Changes In Moment Arms Research Articles

Effect of perturbation timing on recovering whole-body angular momentum during very slow walking

Humans prioritize regulation of the whole-body angular momentum (WBAM) during walking. When perturbed, modulations of the moment arm of the ground reaction force (GRF) with respect to the centre of mass (CoM) assist in recovering WBAM. For sagittal-plane perturbations of the WBAM given at toe off right (TOR), horizontal GRF modulations and not centre of pressure (COP) modulations were mainly responsible for these moment arm modulations. In this study, we aimed to find whether the instant of perturbations affects the contributions of the GRF and/or CoP modulations to the moment arm changes, in balance recovery during very slow walking. Perturbations of the WBAM were applied at three different instants of the gait cycle, namely at TOR, mid-swing (MS), and heel strike right (HSR). Forces equal to 16% of the participant’s body weight were applied simultaneously to the pelvis and upper body in opposite directions for a duration of 150 ms. The results showed that the perturbation onset did not significantly affect the GRF moment arm modulation. However, the contribution of both the CoP and GRF modulation to the moment arm changes did change depending on the perturbation instant. After perturbations resulting in a forward pitch of the trunk a larger contribution was present from the CoP modulation when perturbations were given at MS or HSR, compared to perturbations at TOR. After backward pitch perturbations given at MS and HSR the CoP modulation counteracted the moment arm required for WBAM recovery. Therefore a larger contribution from the horizontal GRF was needed to direct the GRF posterior to the CoM and recover WBAM. In conclusion, the onset of WBAM perturbations does not affect the moment arm modulation needed for WBAM recovery, while it does affect the way CoP and GRF modulation contribute to that recovery.

Relationship between the shape of the elliptical knee joint and jumping height in a leg-type robot driven by pneumatic artificial muscle

The McKibben pneumatic actuator (MPA) is a soft actuator used for performing various practical functions in robots. Particularly, many dynamic robots have been realized using MPAs. However, there is a trade-off between torque generated by MPA and the range of motion of the joint. In this study, we focus on the jumping motion of a leg-type robot and use an elliptical pulley whose moment arm changes depending on the robot’s posture. To confirm the effectiveness of the elliptical pulley, the relationship between the knee joint pulley (patella) shape and jumping height was analyzed by simulation, and the shape of the patella maximizing jumping height was determined. It was shown that an elongated elliptical patella shape is more effective for the jumping motion than a circular one. Furthermore, the effectiveness of the analytically determined patella shape was confirmed by experiments using an actual robot.

Recovery from sagittal-plane whole body angular momentum perturbations during walking

Healthy individuals highly regulate their whole body angular momentum (WBAM) during walking. Since WBAM regulation is essential in maintaining balance, a better understanding is required on how healthy individuals recover from WBAM perturbations. We therefore studied how healthy individuals recover WBAM in the sagittal plane. WBAM can be regulated by adjusting the moment arm of the ground reaction force (GRF) vector with respect to the whole-body centre of mass (CoM). In principle this can be done by centre of pressure (CoP) modulation and/or adjustments of the GRF direction. Two simultaneous perturbations of the same magnitude were applied in opposite direction to the pelvis and upper body (0.34m apart) to perturb WBAM but not the whole body linear momentum (WBLM), while participants walked on a treadmill. The perturbations were given at toe off right, had a magnitude of 4, 8, 12 and 16% of the participant’s body weight, and lasted for 150ms. A recovery of the WBAM was seen directly after the perturbations, induced by adapting the moment arm of the GRF with respect to the CoM. The hip joint of the stance leg played an important role in achieving the WBAM recovery. A change in the direction of the GRF vector and not a contributing CoP modulation, caused the change in moment arm. However, the change in GRF direction came from a change in the horizontal GRF, which also affects the WBLM. This suggest that regulating WBAM may take precedence over the WBLM in early recovery.

Infraspinatus Muscle Fiber Moment Arms During Abduction: A Biomechanical Comparison of Values for Intact Rotator Cuff, Supraspinatus Tear, Superior Capsular Reconstruction, and Reverse Total Shoulder Arthroplasty.

