Mechanical effect of rat flexor carpi ulnaris muscle after tendon transfer: does it generate a wrist extension moment?

The mechanical effect of a muscle following agonist-to-antagonist tendon transfers does not always meet the surgeon's expectations. We tested the hypothesis that after flexor carpi ulnaris (FCU) to extensor carpi radialis (ECR) tendon transfer in the rat, the direction (flexion or extension) of the muscle's joint moment is dependent on joint angle. Five weeks after recovery from surgery (tendon transfer group) and in a control group, wrist angle-moment characteristics of selectively activated FCU muscle were assessed for progressive stages of dissection: 1) with minimally disrupted connective tissues, 2) after distal tenotomy, and 3) after maximal tendon and muscle belly dissection, but leaving blood supply and innervations intact. In addition, force transmission from active FCU onto the distal tendon of passive palmaris longus (PL) muscle (a wrist flexor) was assessed. Excitation of control FCU yielded flexion moments at all wrist angles tested. Tenotomy decreased peak FCU moment substantially (by 93%) but not fully. Only after maximal dissection, FCU wrist moment became negligible. The mechanical effect of transferred FCU was bidirectional: extension moments in flexed wrist positions and flexion moments in extended wrist positions. Tenotomy decreased peak extension moment (by 33%) and increased peak flexion moment of transferred FCU (by 41%). Following subsequent maximal FCU dissection, FCU moments decreased to near zero at all wrist angles tested. We confirmed that, after transfer of FCU towards a wrist extensor insertion, force can be transmitted from active FCU to the distal tendon of passive PL. We conclude that mechanical effects of a muscle after tendon transfer to an antagonistic site can be quite different from those predicted based solely on the sign of the new moment arm at the joint.     

Effects of tendon and muscle belly dissection on muscular force transmission following tendon transfer in the rat

The aim of the present study was to quantify to what extent the scar tissue formation following the transfer of flexor carpi ulnaris (FCU) to the distal tendon of extensor carpi radialis (ECR) affects the force transmission from transferred FCU in the rat. Five weeks after recovery from surgery (tendon transfer group) and in a control group, isometric length-force characteristics of FCU were assessed for progressive stages of dissection: (i) with minimally disrupted connective tissues, (ii) after full dissection of FCU distal tendon exclusively, and (iii) after additional partial dissection of FCU muscle belly. Total and passive length-force characteristics of transferred and control FCU changed significantly by progressive stages of dissection. In both groups, tendon dissection decreased passive FCU force exerted at the distal tendon, as well as the slope of the length-force curve. However, force and slope changes were more pronounced for transferred FCU compared to controls. No additional changes occurred after muscle belly dissection. In contrast, total force increased in transferred FCU following both tendon and muscle belly dissection at all lengths studied, while dissection decreased total force of control FCU. In addition, after tendon and muscle belly dissection, we found decreased muscle belly lengths at equal muscle-tendon complex lengths of transferred FCU. We conclude that scar tissue limits the force transmission from transferred FCU muscle via the tendon of insertion to the skeleton, but that some myofascial connectivity of the muscle should be classified as physiological.     

Assessment of Flexor carpi ulnaris function for tendon transfer surgery

Active and passive length-force curves of spastic flexor carpi ulnaris (FCU) muscles were intra-operatively measured in 10 patients with cerebral palsy to study the variability in FCU muscle function. Maximum active FCU force was in general situated near the neutral position of the wrist and varied between 40 and 135 N. Passive forces varied between 1 and 8 N at maximum active force. The potential active excursion varied between 4 and 7 cm, while patients moved their wrists from flexion to extension along different parts of the active length-force curve. We measured a large inter-individual variety of spastic FCU muscle function in this group of patients. Thus, tailoring the surgical technique of tendon transfer to the specific needs of the desired function requires the assessment of muscle-specific data for each individual patient.     

