Human patellar tendon stiffness is restored following graft harvest for anterior cruciate ligament surgery

Minimising post-operative donor site morbidity is an important consideration when selecting a graft for surgical reconstruction of the torn anterior cruciate ligament (ACL). One of the most common procedures, the bone-patellar tendon-bone (BPTB) graft involves removal of the central third from the tendon. However, it is unknown whether the mechanical properties of the donor site (patellar tendon) recover. The present study investigated the mechanical properties of the human patellar tendon in 12 males (mean±S.D. age: 37±14 years) who had undergone surgical reconstruction of the ACL using a BPTB graft between 1 and 10 years before the study (operated knee; OP). The uninjured contralateral knee served as a control (CTRL). Patellar tendon mechanical properties were assessed in vivo combining dynamometry with ultrasound imaging. Patellar tendon stiffness was calculated from the gradient of the tendon's force–elongation curve. Tendon stiffness was normalised to the tendon's dimensions to obtain the tendon's Young's modulus. Cross-sectional area (CSA) of OP patellar tendons was larger by 21% than CTRL tendons (P<0.01). Patellar tendon stiffness was not significantly different between OP and CTRL tendons, but the Young's modulus was lower by 24% in OP tendons (P<0.01). A compensatory enlargement of the patellar tendon CSA, presumably due to scar tissue formation, enabled a recovery of tendon stiffness in the OP tendons. The newly formed tendon tissue had inferior properties as indicated by the reduced tendon Young's modulus, but it increased to a level that enabled recovery of tendon stiffness.     

Influence of forearm orientation on biceps brachii tendon mechanics and elbow flexor force steadiness

Achilles tendon mechanics influence plantar flexion force steadiness (FS) and balance. In the upper limb, elbow flexor FS is greater in supinated and neutral forearm orientations compared to pronated, with contributions of tendon mechanics remaining unknown in position-dependent FS. This study investigated whether distal biceps brachii (BB) tendon mechanics across supinated, neutral and pronated forearm orientations influence position-dependent FS of the elbow flexors. Eleven males (23 ± 3 years) performed submaximal isometric elbow flexion tasks at low (5, 10% maximal voluntary contraction (MVC)) and high (25, 50, 75% MVC) force levels in supinated, neutral and pronated forearm orientations. Distal BB tendon elongation and CSA were recorded on ultrasound to calculate mechanics of tendon stress, strain and stiffness. Relationships between FS, calculated as coefficient of variation (CV) of force, and tendon mechanics were evaluated with multiple regressions. Supinated and neutral were ∼50% stronger and ∼60% steadier than pronated (p < 0.05). Tendon stress was ∼52% greater in supinated and neutral compared to pronated, tendon strain was ∼36% greater in neutral than pronated (p < 0.05), while tendon stiffness (267.4 ± 78.9 N/mm) did not differ across orientations (p > 0.05). At low forces, CV of force was predicted by MVC (r²: 0.52) in supinated, and MVC and stress in neutral and pronated (r²: 0.65–0.81). At high force levels, CV of force was predicted by MVC and stress in supinated (r²: 0.49), and MVC in neutral (r²: 0.53). Absolute strength and tendon mechanics influence the ability of the BB tendon to distribute forces, and thus are key factors in position-dependent FS.     

In vivo assessment of the interaction of patellar tendon tibial shaft angle and anterior cruciate ligament elongation during flexion