Background:Lines of action of the superior, middle, and inferior infraspinatus muscle fibers work together to produce moment arms that change throughout abduction in an intact shoulder, after a supraspinatus tear, and after superior capsular reconstruction (SCR) and reverse total shoulder arthroplasty (rTSA).Purpose:To use moment arm values to indicate the efficacy of SCR and rTSA to restore infraspinatus function during shoulder abduction.Study Design:Descriptive laboratory study.Methods:A total of 5 human cadaveric shoulders placed in a testing apparatus were each actively abducted (0°-90°) under the following 4 conditions: intact, complete supraspinatus tear, SCR, and rTSA. The 3-dimensional coordinates of points were tracked along the origin and insertion of the superior, middle, and inferior infraspinatus fibers during abduction. Moment arm values were calculated using the origin-insertion method to determine abduction contribution of infraspinatus fiber sections. Analysis of variance and post hoc Tukey testing were used to compare differences in moment arms between the 4 conditions and between fiber sections.Results:In the intact condition, the superior infraspinatus fibers had an abduction moment that decreased with elevation until shifting to adduction. Conversely, the middle and inferior fibers had an adduction moment that turned to abduction (mean moment arm values from 0° to 90°: inferior, from –5.9 to 19.4 mm; middle, from –4.7 to 15.9 mm; superior, from 5.6 to –5.1 mm; P < .05). After a supraspinatus tear, superior fibers lacked any torque, and inferior and middle fibers lost adduction potential (inferior, from 4.8 to 14.0 mm; middle, from –0.2 to 9.6 mm; superior, from 1.0 to 0.7 mm; P < .05). SCR restored the initial superior fiber abduction moment (5.6 mm at 0°; P < .05); middle and inferior fibers had some restoration but were weaker than intact fibers. Loss of abduction moment in all fibers was seen with rTSA (inferior, from –9.6 to –1.6 mm; middle, from –10.5 to –3.6 mm; superior, from –1.7 to –4.6 mm; P < .05).Conclusion:Infraspinatus fiber groups had different and inverse moment arms during scapular plane elevation. SCR most closely resembled the intact shoulder, whereas rTSA transformed the infraspinatus into an adductor.Clinical Relevance:These results support the efficacy of SCR at restoring biomechanical muscle function and suggest that the changes in moment arms for each fiber group be considered when choosing treatment modalities and rehabilitation protocols after rotator cuff tear.

Muscle Fiber Contribution to Rotator Cuff Moment Arms During Abduction for Intact Rotator Cuff, Complete Supraspinatus Tear, Superior Capsular Reconstruction, and Reverse Shoulder Arthroplasty. (225)

Objectives:The Rotator Cuff (RC) is formed from the subscapularis, supraspinatus, infraspinatus, and teres minor muscles and their tendinous extensions. The 4 RC tendons insert on the humeral head such that they contribute to the dynamic stability of the glenohumeral joint along with their rotational actions on the shoulder. The moment arm can be used to demonstrate the work effort potential that a specific muscle is contributing to a musculoskeletal joint rotation. The objective of this study was to break out RC muscles into multiple fibers, providing more clarity as to how individual fibers contribute to a muscle’s overall moment arm during abduction.The aims of this study are: 1.) to illustrate within each RC muscle how multiple muscle fiber lines of action work together to produce abduction in an intact shoulder 2.) to estimate the moment arm changes that take place when the intact rotator cuff goes through surgical repair with either SCR or RSA after complete supraspinatus tear. We hypothesized that the rotator cuff muscles work differently and in combination at the fiber level to bring about a resultant movement that can be assessed through the proposed method of moment arm calculation for intact RC, complete supraspinatus tear, SCR and RSA.Methods:Five fresh cadaveric shoulder specimens were used in an apparatus where each muscle was maintained in tension with the line of action towards its origin on the scapula (Figure 1). An Optotrack camera kept track of digitized points along both the origin and insertion of the rotator cuff muscles as the shoulder was abducted. Using these digitized points, multiple lines of action were created across the breadth of each muscle. Each muscle force action line was then used to calculate moment arm values during 0-90º abduction (Figure 2).Results:Moment arms calculated for multiple fiber lines spanning the tendon attachment site displayed the variance of fiber contribution and function within each muscle during abduction. Our results indicate that rather than providing a return to anatomical shoulder muscle function, RSA and SCR models produce moment arms that vary between muscles, with some contributing more to abduction and some contributing less. Highlighted below are the infraspinatus results for moment arms of individual fiber lines of action (Figure 3) and calculated mean moment arms (Figure 4) over abduction.ANOVA testing demonstrated a significant difference (p<0.001) when analyzing moment arms of intact, complete supraspinatus tear, SCR, and RSA models in teres minor and infraspinatus. There was no significant difference in moment arm values between the models in the subscapularis (p=0.148). Highlighted in Table 1 are the ANOVA testing results for infraspinatus.Conclusions:Our biomechanical analysis demonstrated sufficient sensitivity to detect differences in moment arms of the four rotator cuff muscles across a variety of models, suggesting changes to even one muscle of the shoulder will have significant implications on the function of other shoulder muscles. Furthermore, our analysis of fiber divisions within the same muscle illustrates the complex nature of the shoulder muscles themselves, and future studies should aim to better explore and model their function. The calculated percent differences from intact beautifully illustrated this complexity, as corrective RSA and SCR procedures provided better resemblance of intact anatomy within some rotator cuff muscles while creating a larger percent difference in other muscle groups.By breaking out RC muscles into multiple fibers, more clarity can be gained as to how individual fibers contribute to a muscle’s overall moment arm during abduction. This may further aid surgical decision-making, specifically for RSA where there is continued debate about whether to reconstruct portions of the RC. Given that the supraspinatus tendon is the most frequently torn tendon in the rotator cuff, especially for athletes who apply repetitive stress to the tendon, the results of this study may help inform post-operative rehabilitation by illustrating how abduction and stability are achieved after SCR and RSA.

