. The real glenoid cannot do that. We are aware that none of the simple mechanical models lead to realistic results. A model with realistic assumptions would not permit determinate solutions. Imaging studies of the position of the humeral head relative to the glenoid in abduction show that the head moves first up and then down, in abduction. In Leclerc’s test (radiographed in resisted abduction) the humeral head moves up when the cuff is deficient. Why have Gagey and Hue not discussed these observations? Some shoulders with deficient cuffs do function satisfactorily, if not normally. Gagey and Hue have not explained conclusively why. R. Kölbel, MD G. Bergmann, MD

The role of impact forces and foot pronation: a new paradigm.

This article discusses the possible association between impact forces and foot pronation and the development of running-related injuries, and proposes a new paradigm for impact forces and foot pronation. The article is based on a critical analysis of the literature on heel-toe running addressing kinematics, kinetics, resultant joint movements and forces, muscle activity, subject and material characteristics, epidemiology, and biologic reactions. However, this paper is not a review of the literature but rather an attempt to replace the established concepts of impact forces and movement control with a new paradigm that would allow explaining some of the current contradictions in this topic of research. The analysis included all papers published on this topic over the last 25 years. For the last few years, it concentrated on papers expressing critical concerns on the established concepts of impact and movement control. An attempt was made to find indications in the various publications to support or reject the current concept of impact forces and movement control. Furthermore, the results of the available studies were searched for indications expanding the current understanding of impact forces and movement control in running. Data were synthesized revealing contradictions in the experimental results and the established concepts. Based on the contradictions in the existing research publications, a new paradigm was proposed. Theoretical, experimental, and epidemiological evidence on impact forces showed that one cannot conclude that impact forces are important factors in the development of chronic and/or acute running-related injuries. A new paradigm for impact forces during running proposes that impact forces are input signals that produce muscle tuning shortly before the next contact with the ground to minimize soft tissue vibration and/or reduce joint and tendon loading. Muscle tuning might affect fatigue, comfort, work, and performance. Experimental evidence suggests that the concept of "aligning the skeleton" with shoes, inserts, and orthotics should be reconsidered. They produce only small, not systematic. and subject-specific changes of foot and leg movement. A new paradigm for movement control for the lower extremities proposes that forces acting on the foot during the stance phase act as an input signal producing a muscle reaction. The cost function used in this adaptation process is to maintain a preferred joint movement path for a given movement task. If an intervention counteracts the preferred movement path, muscle activity must be increased. An optimal shoe, insert, or orthotic reduces muscle activity. Thus, shoes, inserts, and orthotics affect general muscle activity and, therefore, fatigue, comfort, work, and performance. The two proposed paradigms suggest that the locomotor system use a similar strategy for "impact" and "movement control." In both cases the locomotor system keeps the general kinematic and kinetic situations similar for a given task. The proposed muscle tuning reaction to impact loading affects the muscle activation before ground contact. The proposed muscle adaptation to provide a constant joint movement pattern affects the muscle activation during ground contact. However, further experimental and theoretical studies are needed to support or reject the proposed paradigms.

RELATIONSHIPS BETWEEN SHOULDER JOINT KINEMATICS AND KINETICS IN PROFESSIONAL BASEBALL PITCHING

10 The upper extremity is vulnerable to injury during throwing due to the repetitious nature of the motion, the extremes in range of motion, and the high angular velocities and torques generated at the shoulder and elbow. The purpose of this study was to investigate the relationships between kinetics and kinematics of the throwing shoulder, and to identify the specific aspects of the throwing motion which place the overhead athlete most at risk. Video data were collected on 40 Major League Baseball pitchers during Spring Training in the 1998 Cactus League. High-speed (120 Hz) cameras were necessary to sufficiently capture the pitching motion for scientific analysis. Twenty body landmarks and the ball were digitized, frame by frame, throughout the pitching motion from two camera views for each athlete. Data from the two cameras were then synchronized in time and mathematically combined to produce a three-dimensional representation of the pitch for one fast ball for each pitcher. Over 50 kinematic variables were computed, and the resultant joint forces and torques at the throwing arm elbow and shoulder were calculated. Average ball speed was 85 mph for the 40 pitchers. Maximum external rotation at the shoulder joint averaged 184 ± 13 deg. Internal rotation velocities in excess of 8500 deg/sec were documented. A significant compressive force (952 ± 162 N), acting to resist distraction, occurred at the shoulder joint near the instant of ball release. A regression equation relating the kinematic parameters of pitching mechanics to shoulder distraction was obtained. Assessment of major league pitching mechanics furnish information regarding a highly dynamic and potentially harmful throwing motion. Parameters of pitching mechanics were identified to reduce joint stress.