A potential cause of non-contact anterior cruciate ligament (ACL) injury is landing on an extended knee. In line with this hypothesis, studies have shown that the ACL is elongated with decreasing knee flexion angle. Furthermore, at low flexion angles the patellar tendon is oriented to increase the anterior shear component of force acting on the tibia. This indicates that knee extension represents a position in which the ACL is taut, and thus may have an increased propensity for injury, particularly in the presence of excessive force acting via the patellar tendon. However, there is very little in vivo data to describe how patellar tendon orientation and ACL elongation interact during flexion. Therefore, this study measured the patellar tendon tibial shaft angle (indicative of the relative magnitude of the shear component of force acting via the patellar tendon) and ACL length in vivo as subjects performed a quasi-static lunge at varying knee flexion angles. Spearman rho rank correlations within each individual revealed that flexion angles were inversely correlated to both ACL length (rho = −0.94 ± 0.07, mean ± standard deviation, p < 0.05) and patellar tendon tibial shaft angle (rho = −0.99 ± 0.01, p < 0.05). These findings indicate that when the knee is extended, the ACL is both elongated and the patellar tendon tibial shaft angle is increased, resulting in a relative increase in anterior shear force on the tibia acting via the patellar tendon. Therefore, these data support the hypothesis that landing with the knee in extension is a high risk scenario for ACL injury.     

Habitual loading results in tendon hypertrophy and increased stiffness of the human patellar tendon.

The purpose of this study was to examine patellar tendon (PT) size and mechanical properties in subjects with a side-to-side strength difference of >/=15% due to sport-induced loading. Seven elite fencers and badminton players were included. Cross-sectional area (CSA) of the PT obtained from MRI and ultrasonography-based measurement of tibial and patellar movement together with PT force during isometric contractions were used to estimate mechanical properties of the PT bilaterally. We found that distal tendon and PT, but not mid-tendon, CSA were greater on the lead extremity compared with the nonlead extremity (distal: 139 +/- 11 vs. 116 +/- 7 mm(2); mid-tendon: 85 +/- 5 vs. 77 +/- 3 mm(2); proximal: 106 +/- 7 vs. 83 +/- 4 mm(2); P < 0.05). Distal tendon CSA was greater than proximal and mid-tendon CSA on both the lead and nonlead extremity (P < 0.05). For a given common force, stress was lower on the lead extremity (52.9 +/- 4.8 MPa) compared with the nonlead extremity (66.0 +/- 8.0 MPa; P < 0.05). PT stiffness was also higher in the lead extremity (4,766 +/- 716 N/mm) compared with the nonlead extremity (3,494 +/- 446 N/mm) (P < 0.05), whereas the modulus did not differ (lead 2.27 +/- 0.27 GPa vs. nonlead 2.16 +/- 0.28 GPa) at a common force. These data show that a habitual loading is associated with a significant increase in PT size and mechanical properties.     

Influence of acetaminophen and ibuprofen on in vivo patellar tendon adaptations to knee extensor resistance exercise in older adults

Millions of older individuals consume acetaminophen or ibuprofen daily and these same individuals are encouraged to participate in resistance training. Several in vitro studies suggest that cyclooxygenase-inhibiting drugs can alter tendon metabolism and may influence adaptations to resistance training. Thirty-six individuals were randomly assigned to a placebo (67 ± 2 yr old), acetaminophen (64 ± 1 yr old; 4,000 mg/day), or ibuprofen (64 ± 1 yr old; 1,200 mg/day) group in a double-blind manner and completed 12 wk of knee extensor resistance training. Before and after training in vivo patellar tendon properties were assessed with MRI [cross-sectional area (CSA) and signal intensity] and ultrasonography of patellar tendon deformation coupled with force measurements to obtain stiffness, modulus, stress, and strain. Mean patellar tendon CSA was unchanged (P > 0.05) with training in the placebo group, and this response was not influenced with ibuprofen consumption. Mean tendon CSA increased with training in the acetaminophen group (3%, P < 0.05), primarily due to increases in the mid (7%, P < 0.05) and distal (8%, P < 0.05) tendon regions. Correspondingly, tendon signal intensity increased with training in the acetaminophen group at the mid (13%, P < 0.05) and distal (15%, P = 0.07) regions. When normalized to pretraining force levels, patellar tendon deformation and strain decreased 11% (P < 0.05) and stiffness, modulus, and stress were unchanged (P > 0.05) with training in the placebo group. These responses were generally uninfluenced by ibuprofen consumption. In the acetaminophen group, tendon deformation and strain increased 20% (P < 0.05) and stiffness (-17%, P < 0.05) and modulus (-20%, P < 0.05) decreased with training. These data suggest that 3 mo of knee extensor resistance training in older adults induces modest changes in the mechanical properties of the patellar tendon. Over-the-counter doses of acetaminophen, but not ibuprofen, have a strong influence on tendon mechanical and material property adaptations to resistance training. These findings add to a growing body of evidence that acetaminophen has profound effects on peripheral tissues in humans.     

