Tendon Force Research Articles

Numerical studies to evaluate crack propagation behaviour of prestressed concrete railway sleepers

The prestressed concrete sleeper is a primary type of sleeper for railway systems around the world. Compared with wooden and composite sleepers, concrete sleepers are of good elasticity and sufficient longitudinal and lateral resistance to ensure the stability of the track. As the railway system is developing to a heavier load and higher speed system, the cracking of concrete sleepers under dynamic load and long-term service is becoming more and more critical. It is essential to evaluate the crack behaviour of prestressed concrete sleepers. In this paper, three-dimensional FEM models have been established using Abaqus based on the brittle cracking model. After the validation by experimental results, the numerical research is performed to evaluate the crack propagation behaviours of prestressed concrete sleepers with different concrete compressive strength, different prestressing force magnitudes, and different loading rates. The results reveal that the cracking resistance of a concrete sleeper turns enhanced by improving the compressive strength of concrete and the initial prestressing force of tendons. It is also observed that the cracking becomes more progressive as the loading rate increases.

Effect of Compliance on Residual Stresses in Manufacturing With Moving Heat Sources

Abstract Manufacturing processes involving moving heat sources include additive manufacturing, welding, laser processing (cladding and heat treatment), machining, and grinding. These processes involve high local thermal stresses that induce plasticity and result in permanent residual stress and distortion. The residual stresses are typically calculated numerically at great computational expense despite the fact that the inelastic fraction of the domain is very small. Efforts to decouple the small plastic part from the large elastic part have led to the development of the tendon force concept. The tendon force can be predicted analytically for the case of infinitely rigid components; however, this limitation has prevented the broader use of the concept in practical applications. This work presents a rigorous mathematical treatment using dimensional analysis, asymptotics, and blending which demonstrates that the effect of geometric compliance depends on a single dimensionless group, the Okerblom number. Closed-form expressions are derived to predict the effect of compliance without the need for empirical ad-hoc fitting or calibration. The proposed expressions require input of only material properties and tabulated process parameters and are thus ideally suited for use in metamodels and design calculations, as well as incorporation into engineering codes and standards.

Resting Tendon Cross-Sectional Area Underestimates Biceps Brachii Tendon Stress: Importance of Measuring During a Contraction.

Force produced by the muscle during contraction is applied to the tendon and distributed through the cross-sectional area (CSA) of the tendon. This ratio of force to the tendon CSA is quantified as the tendon mechanical property of stress. Stress is traditionally calculated using the resting tendon CSA; however, this does not take into account the reductions in the CSA resulting from tendon elongation during the contraction. It is unknown if calculating the tendon stress using instantaneous CSA during a contraction significantly increases the values of in vivo distal biceps brachii (BB) tendon stress in humans compared to stress calculated with the resting CSA. Nine young (22 ± 1 years) and nine old (76 ± 4 years) males, and eight young females (21 ± 1 years) performed submaximal isometric elbow flexion tracking tasks at force levels ranging from 2.5 to 80% maximal voluntary contraction (MVC). The distal BB tendon CSA was recorded on ultrasound at rest and during the submaximal tracking tasks (instantaneous). Tendon stress was calculated as the ratio of tendon force during contraction to CSA using the resting and instantaneous measures of CSA, and statistically evaluated with multi-level modeling (MLM) and Johnson–Neyman regions of significance tests to determine the specific force levels above which the differences between calculation methods and groups became statistically significant. The tendon CSA was greatest at rest and decreased as the force level increased (p < 0.001), and was largest in young males (23.0 ± 2.90 mm2) followed by old males (20.87 ± 2.0 mm2) and young females (17.08 ± 1.54 mm2) (p < 0.001) at rest and across the submaximal force levels. Tendon stress was greater in the instantaneous compared with the resting CSA condition, and young males had the greatest difference in the values of tendon stress between the two conditions (20 ± 4%), followed by old males (19 ± 5%), and young females (17 ± 5%). The specific force at which the difference between the instantaneous and resting CSA stress values became statistically significant was 2.6, 6.6, and 10% MVC for old males, young females, and young males, respectively. The influence of using the instantaneous compared to resting CSA for tendon stress is sex-specific in young adults, and age-specific in the context of males. The instantaneous CSA should be used to provide a more accurate measure of in vivo tendon stress in humans.