Wrist tendon moment arms: Quantification by imaging and experimental techniques

Subject-specific musculoskeletal models require accurate values of muscle moment arms. The aim of this study was to compare moment arms of wrist tendons obtained from non-invasive magnetic resonance imaging (MRI) to those obtained from an in vitro experimental approach. MRI was performed on ten upper limb cadaveric specimens to obtain the centrelines for the flexor carpi radialis (FCR), flexor carpi ulnaris (FCU), extensor carpi radialis longus (ECRL), extensor carpi radialis brevis (ECRB), extensor carpi ulnaris (ECU), and abductor pollicis longus (APL) tendons. From these, the anatomical moment arms about each of the flexion-extension (FE) and radioulnar deviation (RUD) axes of the wrist were calculated. Specimens were mounted on a physiologic wrist simulator to obtain functional measurements of the moment arms using the tendon excursion method. No differences were observed between anatomical and functional values of the FE and RUD moment arms of FCR, ECRL and ECRB, and the RUD moment arm of ECU (p > .075). Scaling the anatomical moment arms relative to ECRB in FE and ECU in RUD reduced differences in the FE moment arm of FCU and the RUD moment arm of APL to less than 15% (p > .139). However, differences persisted in moment arms of FCU in RUD, and ECU and APL in FE (p < .008). This study shows that while measurements of moment arms of wrist tendons using imaging do not always conform to values obtained using in vitro experimental approaches, a stricter protocol could result in the acquisition of subject-specific moment arms to personalise musculoskeletal models.     

Individual muscle force parameters and fiber operating ranges for elbow flexion-extension and forearm pronation-supination

We have quantified individual muscle force and moment contributions to net joint moments and estimated the operating ranges of the individual muscle fibers over the full range of motion for elbow flexion/extension and forearm pronation/supination. A three dimensional computer graphics model was developed in order to estimate individual muscle contributions in each degree of freedom over the full range of motion generated by 17 muscles crossing the elbow and forearm. Optimal fiber length, tendon slack length, and muscle specific tension values were adjusted within the literature range from cadaver studies such that the net isometric joint moments of the model approximated experimental joint moments within one standard deviation. Analysis of the model revealed that the muscles operate on varying portions of the ascending limb, plateau region, and descending limb of the force-length curve. This model can be used to further understand isometric force and moment contributions of individual muscles to net joint moments of the arm and forearm and can serve as a comprehensive reference for the forces and moments generated by 17 major muscles crossing the elbow and wrist.     

Quantification of the forearm muscles mechanical properties using Myotonometer: Intra- and Inter-Examiner reliability and its relation with hand grip strength

The primary objective of this study was to investigate the reliability of the myotonometer in the mechanical properties of the forearm muscles [m. extensor carpi radialis brevis (ECRB), and m. flexor carpi ulnaris (FCU)] in healthy individuals. The secondary objective was to investigate the relationship between the handgrip strength and mechanical properties of these forearm muscles. The mechanical properties (muscle tone, stiffness, and elasticity) of the ECRB and FCU were measured using the MyotonPRO device. Examiner 1 performed two sets of measurements with a time interval of 30 min to determine intra-examiner reliability. Examiner 2 performed measurements during the interval between the two sets of examiner 1. The intra- and inter-examiner reliabilities were excellent (ICC˃0.82) for muscle tone, stiffness, and elasticity of the FCU. Both intra- and inter-examiner reliability in the evaluation of ECRB muscle tone, elasticity, and stiffness was moderate to excellent (ICCs = 0.56–0.98). The muscle tone and stiffness properties of the FCU were positively correlated with the handgrip strength (p <.05). The study findings indicate that the MyotonPRO device is a reliable tool to quantify ECRB, and FCU muscles mechanical properties in healthy individuals.     

Wrist stabilisation and forearm muscle coactivation during freestyle swimming.

The aim of this study was to evaluate the stabilisation of the wrist joint and the ad hoc wrist muscles activations during the two principal phases of the freestyle stroke. Seven male international swimmers performed a maximal semi-tethered power test. A swimming ergometer fixed on the start area of the pool was used to collect maximal power. The electromyography signal (EMG) of the right flexor carpi ulnaris (FCU) and extensor carpi ulnaris (ECU) was recorded with surface electrodes and processed using the integrated EMG (IEMG). Frontal and sagittal video views were digitised frame by frame to determine the wrist angle in the sagittal plane and the principal phases of the stroke (insweep, outsweep). Important stabilisation of the wrist and high antagonist muscle activity were observed during the insweep phase due to the great mechanical constraints. In outsweep, less stabilisation and lower antagonist activities were noted. Factors affecting coactivations in elementary movements, e.g. intensity and instability of the load, accuracy and economy of the movement were confirmed in complex aquatic movement.     