Ultrasound does not provide reliable results for the measurement of the patellar tendon cross sectional area

PurposeThis study examined the reliability of patellar tendon cross sectional area (CSA) measurement using brightness mode ultrasonography.MethodsThe patellar tendon CSA of fourteen participants was examined on two different days and at three different positions (proximal, median and distal). Five trials per day were conducted in each position, replacing the ultrasound probe on every trial. The images were examined by three different and equally experienced observers. We compared the mean of the five trials in each position to examine the day, observer and position effect. Further, Bland and Altman plots, root mean square (RMS) differences and intraclass correlation coefficients (ICC) were calculated.ResultsThere was a significant (p < 0.05) day, observer and position effect on the CSA, while the average ICC was 0.592. The Bland and Altman plots showed that differences between conditions or groups, should be in average lower than 37% or higher than 55% of the patellar tendon CSA to be important for clinical or intervention studies.ConclusionOur findings show low reliability of the method, which resulted from the low clarity and unclear visibility of tissue boundaries in the ultrasound images. Therefore, the measurement of the CSA of the patellar tendon using ultrasound does not provide accurate and reliable results.     

Mechanical properties of the patellar tendon in adults and children

It is not currently known how the mechanical properties of human tendons change with maturation in the two sexes. To address this, the stiffness and Young's modulus of the patellar tendon were measured in men, women, boys and girls (each group, n=10). Patellar tendon force (F_pt) was calculated from the measured joint moment during a ramped voluntary isometric knee extension contraction, the antagonist knee extensor muscle co-activation quantified from its electromyographical activity, and the patellar tendon moment arm measured from magnetic resonance images. Tendon elongation was imaged using the sagittal-plane ultrasound scans throughout the contraction. Tendon cross-sectional area was measured at rest from ultrasound scans in the transverse plane. Maximal F_pt and tendon elongation were (mean±SE) 5453±307 N and 5±0.5 mm for men, 3877±307 N and 4.9±0.6 mm for women, 2017±170 N and 6.2±0.5 mm for boys and 2169±182 N and 5.9±0.7 mm for girls. In all groups, tendon stiffness and Young's modulus were examined at the level that corresponded to the maximal 30% of the weakest participant's F_pt and stress, respectively; these were 925–1321 N and 11.5–16.5 MPa, respectively. Stiffness was 94% greater in men than boys and 84% greater in women than girls (p<0.01), with no differences between men and women, or boys and girls (men 1076±87 N/mm; women 1030±139 N/mm; boys 555±71 N/mm and girls 561.5±57.4 N/mm). Young's modulus was 99% greater in men than boys (p<0.01), and 66% greater in women than girls (p<0.05). There were no differences in modulus between men and women, or boys and girls (men 597±49 MPa; women 549±70 MPa; boys 255±42 MPa and girls 302±33 MPa). These findings indicate that the mechanical stiffness of tendon increases with maturation due to an increased Young's modulus and, in females due to a greater increase in tendon cross-sectional area than tendon length.     