Upper extremity augmentation of lower extremity kinetics during countermovement vertical jumps

Twenty-five volleyball players (14 males, 11 females) were videotaped (60Hz) performing countermovement vertical jumps with and without an arm swing. Ground reaction force and video-based coordinate data were collected simultaneously. The resultant joint force and torque at the hip, knee, ankle and shoulder for two trials per subject per condition were computed and normalized. Average kinematic, resultant joint force and torque data were compared using repeated-measures analysis of variance. Larger values were recorded for the vertical velocity of the centre of mass at take-off in the jumps with (mean 2.75, s=0.3m.s-1) versus without (mean 2.44, s= 0.23m.s-1) an arm swing. The jumps with no arm swing produced larger torques at the hip during the first third of the propulsive phase (from zero to maximum vertical velocity of the centre of mass). During the final two-thirds of the propulsive phase, the arm swing augmented hip extensor torques by slowing the rate of trunk extension and placing the hip extensor muscles in slower concentric conditions that favoured the generation of larger forces and resultant joint torques. During the first two-thirds ofthe propulsive phase, knee extensor torque increased by 28% in the jumps with an arm swing, but maintained a relatively constant magnitude in the jumps with no arm swing.

Repeated measures of adult normal walking using a video tracking system

Abstract The reproducibility of quantitative gait analysis measurements is an important consideration when analyzing data of both normal subjects and patients. Waveform similarity statistics were used to assess the reproducibility of lower extremity kinematic and kinetic data collected on 5 normal adult subjects. For each subject, gait evaluations were done on 3 separate test days with 3 trials collected each day. Pelvis angles were fairly repeatable, however, with notably poor reproducibility in sagittal plane tilt. Re-application errors of the sacral wand coupled with a small range of motion are believed to be the principle contributors to variability in this pelvis angle. The sagittal plane angles for the hip, knee, and ankle demonstrated excellent repeatability within test days and between test days. Frontal and transverse plane angles were fairly repeatable within test days but between-day repeatability was considerably lower. The `downstream' errors accompanying Euler angle calculations coupled with the smaller ranges of motion in the non-sagittal plane angles are suggested to contribute to the ambient variability already in these data and, thus, affect the within-day repeatability. Errors in re-application of midthigh and midcalf wands further contribute to the variability of these data between test days. Net resultant joint forces and moments were repeatable overall with slightly lower between-day statistical values. The exception to this observation was the adduction moment of the ankle which was notably variable between test days. Several explanations for this variability are presented. Results of this study have led to modifications in some marker placement procedures and reinforced the need for others already being used.

Pelvic muscle and acetabular contact forces during gait

Locations, magnitudes, and directions of pelvic muscle and acetabular contact forces are important to model the effects of abnormal conditions (e.g., deformity, surgery) of the hip accurately. Such data have not been reported previously. We computed the three-dimensional locations of all pelvic muscle and acetabular contact forces during level gait. The approach first required computation of the intersegmental joint resultant forces and moments using limb displacement history, foot-floor forces, and estimated limb inertial properties from one subject. The intersegmental resultant moments were then distributed to the muscles using a 47-element muscle model and a non-linear optimization scheme. Muscle forces were vectorally subtracted from the intersegmental resultants to compute the acetabular contact forces. While the peak joint force magnitudes are similar to those reported previously for the femur, the directions of pelvic contact forces and muscle forces varied considerably over the gait cycle. These variations in contact force directions and three-dimensional forces could be as important as the contact force magnitudes in performing experimental or theoretical studies of loads and stresses in the periacetabular region.

Three-Dimensional Kinetics of the Shoulder, Elbow, and Wrist during a Penalty Throw in Water Polo

The purpose of the study was to examine the resultant joint forces (RJFs) and torques (RJTs) at the shoulder, elbow, and wrist during penalty throws and determine the relationships between muscle actions and motions of the throwing arm. Subjects with an overhand (OH) throwing technique created larger maximal and average RJTs at the shoulder and elbow compared to subjects with a sweep (SW) technique (Feltner & Nelson, 1996). Prior to release, OH technique subjects decreased their abduction torque and created adduction torques at the shoulder. Adduction torques and downward vertical motion of the trunk, together with an internal rotation torque at the shoulder, resulted in large internal rotation angular velocities at release for the OH technique subjects. The SW technique subjects did not exhibit these technique characteristics. Additionally, throwing technique exhibited a moderate but positive relationship with several chest, upper arm, and forearm circumference measures. Findings suggest that muscular strength may be a causal determinant of technique style.

A new testing rig for force platform calibration and accuracy tests

Accurate estimation of resultant joint forces and moments from gait analysis data depends heavily on the accuracy of measurement of the ground reaction force (GRF). Typically, multicomponent force platforms are used to measure the components of the GRF and the position of the centre of pressure. A new rig, designed to allow safe and quick static testing of the vertical component and centre of pressure outputs of such force platforms, is described. This rig has been used to test the accuracy of AMTI force platforms and was found to reduce dramatically the testing time. The results of the tests are also given.