Effect of external post-tensioning on steel–concrete composite beams with partial connection

In this paper, an experimental and numerical study was presented for strengthened composite steel–concrete beams using externally post-tensioning tendons with a partial shear connection. Four composite steel–concrete beams under a static monotonic load were tested to validate the numerical model and investigate the different failure mechanisms. The cracking behaviors of the composite beams and overall moment of resistance have been experimentally and numerically studied. The validated numerical model was used to perform a parametric study. It was found that the initial tendon force increased by 18–23% due to the beam bending, which led to an enhancement in the performance of the beam. Moreover, increasing the degree of shear connection (DOSC) from 40% to 100% led to an increase in the mean maximum load by 46% and to decrease the mean maximum deflection by 22.5%. In addition, there was an improvement in the slippage by achieving 28% decrease. Also, there was a decrease in steel flange micro-strain by about 51%.

In vivo human gracilis whole-muscle passive stress-sarcomere strain relationship.

We measured the passive mechanical properties of intact, living human gracilis muscles (n=11 individuals, 10 male and 1 female, age: 33±12 years, mass: 89±23 kg, height: 177±8 cm). Measurements were performed in patients undergoing surgery for free-functioning myocutaneous tissue transfer of the gracilis muscle to restore elbow flexion after brachial plexus injury. Whole-muscle force of the gracilis tendon was measured in four joint configurations (JC1-JC4) with a buckle force transducer placed at the distal tendon. Sarcomere length was also measured by biopsy from the proximal gracilis muscle. After the muscle was removed, a three-dimensional volumetric reconstruction of the muscle was created via photogrammetry. Muscle length from JC1 to JC4 increased by 3.3±1.0, 7.7±1.2, 10.5±1.3 and 13.4±1.2 cm, respectively, corresponding to 15%, 34%, 46% and 59% muscle fiber strain, respectively. Muscle volume and an average optimal fiber length of 23.1±0.7 cm yielded an average muscle physiological cross-sectional area of 6.8±0.7cm2 which is approximately 3 times that measured previously from cadaveric specimens. Absolute passive tension increased from 0.90±0.21N in JC1 to 16.50±2.64N in JC4. As expected, sarcomere length also increased from 3.24±0.08 µm at JC1 to 3.63±0.07 µm at JC4, which are on the descending limb of the human sarcomere length-tension curve. Peak passive muscle stress was 27.8±5.5 kPa in JC4 and muscle modulus ranged from 44.8 MPa in JC1 to 125.7 MPa in JC4. Comparison with other mammalian species indicates that human muscle passive mechanical properties are more similar to rodent muscle than to rabbit muscle. These data provide direct measurements of whole-human muscle passive mechanical properties that can be used in modeling studies and for understanding comparative passive mechanical properties among mammalian muscles.

The immediate effects of foot orthosis geometry on lower limb muscle activity and foot biomechanics

Foot orthoses (FOs) are used to treat clinical conditions by altering the external forces applied to the foot and thereafter the forces of muscles and tendons. However, whether specific geometric design features of FOs affect muscle activation is unknown. The aim of this study was to investigate if medial heel wedging and increased medial arch height have different effects on the electromyography (EMG) amplitude of tibialis posterior, other muscles of the lower limb and the kinematics and kinetics at the rearfoot and ankle. Healthy participants (n = 19) walked in standardised shoes with i) a flat inlay; ii) a standard shape FOs, iii) standard FOs adjusted to incorporate a 6 mm increase in arch height, iv) and standard FOs adjusted to incorporate an 8° medial heel wedging and v) both the 6 mm increase in arch height and 8° increase in medial wedging. EMG was recorded from medial gastrocnemius, peroneus longus, tibialis anterior and in-dwelling tibialis posterior muscles. Motion and ground reaction force data were collected concurrently. Tibialis posterior EMG amplitude reduced in early stance with all FOs (ηp2 = 0.23–1.16). Tibialis posterior EMG amplitude and external ankle eversion moment significantly reduced with FOs incorporating medial wedging. The concurrent reduction in external eversion moment and peak TP EMG amplitude in early stance with medial heel wedging demonstrates the potential for this specific FOs geometric feature to alter TP activation. Medial wedged FOs could facilitate tendon healing in tibialis posterior tendon dysfunction by reducing force going through the TP muscle tendon unit.