Method and strain rate dependence of Achilles tendon stiffness

Tendon stiffness is calculated by dividing changes in tendon force by tendon elongation. For this purpose, participants are commonly asked to perform a maximal muscle contraction (“active” method). Alternatively tendon elongation can be achieved by means of a passive joint rotation (“passive” method). The purpose of this study was to compare Achilles tendon stiffness obtained from both methods across different tendon strain rates. Twenty adults performed a series of ramped maximum isometric plantarflexions of different durations. Passive ankle rotations of different angular velocities were also performed. Achilles tendon stiffness was obtained from a combination of motion analysis, isokinetic dynamometry and ultrasonography and compared across methods at three strain rates. At all strain rates, tendon stiffness obtained from the active method was 6% greater compared to the passive method. In spite of this systematic bias, there was good agreement between the methods. Intraclass correlation coefficients were greater than 0.98, and more than 95% of data points fell into the 95% confidence intervals. This agreement will be acceptable in many research contexts. We also found a linear increase in tendon stiffness with increasing strain rate, which must be taken into consideration when interpreting or reporting tendon stiffness.     

Onset of neonatal locomotor behavior and the mechanical development of Achilles and tail tendons

Tendon tissue engineering approaches are challenged by a limited understanding of the role mechanical loading plays in normal tendon development. We propose that the increased loading that developing postnatal tendons experience with the onset of locomotor behavior impacts tendon formation. The objective of this study was to assess the onset of spontaneous weight-bearing locomotion in postnatal day (P) 1, 5, and 10 rats, and characterize the relationship between locomotion and the mechanical development of weight-bearing and non-weight-bearing tendons. Movement was video recorded and scored to determine non-weight-bearing, partial weight-bearing, and full weight-bearing locomotor behavior at P1, P5, and P10. Achilles tendons, as weight-bearing tendons, and tail tendons, as non-weight-bearing tendons, were mechanically evaluated. We observed a significant increase in locomotor behavior in P10 rats, compared to P1 and P5. We also found corresponding significant differences in the maximum force, stiffness, displacement at maximum force, and cross-sectional area in Achilles tendons, as a function of postnatal age. However, the maximum stress, strain at maximum stress, and elastic modulus remained constant. Tail tendons of P10 rats had significantly higher maximum force, maximum stress, elastic modulus, and stiffness compared to P5. Our results suggest that the onset of locomotor behavior may be providing the mechanical cues regulating postnatal tendon growth, and their mechanical development may proceed differently in weight-bearing and non-weight-bearing tendons. Further analysis of how this loading affects developing tendons in vivo may inform future engineering approaches aiming to apply such mechanical cues to regulate engineered tendon formation in vitro.     

The effect of tendon loading on in-vitro carpal kinematics of the wrist joint

Measurements of in-vitro carpal kinematics of the wrist provide valuable biomechanical data. Tendon loading is often applied during cadaver experiments to simulate natural stabilizing joint compression in the wrist joint. The purpose of this study was to investigate the effect of tendon loading on carpal kinematics in-vitro.A cyclic movement was imposed on 7 cadaveric forearms while the carpal kinematics were acquired by a 4-dimensional rotational X-ray imaging system. The extensor- and flexor tendons were loaded with constant force springs of 50 N, respectively. The measurements were repeated without a load on the tendons. The effect of loading on the kinematics was tested statistically by using a linear mixed model.During flexion and extension, the proximal carpal bones were more extended with tendon loading. The lunate was on the average 2.0° (p=0.012) more extended. With tendon loading the distal carpal bones were more ulnary deviated at each angle of wrist motion. The capitate was on the average 2.4° (p=0.004) more ulnary deviated.During radioulnar deviation, the proximal carpal bones were more radially deviated with the lunate 0.7° more into radial deviation with tendon loading (p<0.001). Conversely, the bones of distal row were more flexed and supinated with the capitate 1.5° more into flexion (p=0.025) and 1.0° more into supination (p=0.011).In conclusion, the application of a constant load onto the flexor and extensor tendons in cadaver experiments has a small but statistically significant effect on the carpal kinematics during flexion–extension and radioulnar deviation.     

Tensile properties of the in vivo human gastrocnemius tendon

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

Influence of 90-day simulated microgravity on human tendon mechanical properties and the effect of resistive countermeasures.