Are joint torques the Holy Grail of human gait analysis?

Despite complex control systems, human gait is characterised by smooth, regular and repeating movements. Such coordinated motion occurs as a direct result of the cyclical activation of many leg muscles. It is the tension in these muscles that serves as the direct cause of the kinematic patterns that we observe and recognise as locomotion. The challenge facing biomechanists, however, is that there are many more muscle activators than independent equations defining the system. This leads to the classic indeterminate problem. Engineers have circumvented this problem by means of a mechanical ruse: all muscle, bone and ligament forces are reduced to a single (vector) resultant joint force and torque. Using an inverse dynamics approach, the joint torques may be estimated and it has been proposed that they are the causative factors that help biomechanists understand how gait is achieved. This is the approach that we followed at the University of Virginia over the past 5 years. In this paper, we will review the data from our own studies, exploring both the advantages and disadvantages of the joint torque approach. Our target populations have included: normal children; cerebral palsy children with orthoses; ankle inversion-eversion in level gait; knee abduction-adduction in stair climbing; and athletes with torn anterior cruciate ligaments. While this quest should be an interesting journey, offering a few fleeting glimpses of biomechanical insight, our final destination does not yield the Holy Grail.

A two-part, viscoelastic foot model for use in gait simulations

A three-dimensional, two-part model of the foot, for use in a simulation of human gait, is presented. Previous simulations of gait have not included the foot segment (e.g. Siegler et al., 1982, J. Biomechanics 15, 415–425) or have fastened it to the ground (e.g. Onyshko and Winter, 1980, J. Biomechanics 13, 361–368). A foot model based on viscoelastic elements (e.g. Meglan, 1991, Ph. D. thesis, Ohio State Univ.), allows more freedom of movement and thus models the physical system more closely. The current model was developed by running simulations of the foot in isolation from just before heel contact to just after toe-off. The driving inputs to the simulation were the resultant ankle joint forces and moments taken from a gait analysis. Nine linear, vertically oriented spring/damper systems, positioned along the midline of the foot were used to model the combined viscoelastic behaviour of the foot, shoe and floor. Associated with each vertical spring/damper system were two orthogonally placed, linear, horizontal dampers used to provide the shear components of the ground reaction force. Torques at the metatarsal-phalangeal joint were supplied by a linear, torsional spring and damper. Control about the vertical axis and the long axis of the foot was achieved by the use of linear, torsional dampers. The predicted kinetic and kinematic values are very similar to those taken from the gait analysis. The model represents an improvement over previous work because the transition from swing to stance was smooth and continuous without the foot being constrained to any specific trajectory.

Assessing the accuracy of 3-D kinematic data: Implications for laboratory setup

The reproducibility of quantitative gait analysis measurements is an important consideration when analyzing data of both normal subjects and patients. Waveform similarity statistics were used to assess the reproducibility of lower extremity kinematic and kinetic data collected on 5 normal adult subjects. For each subject, gait evaluations were done on 3 separate test days with 3 trials collected each day. Pelvis angles were fairly repeatable, however, with notably poor reproducibility in sagittal plane tilt. Re-application errors of the sacral wand coupled with a small range of motion are believed to be the principle contributors to variability in this pelvis angle. The sagittal plane angles for the hip, knee, and ankle demonstrated excellent repeatability within test days and between test days. Frontal and transverse plane angles were fairly repeatable within test days but between-day repeatability was considerably lower. The `downstream' errors accompanying Euler angle calculations coupled with the smaller ranges of motion in the non-sagittal plane angles are suggested to contribute to the ambient variability already in these data and, thus, affect the within-day repeatability. Errors in re-application of midthigh and midcalf wands further contribute to the variability of these data between test days. Net resultant joint forces and moments were repeatable overall with slightly lower between-day statistical values. The exception to this observation was the adduction moment of the ankle which was notably variable between test days. Several explanations for this variability are presented. Results of this study have led to modifications in some marker placement procedures and reinforced the need for others already being used.

Biomechanical reasons for the divergent morphology of the knee joint and the distal epiphyseal suture in hominoids.