Mechanism clarification of mitigating welding induced buckling by transient thermal tensioning based on inherent strain theory

Welding induced buckling in thin plate structures significantly affects productivity and fabrication cost due to post-weld correction works. Transient thermal tensioning (TTT) as an in-process treatment was employed for mitigating welding induced buckling, and its mechanism was clarified by inherent strain theory. Butt welding experiment with 304 L stainless steel was conducted. A finite element (FE) model was established to examine thermal-mechanical response during the entire welding process. Computed results were validated by the measured out-of-plane welding distortion, which was obtained using a three-dimensional optical scanning system. Then, two additional heat sources on both sides of the welding line were applied to achieve TTT treatment with the established numerical model. Moreover, the influence of heating location on mitigating welding induced buckling was discussed. The results show that the maximum deflections along and perpendicular to welding direction were reduced by 82.6% and 74.7%, respectively. The heating location has a decisive influence on the mitigation of welding induced buckling, which should be kept away from the welding line with appropriate distance. The thermal stretching from additional heat sources enhances the self-constraint of the base material on the welding region during the cooling process, resulting in more tensile plastic strain generated to reduce welding inherent strain and tendon force.

The effect of fatigue on peak Achilles tendon force in Irish dancing-specific landing tasks

ABSTRACT Achilles tendinopathy is prevalent among Irish dancers, believed to be due to aesthetic technical requirements and high-impact landing tasks. However, the peak Achilles tendon force during Irish dancing-specific landing tasks has not been quantified. Furthermore, the influence of fatigue is unclear. This study aimed to quantify the peak Achilles tendon force during three common Irish dancing landing tasks and investigate the effects of fatigue on this force. Twelve nationally competitive Irish dancers completed the landing tasks prior to and following a fatigue protocol. A Vicon motion analysis system and AMTI force plates were used to calculate sagittal plane ankle joint kinematics during landing to estimate peak Achilles tendon force. Three independent measures (Rating-of-Fatigue scale, Flight time: Contraction during a counter movement jump and jump height during the landing trials) were used to evaluate participant fatigue between conditions. Results revealed a significant difference in peak Achilles tendon force between the three landing tasks, however, no significant difference was observed between pre- and post-fatigue. Further research is required to investigate the effects of the landing technique used in Irish Dancing on peak Achilles tendon force with the aim to reduce peak Achilles tendon force and the risk of developing Achilles tendinopathy.

Three-dimensional spreading of prestressing forces in box girders

In most applications of post-tensioning, especially for bridge girders, the cross section over which the post-tensioning acts is not rectangular. Since the design of the box girder anchorage zone is not currently addressed in any code provisions, a detailed investigation of the three-dimensional (3D) spreading of the tendon force in flanged sections is required. To accomplish this objective, a computer-based tool is developed for plotting load paths in 3D bodies, from which the flow of forces in the box-girder anchorage zone can be clearly visualized. It is found that the 33.7-degree criterion is appropriate to describe the dispersion of the tendon force in the box-girder. Meanwhile, planar strut-and-tie models are built to approximate the flow of forces in orthogonal planes of the box section. As a useful supplement to the current design codes, direct equations are established for the design of the box-girder anchorage zone. Finally, a case study is carried out to find the cause of cracking in a span-by-span constructed box-girder bridge. The large transverse tensile stress in the bottom flange is primarily caused by the post-tensioning of web tendons, indicating the importance of 3D effects in the box-girder anchorage zone.

Normative Achilles and patellar tendon shear wave speeds and loading patterns during walking in typically developing children.