While microgravity exposure is known to cause deterioration of skeletal muscle performance, little is known regarding its effect on tendon structure and function. Hence, the aims of this study were to investigate the effects of simulated microgravity on the mechanical properties of human tendon and to assess the effectiveness of resistive countermeasures in preventing any detrimental effects. Eighteen men (aged 25-45 yr) underwent 90 days of bed rest: nine performed resistive exercise during this period (BREx group), and nine underwent bed rest only (BR group). Calf-raise and leg-press exercises were performed every third day using a gravity-independent flywheel device. Isometric plantar flexion contractions were performed by using a custom-built dynamometer, and ultrasound imaging was used to determine the tensile deformation of the gastrocnemius tendon during contraction. In the BR group, tendon stiffness estimated from the gradient of the tendon force-deformation relation decreased by 58% (preintervention: 124 +/- 67 N/mm; postintervention: 52 +/- 28 N/mm; P < 0.01), and the tendon Young's modulus decreased by 57% postintervention (P < 0.01). In the BREx group, tendon stiffness decreased by 37% (preintervention: 136 +/- 66 N/mm; postintervention: 86 +/- 47 N/mm; P < 0.01), and the tendon Young's modulus decreased by 38% postintervention (P < 0.01). The relative decline in tendon stiffness and Young's modulus was significantly (P < 0.01) greater in the BR group compared with the BREx group. Unloading decreased gastrocnemius tendon stiffness due to a change in tendon material properties, and, although the exercise countermeasures did attenuate these effects, they did not completely prevent them. It is suggested that the total loading volume was not sufficient to completely prevent alterations in tendon mechanical properties.     

Biceps femoris and semitendinosus tendon/aponeurosis strain during passive and active (isometric) conditions

The purpose of this study was to quantify strain and elongation of the long head of the biceps femoris (BFlh) and the semitendinosus (ST) tendon/aponeurosis. Forty participants performed passive knee extension trials from 90° of knee flexion to full extension (0°) followed by ramp isometric contractions of the knee flexors at 0°, 45° and 90° of knee flexion. Two ultrasound probes were used to visualize the displacement of BFlh and ST tendon/aponeurosis. Three-way analysis of variance designs indicated that: (a) Tendon/aponeurosis (passive) elongation and strain were higher for the BFlh than the ST as the knee was passively extended (p < 0.05), (b) contraction at each angular position was accompanied by a smaller BFlh tendon/aponeurosis (active) strain and elongation than the ST at higher levels of effort (p < 0.05) and (c) combined (passive and active) strain was significantly higher for the BFlh than ST during ramp contraction at 0° but the opposite was observed for the 45° and 90° flexion angle tests (p < 0.05). Passive elongation of tendon/aponeurosis has an important effect on the tendon/aponeurosis behavior of the hamstrings and may contribute to a different loading of muscle fibers and tendinous tissue between BFlh and ST.     

Bone adaptation to altered loading after spinal cord injury: a study of bone and muscle strength

Bone loss from the paralysed limbs after spinal cord injury (SCI) is well documented. Under physiological conditions, bones are adapted to forces which mainly emerge from muscle pull. After spinal cord injury (SCI), muscles can no longer contract voluntarily and are merely activated during spasms. Based on the Ashworth scale, previous research has suggested that these spasms may mitigate bone losses. We therefore wished to assess muscle forces after SCI with a more direct measure and compare it to measures of bone strength. We hypothesized that the bones in SCI patients would be in relation to the loss of muscle forces. Six male patients with SCI 6.4 (SD 4.3) years earlier and 6 age-matched, able-bodied control subjects were investigated. Bone scans from the right knee were obtained by pQCT. The knee extensor muscles were electrically stimulated via the femoral nerve, isometric knee extension torque was measured and patellar tendon force was estimated. Tendon force upon electrical stimulation in the SCI group was 75% lower than in the control subjects (p<0.01). Volumetric bone mineral density of the patella and of the proximal tibia epiphysis were 50% lower in the SCI group than in the control subjects (p<0.01). Cortical area was lower by 43% in the SCI patients at the proximal tibia metaphysis, and by 33% at the distal femur metaphysis. No group differences were found in volumetric cortical density. Close curvilinear relationships were found between stress and volumetric density for the tibia epiphysis (r(2)=0.90) and for the patella (r(2)=0.91). A weaker correlation with the tendon force was found for the cortical area of the proximal tibia metaphysis (r(2)=0.63), and none for the distal femur metaphysis. These data suggest that, under steady state conditions after SCI, epiphyseal bones are well adapted to the muscular forces. For the metaphysis of the long bones, such an adaptation appears to be less evident. The reason for this remains unclear.     