Optimizing the Femoral Offset for Restoring Physiological Hip Muscle Function in Patients With Total Hip Arthroplasty.

ObjectiveFemoral offset (FO) restoration is significantly correlated with functional recovery following total hip arthroplasty (THA). Accurately assessing the effects of FO changes on hip muscles following THA would help improve function and optimize functional outcomes. The present study aimed to (1) identify the impact of FO side difference on the hip muscle moment arms following unilateral THA during gait and (2) propose the optimal FO for a physiological hip muscle function.MethodsIn vivo hip kinematics from eighteen unilateral THA patients during gait were measured with a dual-fluoroscopic imaging system. The moment arms of thirteen hip muscles were calculated using CT-based 3D musculoskeletal models with the hip muscles’ lines of actions. The correlation coefficient (R) between FO and hip muscle moment arm changes compared with the non-implanted hip was calculated. We considered that the FO reconstruction was satisfactory when the abductor moment arms increased, while the extensor, adductor, and flexor moment arms decreased less than 5%.ResultsA decreased FO following THA was significantly correlated with a decrease of the abductor and external rotator moment arms during the whole gait (R > 0.5) and a decrease of extensor moment arms during the stance phase (R > 0.4). An increased FO following THA was significantly associated with shorter flexor moment arms throughout the gait (R < −0.5) and shorter adductor moment arms in the stance phase (R < −0.4). An increase in FO of 2.3–2.9 mm resulted in increased abductor moment arms while maintaining the maximum decrease of the hip muscles at less than 5.0%.ConclusionAn increase of 2–3 mm in FO could improve the abductor and external rotator function following a THA. Accurate surgical planning with optimal FO reconstruction is essential to restoring normal hip muscle function in THA patients.

Achilles tendon moment arm changes with total ankle arthroplasty.

Prior research on total ankle arthroplasty (TAA) has focused on improvements in pain and function following the surgical treatment of ankle arthritis, but its effect on ankle joint mechanics has received relatively little attention. The plantarflexion moment arm of the Achilles tendon is a critical determinant of ankle function with the potential to be altered by TAA. Here we investigate the effect of TAA on Achilles tendon moment arm assessed using two methods. Standing sagittal-plane radiographs were obtained for ten patients presurgery and postsurgery, from which anterior-posterior distance between the posterior calcaneus and the center of the talar dome was measured. Ultrasound imaging and three-dimensional (3D) motion capture were used to obtain moment arm pre- and post-TAA. The absolute changes in moment arm pre- to post-TAA were significantly different from zero for both methods (9.6 mm from ultrasound and 4.6% of the calcaneus length from radiographs). Only 46% of the variance in postoperative 3D Achilles tendon moment arm was explained by the preoperative value (r2 = 0.460; p = .031), while pre- and post-TAA values from radiographs were not correlated (r2 = 0.192, p = .206). While we did not find significant mean differences in Achilles tendon moment arm between pre- and post-TAA, we did find absolute changes in 3D moment arm that were significantly different from zero and these changes were partially explained by a change in location of the talar dome as indicated by measurements from radiographs (r2 = 0.497, p = .023).