Muscle moment arm and normalized moment contributions as reference data for musculoskeletal elbow and wrist joint models

A geometric musculoskeletal model of the elbow and wrist joints was developed to calculate muscle moment arms throughout elbow flexion/extension, forearm pronation/supination, wrist flexion/extension and radial/ulnar deviation. Model moment arms were verified with data from cadaver specimen studies and geometric models available in the literature. Coefficients of polynomial equations were calculated for all moment arms as functions of joint angle, with special consideration to coupled muscles as a function of two joint angles. Additionally, a “normalized potential moment (NPM)” contribution index for each muscle across the elbow and wrist joints in four degrees-of-freedom was determined using each muscle's normalized physiological cross-sectional area (PCSA) and peak moment arm (MA). We hypothesize that (a) a geometric model of the elbow and wrist joints can represent the major attributes of MA versus joint angle from many literature sources of cadaver and model data and (b) an index can represent each muscle's normalized moment contribution to each degree-of-freedom at the elbow and wrist. We believe these data serve as a simple, yet comprehensive, reference for how the primary 16 muscles across the elbow and wrist contribute to joint moment and overall joint performance.     

Three-Dimensional Ankle Moments and Nonlinear Summation of Rat Triceps Surae Muscles

The Achilles tendon and epimuscular connective tissues mechanically link the triceps surae muscles. These pathways may cause joint moments exerted by each muscle individually not to sum linearly, both in magnitude and direction. The aims were (i) to assess effects of sagittal plane ankle angle (varied between 150° and 70°) on isometric ankle moments, in both magnitude and direction, exerted by active rat triceps surae muscles, (ii) to assess ankle moment summation between those muscles for a range of ankle angles and (iii) to assess effects of sagittal plane ankle angle and muscle activation on Achilles tendon length. At each ankle angle, soleus (SO) and gastrocnemius (GA) muscles were first excited separately to assess ankle-angle moment characteristics and subsequently both muscles were excited simultaneously to investigate moment summation. The magnitude of ankle moment exerted by SO and GA, the SO direction in the transverse and sagittal planes, and the GA direction in the transverse plane were significantly affected by ankle angle. SO moment direction in the frontal and sagittal planes were significantly different from that of GA. Nonlinear magnitude summation varied between 0.6±2.9% and −3.6±2.9%, while the nonlinear direction summation varied between 0.3±0.4° and −0.4±0.7° in the transverse plane, between 0.5±0.4° and 0.1±0.4° in the frontal plane, and between 3.0±7.9° and 0.3±2.3° in the sagittal plane. Changes in tendon length caused by SO contraction were significantly lower than those during contraction of GA and GA+SO simultaneously. Thus, moments exerted by GA and SO sum nonlinearly both in the magnitude and direction. The limited degree of nonlinear summation may be explained by different mechanisms acting in opposite directions.     

Are skeletal muscles independent actuators? Force transmission from soleus muscle in the cat

It is unclear if skeletal muscles act mechanically as independent actuators. The purpose of the present study was to investigate force transmission from soleus (SO) muscle for physiological lengths as well as relative positions in the intact cat hindlimb. We hypothesized that force transmission from SO fibers will be affected by length changes of its two-joint synergists. Ankle plantar flexor moment on excitation of the SO was measured for various knee angles (70-140 degrees ). This involved substantial length changes of gastrocnemius and plantaris muscles. Ankle angle was kept constant (80 degrees -90 degrees ). However, SO ankle moment was not significantly affected by changes in knee angle; neither were half-relaxation time and the maximal rate of relaxation (P > 0.05). Following tenotomy, SO ankle moment decreased substantially (55 +/- 16%) but did not reach zero, indicating force transmission via connective tissues to the Achilles tendon (i.e., epimuscular myofascial force transmission). During contraction SO muscle shortened to a much greater extent than in the intact case (16.0 +/- 0.6 vs. 1.0 +/- 0.1 mm), which resulted in a major position shift relative to its synergists. If the SO was moved back to its position corresponding to the intact condition, SO ankle moment approached zero, and most muscle force was exerted at the distal SO tendon. Our results also suggested that in vivo the lumped intact tissues linking SO to its synergists are slack or are operating on the toe region of the stress-strain curve. Thus, within the experimental conditions of the present study, the intact cat soleus muscle appears to act mechanically as an independent actuator.     