Non-uniformity in the healthy patellar tendon is greater in males and similar in different age groups

There is increasing evidence that tendons are heterogeneous and take advantage of structural mechanisms to enhance performance and reduce injury. Fascicle-sliding, for example, is used by energy-storing tendons to enable them to undergo large extensions while protecting the fascicles from damage. Reductions in fascicle-sliding capacity may thus predispose certain populations to tendinopathy. Evidence from the Achilles tendon of significant superficial-to-deep non-uniformity that is reduced with age supports this theory. Similar patellar tendon non-uniformity has been observed, but the effects of age and sex have yet to be assessed. Healthy adults (n = 50, 25M/25F) from a broad range of ages (23–80) were recruited and non-uniformity was quantified using ultrasound speckle-tracking during passive knee extension. Significant superficial-to-deep non-uniformity and proximal/distal variations were observed. No effect of age was found, but males exhibited significantly greater non-uniformity than females (p < 0.05). The results contrast with previous findings in the Achilles tendon; in this study, tendons and tendon regions at high risk for tendinopathy (i.e. males and proximal regions, respectively) exhibited greater non-uniformity, whereas high-risk Achilles tendons (i.e. older adults) previously showed reduced non-uniformity. This suggests that non-uniformity may be dominated by factors other than fascicle-sliding. Anatomically, the varied proximal attachment of the patellar tendon may influence non-uniformity, with quadriceps passive resistance limiting superficial tendon movement, thus linking flexibility, non-uniformity and injury risk. This study also provides evidence of a differential effect of aging on the patellar tendon compared with evidence from prior studies on other tendons necessitating further study to elucidate links between non-uniformity and injury.     

Effect of muscle contraction levels on the force-length relationship of the human Achilles tendon during lengthening of the triceps surae muscle-tendon unit

Findings from animal experiments are sometimes contradictory to the idea that the tendon structure is a simple elastic spring in series with muscle fibers, and suggest influence of muscle contraction on the tendon mechanical properties. The purpose of the present study was to investigate the influence of muscle contraction levels on the force-length relationship of the human Achilles tendon during lengthening of the triceps surae muscle-tendon unit. For seven subjects, ankle dorsiflexion was performed without (passive condition) and with contraction of plantar flexor muscles (eccentric conditions, at 3 contraction levels) on an isokinetic dynamometer. Deformation of the Achilles tendon during each trial was measured using ultrasonography. The Achilles tendon force corresponding to the tendon elongation of 10mm in the passive condition was significantly smaller than those in the eccentric conditions (p<0.05 or p<0.01). Within the eccentric conditions, the Achilles tendon force corresponding to the tendon elongation of 10mm was significantly greater in the maximal contraction level than those in submaximal eccentric conditions (p<0.05 or p<0.01). In addition, the tendon stiffness was greater in higher contraction levels (p<0.05 or p<0.01). Present results suggest that the human tendon structure is not a simple elastic spring in series with muscle fibers.     