Why do muscles lose torque potential when activated within their agonistic group?

Agonistic muscles lose approximately 20% of their individual torque-generating capacity when activated with their agonistic muscles compared with when stimulated in isolation. In this study, we (1) tested if this loss in torque was accompanied by a corresponding loss in force, thereby testing the potential role of changes in moment arms between conditions; (2) removed all inter-muscular connections between the quadriceps muscles, thus determining the potential role of inter-muscular force transmission; and (3) systematically changed the inter-muscular pressure by performing experiments at different activation/force levels, thereby exploring the possible role of inter-muscular pressure in the loss of torque capacity with simultaneous muscle activation. Experiments were performed in a New Zealand white rabbit quadriceps model (N=5). Torque and force were measured during activation of femoral nerve branches that supply the individual quadriceps muscles while activating these branches simultaneously and in isolation. Regardless of joint angle and inter-muscular connections between muscles, the differences in torque values between the simultaneous and the isolated activation of the quadriceps muscles were also observed for the directly measured force values. Mean differences in simultaneous and isolated muscle activation remained similar between the intact and separated conditions: torque difference 21±5% of maximum isometric torque of intact condition (MICtorque), versus 19±6% MICtorque, respectively, and force difference 18±3% MICforce versus 19±7% MICforce, respectively. The absolute torque loss was independent of the force, and thus presumably the inter-muscular pressures. Based on these results, we conclude that the torque deficit observed during simultaneous compared with isolated muscle activation is not primarily caused by moment arm, inter-muscular pressure or inter-muscular force transmission. The mechanisms underlying loss of force capacity during agonistic muscle contraction remain unknown.

An Analytical Approach to Posture-Dependent Muscle Force and Muscle Activation Patterns.

Personalized training by taking into account individual anatomy to improve performance is a research frontier. In this paper, we first introduce an analytical method to study the pattern of changes in muscle forces as a function of posture. Our method is also able to analyze variation of maximal muscle force and muscle activation values (in various postures) as a result of posture-dependent changes in moment arms. This method also helps us evaluate the utility of person specific training. It also provides us with model based approximations for activation and muscle force patterns during different motions without a need for subject recordings, which enables athletes to have a better understanding of how each muscle contributes during each posture, in a fast and efficient way. Second, we analyze the results of this method for a simple squat move. Our results show that both maximal muscle force and muscle activation values have variable sensitivity to the moment arm values for different postures and muscles. It suggests that individually modified training plans could likely improve performance for some sets of movements.

Aging effects on the Achilles tendon moment arm during walking

The Achilles tendon (AT) moment arm transforms triceps surae muscle forces into a moment about the ankle which is critical for functional activities like walking. Moreover, the AT moment arm changes continuously during walking, as it depends on both ankle joint rotation and triceps surae muscle loading (presumably due to bulging of the muscle belly). Here, we posit that aging negatively effects the architecturally complex AT moment arm during walking, which thereby contributes to well-documented reductions in ankle moment generation during push-off. We used motion capture-guided ultrasound imaging to quantify instantaneous variations in the AT moment arms of young (23.9 ± 4.3 years) and older (69.9 ± 2.6 years) adults during walking, their dependence on triceps surae muscle loading, and their association with ankle moment generation during push-off. Older adults walked with 11% smaller AT moment arms and 11% smaller peak ankle moments during push-off than young adults. Moreover, as hypothesized, these unfavourable changes were significantly and positively correlated (r2 = 0.38, p < 0.01). More surprisingly, aging attenuated load-dependent increases in the AT moment arm (i.e., those between heel-strike and push-off at the same ankle angle); only young adults exhibited a significant increase in their AT moment arm due to triceps surae muscle-loading. Age-associated reductions in triceps surae volume or activation, and thus muscle bulging during force generation, may compromise the mechanical advantage of the AT during the critical push-off phase of walking in older adults. Thus, strategies to restore and/or improve locomotor performance in our aging population should consider these functionally important changes in musculoskeletal behavior.

Impact of individual anatomy on the benefit of cup medialisation in total hip arthroplasty.