Motion analysis is commonly used to evaluate joint kinetics in children with cerebral palsy who exhibit gait disorders. However, one cannot readily infer muscle-tendon forces from joint kinetics. This study investigates the use of shear wave tensiometry to characterize Achilles and patellar tendon forces during gait. How do Achilles and patellar tendon wave speed and loading modulate with walking speed in typically developing children? Twelve typically developing children (9-16 years old) walked on an instrumented treadmill with shear wave tensiometers over their Achilles (n = 11) and patellar (n = 9) tendons. Wave speeds were recorded at five leg length-normalized walking speeds (very slow to very fast). Achilles and patellar tendon moment arms were measured with synchronized ultrasound and motion capture. The tendon wave speed-load relationship was calibrated at the typical walking speed and used to estimate tendon loading at other walking speeds. Characteristic Achilles and patellar tendon wave speed trajectories exhibited two peaks over a gait cycle. Peak Achilles tendon force closely aligned with peak ankle plantarflexor moment during pushoff, though force exhibited less modulation with walking speed. A second peak in late swing Achilles loading, which was not evident from the ankle moment, increased significantly with walking speed (p < 0.001). The two peaks in patellar tendon loading occurred at 12 ± 1% and 68 ± 6% of the gait cycle, matching the timing of peak knee extension moment in early stance and early swing. Both patellar tendon load peaks increased significantly with walking speed (p < 0.05). This is the first study to use shear wave tensiometry to characterize Achilles and patellar tendon loading during gait in children. These data could serve as a normative comparison when using tensiometry to identify abnormal tendon loading patterns in individuals who exhibit equinus and/or crouch gait.

Patella Apex Influences Patellar Ligament Forces and Ratio.

The relationship between three-dimensional shape and patellofemoral mechanics is complicated. The Wiberg patella classification is a method of distinguishing shape differences in the axial plane of the patella that can be used to connect shape differences to observed mechanics. This study uses the Wiberg patella classification to differentiate variance in a statistical shape model describing changes in patella morphology and height. We investigate how patella morphology influences force distribution within the patellofemoral joint. The Wiberg type I patella has a more symmetrical medial and lateral facet while the type III patella has a larger lateral facet compared to medial. The second principal component of the statistical shape model described shape variation that qualitatively resembled the different Wiberg patellas. We generated patellofemoral morphologies from the statistical shape model and integrated them into a musculoskeletal model with a twelve degrees-of-freedom knee. We simulated an overground walking trial with these morphologies and recorded patellofemoral mechanics and ligament forces. An increase in patellar ligament force corresponded with an increase in patella height. Wiberg type III patellas had a sharper patella apex which related to lower ratios of quadriceps tendon forces to patellar ligament forces. The change in pivot point of the patella affects the ratio of forces as well as the patellofemoral reaction force. This study provides a better understanding of how patella morphology affects fundamental patella mechanics which may help identify at-risk populations for pathology development.

Concept and analysis of a low-cost energy dissipater with partially self-centering mechanism

To achieve a good self-centering ability in the ‘rocking’ connection, the applied initial prestressed force of the post-tensioned tendons should be high enough to provide sufficient restoring force for the connection. However, a too high initial prestress level of the post-tensioned tendons weakens the deformation capacity of the connection. In answer to this problem, a low-cost and easy-construction bamboo-shaped energy dissipater with parallel elastic spring (PS-SBED) was proposed in this paper, which aims at sustaining good self-centering ability, obtaining enough energy dissipation capacity and improving deformation capacity. The proposed energy dissipater is featured as partially self-centering mechanism which was proved through theoretical and numerical ways. Explicit expressions for the self-centering ability and energy dissipation capacity of the PS-SBED were derived based on the theoretical hysteretic model. The effects of the stiffness, initial deformation and application conditions of the spring on the performance of the PS-SBED were evaluated based on parametric studies through validated numerical model. The bearing capacity of the PS-SBED was positively affected by the spring's stiffness and initial deformation. The residual deformation of the PS-SBED could be effectively reduced by the application of the elastic spring, compared with the energy dissipater without spring, but the energy dissipation capacity of the PS-SBED was not affected by the application of the spring. The application of the PS-SBED was able to increase the bearing capacity and improve the self-centering ability of the ‘rocking’ connection with low initial prestress level, which proved the effectiveness of the PS-SBED.