Modeling of the knee joint load in rehabilitative knee extension exercises under water

A biomechanical and hydrodynamic theoretical model has been developed in order to calculate the knee joint load during underwater knee extension exercises. The hydrodynamic force has been evaluated within the framework of a strip-theory approach, when a blunt rectangular resistive device is applied proximally to the shank to increase its frontal area. Analytical expressions of the patellar tendon force (F(PT)), the axial (phi(n)) and the shear (phi(t)) component of the tibiofemoral joint load have been derived as a function of joint angle (theta), angular velocity (theta ), angular acceleration (theta ), resistive device density, length (L(x)), width (L(z)) and thickness, and average hydrodynamic drag and added mass coefficients. An inverse dynamic problem has been solved, assuming for theta and theta a dependence on theta consistent with the experimental kinematic data available in the literature. The results highlight that the characteristics of the resistive device and the level of muscular activation can be adjusted reciprocally in order to control the peak value of F(PT), phi(n) and phi(t), and the position of these peaks within the joint range of motion (ROM). No anterior cruciate ligament (ACL) stress is observed (phi(t)>0) over the whole ROM, independent of the level of muscular activation, for a light resistive device with L(x) < or = 0.3 m and L(z) < or = 0.4 m. This work highlights that aquatic exercises can be usefully and safely implemented in the rehabilitation program following ACL surgery, and whenever it is important to avoid excessive shear joint forces that constrain the tibial plateau anterior translation with respect to the femur.     

Influence of aging on the in vivo properties of human patellar tendon

Tendons are important for optimal muscle force transfer to bone and play a key role in functional ability. Changes in tendon properties with aging could contribute to declines in physical function commonly associated with aging. We investigated the in vivo mechanical properties of the patellar tendon in 37 men and women [11 young (27 +/- 1 yr) and 26 old (65 +/- 1 yr)] using ultrasonography and magnetic resonance imaging (MRI). Patella displacement relative to the tibia was monitored with ultrasonography during ramped isometric contractions of the knee extensors, and MRI was used to determine tendon cross-sectional area (CSA) and signal intensity. At peak force, patellar tendon deformation, stress, and strain were 13 (P = 0.05), 19, and 12% less in old compared with young (P < 0.05). Additionally, deformation, stiffness, stress, CSA, and length were 18, 35, 41, 28, and 11% greater (P < 0.05), respectively, in men compared with women. After normalization of mechanical properties to a common force, no age differences were apparent; however, stress and strain were 26 and 22% higher, respectively, in women compared with men (P < 0.05). CSA and signal intensity decreased 12 and 24%, respectively, with aging (P < 0.05) in the midregion of the tendon. These data suggest that differences in patellar tendon in vivo mechanical properties with aging are more related to force output rather than an age effect. In contrast, the decrease in signal intensity indirectly suggests that the internal milieu of the tendon is altered with aging; however, the physiological and functional consequence of this finding requires further study.     

Mechanical properties of the triceps surae tendon and aponeurosis in relation to intensity of sport activity

The purpose of the present study was to investigate whether the mechanical properties (i.e. force strain relationship) of the triceps surae tendon and aponeurosis relate to the performed sport activity in an intensity-dependent manner. This was done by comparing sprinters with endurance runners and subjects not active in sports. Sixty-six young male subjects (26+/-5 yr; 183+/-6 cm; 77.6+/-6.7 kg) participated in the study. Ten of these subjects were adults not active in sports, 28 were endurance runners and 28 sprinters. All subjects performed isometric maximal voluntary plantar flexion contractions (MVC) on a dynamometer. The distal aponeuroses of the gastrocnemius medialis (GM) was visualised by ultrasound during the MVC. The results showed that only the sprinters had higher normalised stiffness (relationship between tendon force and tendon strain) of the triceps surae tendon and aponeurosis and maximal calculated tendon forces than the endurance runners and the subjects not active in sports. Furthermore, including the data of all 66 examined participants tendon stiffness correlated significantly (r=0.817, P<0.001) with the maximal tendon force achieved during the MVC. It has been concluded that the mechanical properties of the triceps surae tendon and aponeurosis do not show a graded response to the intensity of the performed sport activity but rather remain at control level in a wide range of applied strains and that strain amplitude and/or frequency should exceed a given threshold in order to trigger additional adaptation effects. The results further indicate that subjects with higher muscle strength possibly increase the margin of tolerated mechanical loading of the tendon due to the greater stiffness of their triceps surae tendon and aponeurosis.