Neuromechanical control of the forearm muscles during gripping with sudden flexion and extension wrist perturbations

The purpose of this study was to investigate how gripping modulates forearm muscle co-contraction prior to and during sudden wrist perturbations. Ten males performed a sub-maximal gripping task (no grip, 5% and 10% of maximum) while a perturbation forced wrist flexion or extension. Wrist joint angles and activity from 11 muscles were used to determine forearm co-contraction and muscle contributions to wrist joint stiffness. Co-contraction increased in all pairs as grip force increased (from no grip to 10% grip), corresponding to a 36% increase in overall wrist joint stiffness. Inclusion of individual muscle contributions to wrist joint stiffness enhanced the understanding of forearm co-contraction. The extensor carpi radialis longus (ECRL) and brevis had the largest stiffness contributions (34.5 ± 1.3% and 20.5 ± 2.3%, respectively), yet muscle pairs including ECRL produced the lowest co-contraction. The muscles contributing most to wrist stiffness were consistent across conditions (ECRL for extensors; Flexor Digitorum Superficialis for flexors), suggesting enhanced contributions rather than muscular redistribution. This work provides investigation of the neuromuscular response to wrist perturbations and gripping demands by considering both co-contraction and muscle contributions to joint stiffness. Individual muscle stiffness contributions can be used to enhance the understanding of forearm muscle control during complex tasks.     

Effects of postural muscle fatigue on the relation between segmental posture and movement.

The purpose of this study was to examine whether fatigue of postural muscles might influence the coordination between segmental posture and movement. Seven healthy adults performed series of fifteen fast wrist flexions and extensions while being instructed to keep a dominant upper limb posture as constant as possible. These series of voluntary movements were performed before and after a fatiguing submaximal isometric elbow flexion, and also with or without the help of an elbow support. Surface EMG from muscles Delto?deus anterior, Biceps brachii, Triceps brachii, Flexor carpi ulnaris, Extensor carpi radialis were recorded simultaneously with wrist, elbow and shoulder accelerations and wrist and elbow displacements. Fatigue was evidenced by a shift of the elbow and shoulder muscles EMG spectra towards low frequencies. Kinematics of wrist movements and corresponding activations of wrist prime-movers, as well as the background of postural muscle activation before wrist movement were not modified. There were only slight changes in timing of postural muscle activations. These data indicate that postural fatigue induced by a low-level isometric contraction has no effect on voluntary movement and requires no dramatic adaptation in postural control.     

The importance of abductor pollicis longus in wrist motions: A physiological wrist simulator study

The abductor pollicis longus (APL) is one of the primary radial deviators of the wrist, owing to its insertion at the base of the first metacarpal and its large moment arm about the radioulnar deviation axis. Although it plays a vital role in surgical reconstructions of the wrist and hand, it is often neglected while simulating wrist motions in vitro. The aim of this study was to observe the effects of the absence of APL on the distribution of muscle forces during wrist motions. A validated physiological wrist simulator was used to replicate cyclic planar and complex wrist motions in cadaveric specimens by applying tensile loads to six wrist muscles – flexor carpi radialis (FCR), flexor carpi ulnaris, extensor carpi radialis longus (ECRL), extensor carpi radialis brevis, extensor carpi ulnaris (ECU) and APL. Resultant muscle forces for active wrist motions with and without actuating the APL were compared. The absence of APL resulted in higher forces in FCR and ECRL – the synergists of APL – and lower forces in ECU – the antagonist of APL. The altered distribution of wrist muscle forces observed in the absence of active APL control could significantly alter the efficacy of in vitro experiments conducted on wrist simulators, in particular when investigating those surgical reconstructions or rehabilitation of the wrist heavily reliant on the APL, such as treatments for basal thumb osteoarthritis.     

The neuromuscular compartments of the flexor carpi ulnaris

Learning Objectives: After studying this article, the participant should be able to: 1. Report on the vascular supply and innervation pattern of the flexor carpi ulnaris. 2. Describe the muscle architecture of the flexor carpi ulnaris, including the physiological cross-sectional area and fiber length. 3. State the uses of the flexor carpi ulnaris both for resurfacing defects in the vicinity of the elbow and in local functional tendon transfers. 4. Understand the principles of splitting skeletal muscles based on neurovascular supply to enhance its utilization in reconstructive procedures. The aim of this study was to describe the intramuscular innervation and vascular supply of the human flexor carpi ulnaris, with confirmation of findings by a similar study in the primate. Two distinct intramuscular nerve branches running parallel to each other, on either side of a central tendon, from the proximal quarter of the muscle belly to its insertion were found. The muscle could then be split into a humeral and an ulnar compartment, each with its own primary nerve branch. Perfusion studies confirmed the adequacy of circulation to the two compartments. In the primate flexor carpi ulnaris, electrical stimulation of the respective branches revealed independent contraction of each compartment. This study provides useful information for enabling the local transfer of the muscle as a whole, both for resurfacing in the vicinity of the elbow and for functional tendon transfers. It will also enable the transfer of the muscle as one or two separate compartments (for resurfacing, in tendon transfers for muscle paralysis, congenital defects, and muscle defects resulting from trauma, and after resections for neoplasm and infection).     