A General Expression for the Welding Tendon Force

Abstract This study presents a novel expression for the tendon force associated with residual stresses produced during welding of large, thin sections. A general engineering equation is presented as the combination of a closed-form expression, based on idealized treatment, and correction factors to account for the effects of temperature-dependent thermal and mechanical material properties. The closed-form expression corresponds to the assumption of constant material properties. A rigorous mathematical treatment is utilized to derive explicit, exact expressions for the temperature-dependent correction factors without the need for empirical correlations. The temperature-dependent behavior of materials is captured accurately using four dimensionless groups. The analysis was validated through numerical simulations with common structural grades of low-carbon steel, stainless steel, aluminum, and titanium. The idealized treatment resulted in predictions with a mean difference of 18%, which was reduced to 7% by incorporating the correction factors. The remaining error is a systematic overestimate, which can be attributed to compliance effects of the finite plate used in the simulations, and is the focus of ongoing research. The utility of applying the novel tendon force equation to problems in fabrication procedure design is demonstrated with an example predicting distortion during manufacturing of hollow structural sections.

Functional Ankle Range of Motion but Not Peak Achilles Tendon Force Diminished With Heel-Rise and Jumping Tasks After Achilles Tendon Repair

Background: Deficits in sporting performance after Achilles tendon repair may be due to changes in musculotendinous unit structure, including tendon elongation and muscle fascicle shortening. Purpose/Hypothesis: The purpose was to discern whether Achilles tendon rupture reduces triceps surae muscle force generation, alters functional ankle range of motion, or both during sports-related tasks. We hypothesized that individuals who have undergone Achilles tendon repair lack the functional ankle range of motion needed to complete sports-related tasks. Study Design: Descriptive laboratory study. Methods: The study included individuals 1 to 3 years after treatment of Achilles tendon rupture with open repair. Participants (n = 11) completed a heel-rise task and 3 jumping tasks. Lower extremity biomechanics were analyzed using motion capture. Between-limb differences were tested using paired t test. Results: Pelvic vertical displacement was reduced during the heel-rise (mean difference, −12.8%; P = .026) but not during the jumping task (P > .1). In the concentric phase of all tasks, peak ankle plantarflexion angle (range of mean difference, −19.2% to −48.8%; P < .05) and total plantar flexor work (defined as the area under the plantar flexor torque – ankle angle curve) (range of mean difference, −9.5% to −25.7%; P < .05) were lower on the repaired side relative to the uninjured side. No significant differences were seen in peak Achilles tendon load or impulse with any of the tasks. There were no differences in plantar flexor work or Achilles tendon load parameters during eccentric phases. Conclusion: Impaired task performance or increased demands on proximal joints were observed on the repaired side in tasks isolating ankle function. Tasks that did not isolate ankle function appeared to be well recovered, although functional ankle range of motion was reduced with rupture. Reduced plantar flexor muscle-tendon unit work supports previous reports that an elongated tendon and shorter muscle fascicles caused by Achilles tendon rupture constrain functional capacity. Achilles tendon peak load and impulse were not decreased, suggesting that reduced and shifted functional ankle range of motion (favoring dorsiflexion) underlies performance deficits. Clinical Relevance: These findings point to the need to reduce tendon elongation and restore muscle length of the triceps surae after Achilles tendon rupture in order to address musculature that is short but not necessarily weak for improved performance with sports-related activities.