The obliquity of the femoral diaphysis accounts for the valgus position of the human knee joint and reduces bending moments in the frontal plane. A high angle of obliquity is considered a hallmark of hominid bipedality, but its functional importance has rarely been identified correctly. A biostatic investigation of the knee joint in various realistic positions unveils resultant joint forces which do not deviate greatly from the long axis of the femoral shaft. This is due to the length of the femur and to the shortness of the human foot. The flat epiphyseal suture is more or less perpendicular to these joint forces, and the equal size of the femoral condyles reflects the even distribution of forces between them. In great apes the resultant forces acting in the knee joint vary considerably in dependence on the degree of flexion and rotation of the knee joint. The resultant joint force may be line with the femur shaft or diverge. The epiphyseal surfaces offer facets to all joint forces found in the course of the study. Due to the pronounced varus position of the knee joint, the joint itself and the adjacent part of the femur are under medially concave bending moments, which lead to higher compressive forces at the medial than at the lateral condyle. The enlarged medial condyle allows the distribution of medially displaced joint forces over a relatively large area, and the elliptic cross-section yields high bending resistance in the frontal plane. A human-like angle of obliquity is present in the early australopithecines, the values being mostly within the range of variation of children. The valgus position of the australopithecine knee joint is considered to be a functional, and epigenetic consequence of habitual bipedality. It is particularly pronounced because of the short length of the femur and the great bitrochanteric width.

A Video Technique for Obtaining 3-D Coordinates in Alpine Skiing

A video technique to obtain 3-D data in an Alpine skiing competition was investigated. The flight and landing phases of a jump were recorded during the 1994 Olympic combined downhill race. A direct linear transformation (DLT) implementation was applied, which computes the DLT parameters for each video image of each camera separately. As a consequence, one is able to pan and tilt the cameras and zoom the lenses. The problem of distributing control points in the large object space could be solved satisfactorily. The method proved to be suitable for obtaining 3-D data with reasonable accuracy, which is even sufficient for inverse dynamics. The computed resultant knee joint forces and moments compare well with results reported by other authors.

Finger joint force minimization in pianists using optimization techniques.

A numerical optimization procedure was used to determine finger positions that minimize and maximize finger tendon and joint force objective functions during piano play. A biomechanical finger model for sagittal plane motion, based on finger anatomy, was used to investigate finger tendon tensions and joint reaction forces for finger positions used in playing the piano. For commonly used piano key strike positions, flexor and intrinsic muscle tendon tensions ranged from 0.7 to 3.2 times the fingertip key strike force, while resultant inter-joint compressive forces ranged from 2 to 7 times the magnitude of the fingertip force. In general, use of a curved finger position, with a large metacarpophalangeal joint flexion angle and a small proximal interphalangeal joint flexion angle, reduces flexor tendon tension and resultant finger joint force.

Anterior Cruciate Ligament Forces in Alpine Skiing.

The purpose of this study was to estimate cruciate ligament forces in Alpine skiing during a movement that has been associated with anterior cruciate ligament (ACL) tears. Resultant knee joint forces and moments were obtained from two skiers during a World Cup Downhill race using an inverse dynamics approach and a 2-D bilaterally symmetric system model. It was found that ACL forces were typically small for both skiers throughout the movement analyzed because quadriceps forces prevented anterior displacement of the tibia relative to the femur at the knee joint angles observed. However, for about 10 ms, loading conditions in the knee joint of Subject 2 (who displayed poor form) were such that large ACL forces may have been present. These particular loading conditions were never observed in Subject 1, who displayed good form. Since neither of the skiers was injured, it is not possible to draw firm conclusions about isolated ACL tears in Alpine skiing from the data at hand.

External Loading at the Knee Joint for Landing Movements in Alpine Skiing

The purpose of this study was to determine resultant knee joint forces and moments during a specific movement in Alpine ski racers. The movement analyzed consisted of a landing from a bump and the initiation of recovery (if necessary). Resultant loads were obtained using an inverse dynamics approach. Results of two specific skiers are contrasted, one skier landing in good form, the second skier landing in poor form. The skier landing in poor form exhibited larger knee flexion, and larger knee joint resultant forces and moments than the skier landing in good form. The movement of the skier landing in poor form has been associated with isolated anterior cruciate ligament (ACL) injury. However, the data obtained in this study do not indicate that either skier was in danger of ACL injury.

In vitro study of initial stability of a conical collared femoral component

This in vitro experimental study compared the initial stability of an uncemented conical collared femoral component to that of the same component with the collar removed. The two configurations examined simulated joint resultant forces encountered in single leg stance and stair climbing. For the simulated single leg stance loads, the data do not allow any inferences about relative component stability. With the exception of one collarless control, all micromotion for single leg stance loading was under 150 μm, measured approximately 1.5 cm below the resection line. For scaled stair-climbing loads, however, the conical collared component group was significantly more stable than the collarless control group in transverse (primarily rotational) micromotion. The overall average measured transverse motion for the collarless control group was more than 3.7 times greater than that of the conical collar group at scaled stair-climbing loads. The two conical collared components loaded to full peak stair-climbing load (2,100 N) exhibited micromotion under 160 μm. The results suggest that the conical collar may improve stability of an uncemented prosthesis under loads that include an out-of-plane (rotational) component.