In total hip arthroplasty, cup medialisation with compensatory increase of femoral offset has been proposed to increase the moment arms of abductor muscles. However, this technique is associated with a loss of acetabular bone stock. Previous data indicates that the potential benefit is not constant among patients and is likely related to patient anatomy. Therefore, to be able to select patients who would benefit most from this technique, we measured several anatomical parameters of the pelvis and femur in 15 patients; and evaluated correlations between them and the changes of moment arms associated with cup medialisation. The anatomical measurements were performed on 3-D reconstructions of preoperative CT scans. The moment arms of the gluteus medius and gluteus minimus were calculated for an abduction and flexion movement using patient-specific finite element models. 6 anatomical parameters were correlated with muscle moment arm variations after cup medialisation. This effect was not systematic for either muscles or movements. Among all parameters, femoral anteversion was the most important parameter in explaining the effect of cup medialisation. Patients with small femoral anteversion or low-riding greater trochanter benefit more from cup medialisation in terms of moment arm increase in abduction motion.

Impact of the Femoral Head Position on Moment Arms in Total Hip Arthroplasty: A Parametric Finite Element Study

BackgroundAlthough the importance of accurate femoral reconstruction to achieve a good functional outcome is well documented, quantitative data on the effects of a displacement of the femoral center of rotation on moment arms are scarce. The purpose of this study was to calculate moment arms after nonanatomical femoral reconstruction. MethodsFinite element models of 15 patients including the pelvis, the femur, and the gluteal muscles were developed. Moment arms were calculated within the native anatomy and compared to distinct displacement of the femoral center of rotation (leg lengthening of 10 mm, loss of femoral offset of 20%, anteversion ±10°, and fixed anteversion at 15°). Calculations were performed within the range of motion observed during a normal gait cycle. ResultsAlthough with all evaluated displacements of the femoral center of rotation, the abductor moment arm remained positive, some fibers initially contributing to extension became antagonists (flexors) and vice versa. A loss of 20% of femoral offset led to an average decrease of 15% of abductor moment. Femoral lengthening and changes in femoral anteversion (±10°, fixed at 15°) led to minimal changes in abductor moment arms (maximum change of 5%). Native femoral anteversion correlated with the changes in moment arms induced by the 5 variations of reconstruction. ConclusionAccurate reconstruction of offset is important to maintaining abductor moment arms, while changes of femoral rotation had minimal effects. Patients with larger native femoral anteversion appear to be more susceptible to femoral head displacements.

Effect of six degrees of freedom knee kinematics on ligament length and moment arm in an intact knee model.

Biomechanics studies can help improve athletic performance. However, the biomechanics of knee joint ligament length changes and moment arms over six degrees of freedom (DOF) have yet to be established. To construct a knee model to investigate the length and moment arm changes of the anterior cruciate ligament (ACL), posterior cruciate ligament (PCL), medial collateral ligament (MCL), and lateral collateral ligament (LCL). Six DOF joint modeling and analysis were performed using specialized modeling software. The length of all ligaments varied with tibiofemoral flexion angle, contributed to joint motion, and restrained the joint in different positions. The ACL, MCL, and LCL lengths decreased, the PCL increased, the posterior tibial translation increased, the MCL moment arm increased, and the LCL moment arm decreased between 0° and 90°. The primary ligament restraints were the PCL (0° to 30°), MCL (30° to 60°), and PCL (60° to 90°). The restraining function of each ligament during motion can be modeled based on changes in ligament lengths during tibial translations and rotations during flexion. Understanding the correlations between ligament lengths and moment arm changes over a wide range of motion will help improve our understanding of joint kinematics and may be useful for the diagnosis and treatment of sports injury.

Evolution of the human hip. Part 2: muscling the double extension.

Part 1 of this article outlined the extensive osseous adaptations around the hip that occurred in the development of a habitual bipedal gait in modern humans. The shortest summary of these osseous changes is ‘double extension’, i.e. extension of both the hip joint and the lumbar spine. Not surprisingly, these osseous changes went hand in hand with major muscular changes. The primary changes that accompanied the double extension were changes in relative muscle volume for the quadriceps, gluteus maximus and hamstrings, changes in moment arms for the iliopsoas, gluteus maximus and hamstrings, a change in function for the gluteus medius and minimus, while the functional anatomy of the adductors and hip rotators changed only slightly. The effect of these osseous and muscular changes was improved energy efficiency of human bipedal walking and (long distance) running. However, this occurred at the expense of maximum power, characteristic for activities such as tree climbing (in the apes), but equally so for sprinting. Recognizing these changes and their consequences may help us better understand and treat soft-tissue disorders around the hip.