Internal Mechanics of a Subject-Specific Wrist in the Sagittal versus Dart-Throwing Motion Plane in Adult and Elder Models: Finite Element Analyses

Introduction: Most of the wrist motions occur in a diagonal plane of motion, termed the dart-throwing motion (DTM) plane; it is thought to be more stable compared with movement in the sagittal plane. However, the effect of the altered carpus motion during DTM on the stress distribution at the radiocarpal joint has yet to be explored. Aim: To calculate and compare the stresses between the radius and two carpal bones (the scaphoid and the lunate) in two wrist positions, extension and radial extension (position in DTM), and between an adult and an elder model. Methods: A healthy wrist of a 40-year-old female was scanned using Magnetic Resonance Imaging in two wrist positions (extension, radial extension). The scans were transformed into three-dimensional models and meshed. Finite element (FE) analyses in each position of the wrist were conducted for both adult and elder models, which were differentiated by the mechanical properties of the ligaments. The distal surfaces of the carpal bones articulating with the metacarpals were loaded by physically accurate tendon forces for each wrist position. Results: The von Mises, shear stresses and contact stresses were higher in the extension model compared with the radial-extension model and were higher for the radius-scaphoid interface in the adult model compared with the elder model. In the radius-scaphoid interface, the stress differences between the two wrist positions were smaller in the elder model (11.5% to 22.5%) compared with the adult model (33.6–41.5%). During radial extension, the contact area at the radius-lunate interface was increased, more so in the adult model (222.2%) compared with the elder model (127.9%), while the contact area at the radius-scaphoid was not affected by the position of the wrist in the adult model (100.9%) but decreased in the elder model (50.2%) during radial extension. Conclusion: The reduced stresses during radial extension might provide an explanation to our frequent use of this movement pattern, as the reduced stresses decrease the risk of overuse injury. Our results suggest that this conclusion is relevant to both adults and elder individuals.

Seismic Behaviour of Spring Anchored Unbonded Post-Tensioned Rocking Systems

ABSTRACT The construction of low damage, self-centering, unbonded post-tensioned seismic systems requires special techniques and equipment such as hydraulic jacks to stress tendons that may not be readily available in developing communities. The recently proposed spring anchored rocking system (SARS) uses a spring at the anchorage of the post-tensioning tendon to facilitate applying force to the tendons using simple techniques such as torque wrenches. The seismic behaviour of the SARS is investigated using finite element models developed in OpenSees. The self-centering lateral force resisting system of a 3-story timber structure was anchored with springs having four different configurations, and subjected to a set of 22-pair ground motions. The finite element models consisted of elastic beam column elements to model the frame members, nonlinear truss element to model the post-tensioning tendon, spring element to model the anchorage of the tendon, and gap element to model the spring closure. It is found that the spring at the post-tensioning anchorage introduces flexibility, reduces the tendon force, and generally increases the rigid body deformations of the SARS compared to conventionally anchored systems using nuts set on bearing plates.

Prevention of strain-induced impairments of patellar tendon micromorphology in adolescent athletes.

High-level patellar tendon strain may cause impairments of the tendon's micromorphological integrity in growing athletes and increase the risk for tendinopathy. This study investigated if an evidence-based tendon exercise intervention prevents high-level patellar tendon strain, impairments of micromorphology and pain in adolescent basketball players (male, 13-15years). At three time points over a season (M1-3), tendon mechanical properties were measured using ultrasound and dynamometry, proximal tendon micromorphology with a spatial frequency analysis and pain and disability using VISA-P scores. The control group (CON, n=19) followed the usual strength training plan, including sprint and change-of-direction drills. In the intervention group (INT, n=14), three sessions per week with functional exercises were integrated into the training, providing repetitive high-magnitude tendon loading for at least 3s per repetition. The frequency of high-level strain (ie, ≥9%) continuously decreased in INT, while tending to increase in CON since tendon force increased in both (p<0.001), yet tendon stiffness only in INT (p=0.004). In CON, tendon strain was inversely associated with tendon peak spatial frequency at all time points (p<0.05), indicating impairments of tendon micromorphological integrity with higher strain, but not at M2 and M3 in INT. Descriptively, the fraction of asymptomatic athletes at baseline was similar in both groups (~70%) and increased to 100% in M3 in INT, while remaining unchanged in CON. We suggest that functional high-load tendon exercises could reduce the prevalence of high-level patellar tendon strain and associated impairments of its micromorphology in adolescent athletes, providing new opportunities for tendinopathy prevention.

