Changes in Achilles tendon moment arm from rest to maximum isometric plantarflexion: in vivo observations in man

Achilles tendinopathy has an aberrant strain response to eccentric exercise

PURPOSE

Eccentric exercise has become the treatment of choice for Achilles tendinopathy. However, little is known about the acute response of tendons to eccentric exercise or the mechanisms underlying its clinical benefit. This research evaluated the sonographic characteristics and acute anteroposterior (AP) strain response of control (healthy), asymptomatic, and symptomatic Achilles tendons to eccentric exercise.

METHODS

Eleven male adults with unilateral midportion Achilles tendinopathy and nine control male adults without tendinopathy participated in the research. Sagittal sonograms of the Achilles tendon were acquired immediately before and after completion of a common eccentric rehabilitation exercise protocol and again 24 h later. Tendon thickness, echogenicity, and AP strain were determined 40 mm proximal to the calcaneal insertion.

RESULTS

Compared with the control tendon, both the asymptomatic and symptomatic tendons were thicker (P < 0.05) and hypoechoic (P < 0.05) at baseline. All tendons decreased in thickness immediately after eccentric exercise (P < 0.05). The symptomatic tendon was characterized by a significantly lower AP strain response to eccentric exercise compared with both the asymptomatic and control tendons (P < 0.05). AP strains did not differ in the control and asymptomatic tendons. For all tendons, preexercise thickness was restored 24 h after exercise completion.

CONCLUSIONS

These observations support the concept that Achilles tendinopathy is a bilateral or systemic process and structural changes associated with symptomatic tendinopathy alter fluid movement within the tendon matrix. Altered fluid movement may disrupt remodeling and homeostatic processes and represents a plausible mechanism underlying the progression of tendinopathy.

In vivo measurements of moment arm lengths of three elbow flexors at rest and during isometric contractions

The purpose of this study was to determine in vivo moment arm lengths (MAs) of three elbow flexors at rest and during low- and relatively high-intensity contractions, and to examine the contraction intensity dependence of MAs at different joint positions. At 50°, 80° and 110° of elbow flexion, MAs of the biceps brachii, brachialis and brachioradialis were measured in 10 young men using sagittal images of the right arm obtained by magnetic resonance imaging, at rest and during 20% and 60% of isometric maximal voluntary elbow flexion. In most conditions, MAs increased with isometric contractions, which is presumably due to the contraction-induced thickening of the muscles. This phenomenon was especially evident in the flexed elbow positions. The influence of the contraction intensities on the increases in MAs varied across the muscles. These results suggest that in vivo measurements of each elbow flexor MA during contractions are essential to properly examine the effects on the interrelationships between elbow flexion torque and individual muscle forces.

Use of a three-curved rocker sole shoe modification to improve intermittent claudication calf pain--a pilot study

This was a pilot study utilizing hospital-based walking trials to compare two footwear conditions. Two pairs of identical therapeutic shoes were ordered for volunteer claudicant subjects. One pair was adapted with a specifically designed three-curve rocker sole. Volunteer claudicant subjects (n = 8, mean age 66 +/- 9.9 years) with stable pain-free distances of 10-400 meters to calf claudication pain were recruited into the study. Walking trials were used to compare differences in both the pain-free distance to claudication and the intensity of the calf pain once claudicating exhibited by each subject while walking separately with the two footwear conditions during the same clinical session. The results demonstrated that claudicant pain-free walking distance was increased, and the intensity of claudication calf pain reduced when walking with the rocker-soled in comparison with baseline. A specifically designed rocker sole has the potential to offer a reduction in the intensity of calf pain experienced by claudicants. A more extensive study is planned to determine the efficacy of this new footwear adaptation when the footwear is worn for extended periods.