Benefit of Cup Medialization in Total Hip Arthroplasty is Associated With Femoral Anatomy

Medialization of the cup with a respective increase in femoral offset has been proposed in THA to increase abductor moment arms. Insofar as there are potential disadvantages to cup medialization, it is important to ascertain whether the purported biomechanical benefits of cup medialization are large enough to warrant the downsides; to date, studies regarding this question have disagreed. The purpose of this study was to quantify the effect of cup medialization with a compensatory increase in femoral offset compared with anatomic reconstruction for patients undergoing THA. We tested the hypothesis that there is a (linear) correlation between preoperative anatomic parameters and muscle moment arm increase caused by cup medialization. Fifteen patients undergoing THA were selected, covering a typical range of preoperative femoral offsets. For each patient, a finite element model was built based on a preoperative CT scan. The model included the pelvis, femur, gluteus minimus, medius, and maximus. Two reconstructions were compared: (1) anatomic position of the acetabular center of rotation, and (2) cup medialization compensated by an increase in the femoral offset. Passive abduction-adduction and flexion-extension were simulated in the range of normal gait. Muscle moment arms were evaluated and correlated to preoperative femoral offset, acetabular offset, height of the greater trochanter (relative to femoral center of rotation), and femoral antetorsion angle. The increase of muscle moment arms caused by cup medialization varied among patients. Muscle moment arms increase by 10% to 85% of the amount of cup medialization for abduction-adduction and from -35% (decrease) to 50% for flexion-extension. The change in moment arm was inversely correlated (R(2) = 0.588, p = 0.001) to femoral antetorsion (anteversion), such that patients with less femoral antetorsion gained more in terms of hip muscle moments. No linear correlation was observed between changes in moment arm and other preoperative parameters in this series. The benefit of cup medialization is variable and depends on the individual anatomy. Cup medialization with compensatory increase of the femoral offset may be particularly effective in patients with less femoral antetorsion. However, cup medialization must be balanced against its tradeoffs, including the additional loss of medial acetabular bone stock, and eventual proprioceptive implications of the nonanatomic center of rotation and perhaps joint reaction forces. Clinical studies should better determine the relevance of small changes of moment arms on function and joint reaction forces.

Internal Foot Kinematics Using High‐Speed Fluoroscopy

IntroductionThe navicular bone (NB) and the deep digital flexor tendon (DDFT) are common sites of lameness. Greater knowledge of foot mechanics will aid in understanding of the aetiopathogenesis and may provide new opportunities for treatment and prevention. It was hypothesised that changes in the distal interphalangeal joint (DIPj) and movement of the NB and DDFT during the stance phase will lead to changes in the moment arm of the DDFT around the DIPj.MethodsInternal hoof kinematics were determined using a high‐speed fluoroscopy system. Six horses were walked and trotted over a raised platform consisting of four force plates, through the field of view of the fluoroscopy system. The angles of the DIPj and the DDFT around the navicular bone and the moment arm of the DDFT were measured. Changes in these parameters during stance and differences between strides, gaits and horses were assessed using an ANOVA or paired T‐test as appropriate (P&lt;0.05).ResultsThe range of motion (ROM, mean ± SD) during stance of the DIPJ was 28.6 ± 4.6° (walk) and 26.5 ± 6.3 (trot) with significant differences between gaits, strides and horses (P&lt;0.001). The angle change of the DDFT around the NB was 21.9 ± 6.0° in walk and 26.6 ± 6.5 in trot. The change in moment arm of the DDFT around the DIPj was 3.7 ± 1.3 mm in walk and 4.1 ± 2.2 mm in trot. There were no significant differences between the different time points during stance or between gaits.ConclusionsHigh‐speed fluoroscopy is a novel method that allows assessment of internal foot kinematics. The observed changes between gaits during stance may result in altered stress distribution in the DDFT. Further studies will evaluate the effect of farriery intervention on internal foot kinematics.Ethical Animal Researchthis study was approved by the authors′ institution's ethics and welfare committee. Sources of funding: Royal Veterinary College. Competing interests: none.