Research on seismic behavior of precast self-centering concrete walls with dry slip-friction connectors

A new precast self-centering concrete wall structure is presented, namely a set of slip-friction energy dissipating devices is installed at the wall toe. The opening and closing of the walls' corner joints under earthquakes drives slip-friction connectors to provide frictional energy dissipation capacities. In this paper, the influences of four parameters including bolt's slot length (SL), friction force, connection plate thickness and initial prestress force (fp0) of PT tendons on the seismic performance of the new precast self-centering concrete wall are systematically investigated. The results show that: 1) The bolt's SL has no effect on the initial stiffness and critical load of the wall, but the ultimate lateral-load capacity continues to decrease with the increase of SL. 2) The larger the friction force of the connectors, the worse the self-centering capacity, but the better the energy dissipation capacity of the specimen. 3) The connection plate thickness t has little effect on the self-centering capacity and energy dissipation capacity of wall. However, the wall's ultimate lateral-load capacity increases continuously with the increase of the plate thickness. 4) With the increase of fp0, the self-centering capacity of specimen is significantly improved, but the energy dissipation capacity is reduced. It is necessary to reasonably control the initial prestress level of the PTs during design, which is very important to obtain the balance between the self-centering capacity and the energy dissipation capacity of the wall for the best behavior.

The effect of speed on Achilles tendon forces and patellofemoral joint stresses in high-performing endurance runners.

Achilles tendinopathy and patellofemoral pain are common running injuries associated with increased Achilles tendon (AT) forces and patellofemoral joint (PFJ) stresses. This study examined AT forces and PFJ stresses at different running speeds in high-performing endurance runners. Twenty runners ran overground at four running speeds (3.3, 3.9, 4.8, and 5.6m/s). AT forces and PFJ stresses were estimated from kinematic and kinetic data. Repeated measures ANOVA with partial eta squared effect sizes was conducted to assess differences between running speeds. Increased peak AT forces (19.5%; p<0.001) and loading rates (57.3%; p<0.001) from 3.3m/s to 5.6m/s were observed. Cumulative AT loading was greater in the faster speeds compared to the slower speeds. Faster running speeds resulted in increased peak plantar flexor moments, increased peak plantar flexion angles, and a more flexed knee and an anterior center of pressure position at touchdown. Peak PFJ stress was lower in the slowest speed (3.3m/s) compared to the faster running speeds (3.9-5.6m/s; p=0.005). PFJ stress loading rate significantly increased (43.6%; p<0.001). Greater AT loading observed could be associated with strategies such as increased plantar flexor moments and altered lower body position at touchdown which are commonly employed to generate greater ground contact forces. Greater AT and PFJ loading rates were likely due to shorter ground contact times and therefore less time available to reach the peak. Running at faster speeds could increase the risk of developing Achilles tendinopathy and patellofemoral pain or limit recovery from these injuries without sufficient recovery.

Maximum dorsiflexion increases Achilles tendon force during exercise for midportion Achilles tendinopathy.

Rehabilitation is an important treatment for non-insertional Achilles tendinopathy. To date, eccentric loading exercises (ECC) have been the predominant choice; however, mechanical evidence underlying their use remains unclear. Other protocols, such as heavy slow resistance loading (HSR), have shown comparable outcomes, but with less training time. This study aims to identify the effect of external loading and other variables that influence Achilles tendon (AT) force in ECC and HSR. Ground reaction force and kinematic data during ECC and HSR were collected from 18 healthy participants for four loading conditions. The moment arms of the AT were estimated from MRIs of each participant. AT force then was calculated using the ankle torque obtained from inverse dynamics. In the eccentric phase, the AT force was not larger than in the concentric phase in both ECC and HSR. Under the same external load, the force through the AT was larger in ECC with the knee bent than in HSR with the knee straight due to increased dorsiflexion angle of the ankle. Multivariate regression analysis showed that external load and maximum dorsiflexion angle were significant predictors of peak AT force in both standing and seated positions. Therefore, to increase the effectiveness of loading the AT, exercises should apply adequate external load and reach maximum dorsiflexion during the movement. Peak dorsiflexion angle affected the AT force in a standing position at twice the rate of a seated position, suggesting standing could prove more effective for the same external loading and peak dorsiflexion angle.