Muscle power attenuation by tendon during energy dissipation

An important function of skeletal muscle is deceleration via active muscle fascicle lengthening, which dissipates movement energy. The mechanical interplay between muscle contraction and tendon elasticity is critical when muscles produce energy. However, the role of tendon elasticity during muscular energy dissipation remains unknown. We tested the hypothesis that tendon elasticity functions as a mechanical buffer, preventing high (and probably damaging) velocities and powers during active muscle fascicle lengthening. We directly measured lateral gastrocnemius muscle force and length in wild turkeys during controlled landings requiring rapid energy dissipation. Muscle-tendon unit (MTU) strain was measured via video kinematics, independent of muscle fascicle strain (measured via sonomicrometry). We found that rapid MTU lengthening immediately following impact involved little or no muscle fascicle lengthening. Therefore, joint flexion had to be accommodated by tendon stretch. After the early contact period, muscle fascicles lengthened and absorbed energy. This late lengthening occurred after most of the joint flexion, and was thus mainly driven by tendon recoil. Temporary tendon energy storage led to a significant reduction in muscle fascicle lengthening velocity and the rate of energy absorption. We conclude that tendons function as power attenuators that probably protect muscles against damage from rapid and forceful lengthening during energy dissipation.     

Mechanisms of improved knee flexion after rectus femoris transfer surgery

Rectus femoris transfer is frequently performed to treat stiff-knee gait in subjects with cerebral palsy. In this surgery, the distal tendon is released from the patella and re-attached to one of several sites, such as the sartorius or the iliotibial band. Surgical outcomes vary, and the mechanisms by which the surgery improves knee motion are unclear. The purpose of this study was to clarify the mechanism by which the transferred muscle improves knee flexion by examining three types of transfers. Muscle-actuated dynamic simulations were created of ten children diagnosed with cerebral palsy and stiff-knee gait. These simulations were altered to represent surgical transfers of the rectus femoris to the sartorius and the iliotibial band. Rectus femoris transfers in which the muscle remained attached to the underlying vasti through scar tissue were also simulated by reducing but not eliminating the muscle's knee extension moment. Simulated transfer to the sartorius, which converted the rectus femoris’ knee extension moment to a flexion moment, produced 32±8° improvement in peak knee flexion on average. Simulated transfer to the iliotibial band, which completely eliminated the muscle's knee extension moment, predicted only slightly less improvement in peak knee flexion (28±8°). Scarred transfer simulations, which reduced the muscle's knee extension moment, predicted significantly less (p<0.001) improvement in peak knee flexion (14±5°). Simulations revealed that improved knee flexion following rectus femoris transfer is achieved primarily by reduction of the muscle's knee extension moment. Reduction of scarring of the rectus femoris to underlying muscles has the potential to enhance knee flexion.     

Modeling of the muscle/tendon excursions and moment arms in the thumb using the commercial software anybody

A biomechanical model of a thumb would be useful for exploring the mechanical loadings in the musculoskeletal system, which cannot be measured in vivo. The purpose of the current study is to develop a practical kinematic thumb model using the commercial software Anybody (Anybody Technology, Aalborg, Denmark), which includes real CT-scans of the bony sections and realistic tendon/muscle attachments on the bones. The thumb model consists of a trapezium, a metacarpal bone, a proximal and a distal phalanx. These four bony sections are linked via three joints, i.e., IP (interphalangeal), MP (metacarpophalangeal) and CMC (carpometacarpal) joints. Nine muscles were included in the proposed model. The theoretically calculated moment arms of the tendons are compared with the corresponding experimental data by Smutz et al. [1998. Mechanical advantage of the thumb muscles. J. Biomech. 31(6), 565–570]. The predicted muscle moment arms of the majority of the muscle/tendon units agree well with the experimental data in the entire range of motion. Close to the end of the motion range, the predicted moment arms of several muscles (i.e., ADPt and ADPo (transverse and oblique heads of the adductor pollicis, respectively) muscles for CMC abduction/adduction and ADPt and FPB (flexor pollicis brevis) muscle for MP extension/flexion) deviate from the experimental data. The predicted moment potentials for all muscles are consistent with the experimental data. The findings thus suggest that, in a biomechanical model of the thumb, the mechanical functions of muscle–tendon units with small physiological cross-sectional areas (PCSAs) can be well represented using single strings, while those with large PCSAs (flat-wide attachments, e.g., ADPt and ADPo) can be represented by the averaged excursions of two strings. Our results show that the tendons with large PCSAs can be well represented biomechanically using the proposed approach in the major range of motion.     