Static and dynamic loading of mandibular condyles and their positional changes after bilateral sagittal split advancement osteotomies

This study analysed the effects of change of direction of masseter (MAS) and medial pterygoid muscles (MPM) and changes of moment arms of MAS, MPM and bite force on static and dynamic loading of the condyles after surgical mandibular advancement. Rotations of the condyles were assessed on axial MRIs. 16 adult patients with mandibular hypoplasia were studied. The mandibular plane angle (MPA) was <39° in Group I (n=8) and >39° in Group II (n=8). All mandibles were advanced with a bilateral sagittal split osteotomy (BSSO). In Group II, BSSO was combined with Le Fort I osteotomy. Pre and postoperative moment arms of MAS, MPM and bite force were used in a two-dimensional model to assess static loading of the condyles. Pre and postoperative data on muscle cross-sectional area, volume and direction were introduced in three-dimensional dynamic models of the masticatory system to assess the loading of the condyles during opening and closing. Postsurgically, small increases of static condylar loading were calculated. Dynamic loading decreased slightly. Minor rotations of the condyles were observed. The results do not support the idea that increased postoperative condylar loading is a serious cause for condylar resorption or relapse.

Sensitivity of model predictions of muscle function to changes in moment arms and muscle–tendon properties: A Monte-Carlo analysis

Hill-type muscle models are commonly used in musculoskeletal models to estimate muscle forces during human movement. However, the sensitivity of model predictions of muscle function to changes in muscle moment arms and muscle–tendon properties is not well understood. In the present study, a three-dimensional muscle-actuated model of the body was used to evaluate the sensitivity of the function of the major lower limb muscles in accelerating the whole-body center of mass during gait. Monte-Carlo analyses were used to quantify the effects of entire distributions of perturbations in the moment arms and architectural properties of muscles. In most cases, varying the moment arm and architectural properties of a muscle affected the torque generated by that muscle about the joint(s) it spanned as well as the torques generated by adjacent muscles. Muscle function was most sensitive to changes in tendon slack length and least sensitive to changes in muscle moment arm. However, the sensitivity of muscle function to changes in moment arms and architectural properties was highly muscle-specific; muscle function was most sensitive in the cases of gastrocnemius and rectus femoris and insensitive in the cases of hamstrings and the medial sub-region of gluteus maximus. The sensitivity of a muscle's function was influenced by the magnitude of the muscle's force as well as the operating region of the muscle on its force-length curve. These findings have implications for the development of subject-specific models of the human musculoskeletal system.

Positional changes of the masseter and medial pterygoid muscles after surgical mandibular advancement procedures: an MRI study

This study evaluated whether surgical mandibular advancement procedures induced a change in the direction and the moment arms of the masseter (MAS) and medial pterygoid (MPM) muscles. Sixteen adults participated in this study. The sample was divided in two groups: Group I (n=8) with a mandibular plane angle (mpa) <39° and Group II (n=8) with an mpa >39°. Group I patients were treated with a bilateral sagittal split osteotomy (BSSO). Those in Group II were treated with a BSSO combined with a Le Fort I osteotomy. Pre- and postoperative direction and moment arms of MAS and MPM were compared in these groups. Postsurgically, MAS and MPM in Group II showed a significantly more vertical direction in the sagittal plane. Changes of direction in the frontal plane and changes of moment arms were insignificant in both groups. This study demonstrated that bimaxillary surgery in patients with an mpa >39° leads to a significant change of direction of MAS and MPM in the sagittal plane.

The effect of clavicle malunion on shoulder biomechanics; A computational study

BackgroundClavicle malunion affects the biomechanics of the shoulder joint. The purpose of this study is to establish the abduction, flexion, and internal (medial) rotation biomechanics of the shoulder after clavicle malunion. MethodsA computational study was performed utilizing a three-dimensional, validated computational model of the upper extremity. Sequential shortening of the clavicle up to 20% was simulated. Muscle forces, moment arms, and moments were calculated for the surrounding musculature through a range of flexion, abduction, and internal rotation during the simulated shortening. FindingsShortening of the clavicle decreases the shoulder elevation moments of the upper extremity muscles during abduction. Internal rotation moments are also decreased with shortening. Flexion moments were affected less through physiologic range of motion. The observed effects are due to a combination of changes in moment arms of the individual muscles as well as a decrease in the force generating capacity of the muscles. Additionally, shortening of the clavicle increases coronal angulation of the clavicle at the sternoclavicular joint. InterpretationShortening causes a decrease in the moment generating capacity as well as the total force generating capacity of the shoulder girdle muscles. The clinical significance of these computational results, which are consistent with recent clinical studies, is validation of the proposed functional deficit caused by clavicle malunion.