Wrist and digital joint motion produce unique flexor tendon force and excursion in the canine forelimb

The force and excursion within the canine digital flexor tendons were measured during passive joint manipulations that simulate those used during rehabilitation after flexor tendon repair and during active muscle contraction, simulating the active rehabilitation protocol. Tendon force was measured using a small buckle placed upon the tendon while excursion was measured using a suture marker and video analysis method. Passive finger motion imposed with the wrist flexed resulted in dramatically lower tendon force (approximately 5 N) compared to passive motion imposed with the wrist extended (approximately 17 N). Lower excursions were seen at the level of the proximal interphalangeal joint with the wrist flexed (approximately 1.5 mm) while high excursion was observed when the wrist was extended or when synergistic finger and wrist motion were imposed (approximately 3.5 mm). Bivariate discriminant analysis of both force and excursion data revealed a natural clustering of the data into three general mechanical paradigms. With the wrist extended and with either one finger or four fingers manipulated, tendons experienced high loads of approximately 1500 g and high excursions of approximately 3.5 mm. In contrast, the same manipulations performed with the wrist flexed resulted in low tendon forces (4-8 N) and low tendon excursions of approximately 1.5 mm. Synergistic wrist and finger manipulation provided the third paradigm where tendon force was relatively low (approximately 4 N) but excursion was as high as those seen in the groups which were manipulated with the wrist extended. Active muscle contraction produced a modest tendon excursion (approximately 1 mm) and high or low tendon force with the wrist extended or flexed, respectively. These data provide the basis for experimentally testable hypotheses with regard to the factors that most significantly affect functional recovery after digital flexor tendon injury and define the normal mechanical operating characteristics of these tendons.     

Joit torque and energy patterns in normal gait

Experiments were conducted on normal level gait to determine the synergistic patterns present in the forces causing joint moments and those associated with the generation, absorption and transfer of mechanical energy. The following generalizations can be made about the patterns: (i) During swing phase three forces (gravitational, muscle and knee joint acceleration) are responsible for shank rotation, and are shown to act together during both acceleration and deceleration.—(ii) The patterns of generation, absorption and transfer of mechanical energy at the joints are detailed. These patterns demonstrate inter-segment transfers of energy through the joint centres, and through the muscles, as well as the more recognized generation and absorption by the muscles themselves.—As a result of the complexity shown in these patterns it is cautioned that fundamental relationships that may have been derived from controlled biomechanical experiments (such as horizontal flexion and extension of the forearm) are not likely to apply to more normal movements such as gait.     

Biarticular hip extensor and knee flexor muscle moment arms of the feline hindlimb

Moment arms are important for understanding muscular behavior and for calculating internal muscle forces in musculoskeletal simulations. Biarticular muscles cross two joints and have moment arms that depend on the angle of both joints the muscles cross. The tendon excursion method was used to measure the joint angle-dependence of hamstring (biceps femoris, semimembranosus and semitendinosus) moment arm magnitudes of the feline hindlimb at the knee and hip joints. Knee angle influenced hamstring moment arm magnitudes at the hip joint; compared to a flexed knee joint, the moment arm for semimembranosus posterior at the hip was at most 7.4 mm (25%) larger when the knee was extended. On average, hamstring moment arms at the hip increased by 4.9 mm when the knee was more extended. In contrast, moment arm magnitudes at the knee varied by less than 2.8 mm (mean=1.6 mm) for all hamstring muscles at the two hip joint angles tested. Thus, hamstring moment arms at the hip were dependent on knee position, while hamstring moment arms at the knee were not as strongly associated with relative hip position. Additionally, the feline hamstring muscle group had a larger mechanical advantage at the hip than at the knee joint.