The minimum required muscle force for a sit-to-stand task

The purpose of this study was to reveal the minimum required muscle force for a sit-to-stand task. Combining experimental procedures and computational processing, movements of various sit-to-stand patterns were obtained. Muscle forces and activations during a movement were calculated with an inverse dynamics method and a static numerical optimization method. The required muscle force for each movement was calculated with peak muscle activation, muscle physiological cross sectional area and specific tension. The robustness of the results was quantitatively evaluated with sensitivity analyses. From the results, a distinct threshold was found for the total required muscle force of the hip and knee extensors. Specifically, two findings were revealed: (1) the total force of hip and knee extensors is appropriate as the index of minimum required muscle force for a sit-to-stand task and (2) the minimum required total force is within the range of 35.3-49.2 N/kg. A muscle is not mechanically independent from other muscles, since each muscle has some synergetic or antagonistic muscles. This means that the mechanical threshold of one muscle varies with the force exertion abilities of other muscles and cannot be evaluated independently. At the same time, some kinds of mechanical threshold necessarily exist in the sit-to-stand task, since a muscle force is an only force to drive the body and people cannot stand up from a chair without muscles. These indicate that the existence of the distinct threshold in the result of the total required muscle force is reasonable.

Ankle joint mechanics and foot proportions differ between human sprinters and non-sprinters

Recent studies of sprinters and distance runners have suggested that variations in human foot proportions and plantarflexor muscle moment arm correspond to the level of sprint performance or running economy. Less clear, however, is whether differences in muscle moment arm are mediated by altered tendon paths or by variation in the centre of ankle joint rotation. Previous measurements of these differences have relied upon assumed joint centres and measurements of bone geometry made externally, such that they would be affected by the thickness of the overlying soft tissue. Using magnetic resonance imaging, we found that trained sprinters have shorter plantarflexor moment arms (p = 0.011) and longer forefoot bones (p = 0.019) than non-sprinters. The shorter moment arms of sprinters are attributable to differences in the location of the centre of rotation (p < 0.001) rather than to differences in the path of the Achilles tendon. A simple computer model suggests that increasing the ratio of forefoot to rearfoot length permits more plantarflexor muscle work during plantarflexion that occurs at rates expected during the acceleration phase following the sprint start.

Implications of Muscle Relative Position as a Co-Determinant of Isometric Muscle Force

Force is transmitted from muscle fiber to bone via several pathways: (1) via the tendons (i.e. myotendinous force transmission), (2) via intermuscular connective tissue to adjacent muscles (i.e. intermuscular myofascial force transmission), (3) via structures other than muscles (i.e. extramuscular myofascial force transmission). In vivo, the position of a muscle relative to adjacent muscles changes due to differences in moment arm between synergists as well as due to the fact that some muscles span only one joint and other muscles more than one joint. The position of a muscle relative to non-muscular structures within a compartment is altered with each change of the length of the muscle.

The aim of this article is to describe recent experimental results, as well as some new experimental data, that have elucidated the role of muscle relative position on force transmission from muscle. Furthermore, relevant literature is discussed, taking into consideration these new insights of muscle functioning. It is concluded that the position of a muscle relative to surrounding tissues is a major co-determinant of isometric muscle force. For muscles operating within their in vivo context of connective tissue, such position effects should be taken into account.

Length changes of human tibialis anterior central aponeurosis during passive movements and isometric, concentric, and eccentric contractions

The behavior of aponeuroses during voluntary contractions is still poorly understood and results provided in the literature are controversial. Therefore, the aim of this study was to investigate the behavior of the tibialis anterior aponeurosis during passive movements and active isometric, concentric, and eccentric contractions in vivo. Based on previous findings, we hypothesized that the aponeurosis exhibits behavior that is not consistent with a serial alignment with the contractile element of the muscle. Nine subjects participated in the study and performed contractions on a Biodex-dynamometer. Two ultrasound probes were used to visualize the proximal and distal ends of the tibialis anterior aponeurosis from which length changes were calculated. The main findings were that: (1) During isometric contractions, aponeurosis lengths increased and decreased with increasing and decreasing forces by about 2.8 (±1.5) mm while tendon length changes were much greater and averaged 15 (±3.3) mm, (2) during passive movements, aponeurosis lengths did not change significantly, and (3) despite similar average torque changes during concentric and eccentric contractions, aponeurosis lengths changed by 2.2 mm in concentric but only by 1.2 mm in eccentric contractions. The results of this study did not provide clear evidence for or against a serial alignment of the aponeurosis with the contractile elements. However, at low activation levels during force ramp contractions, there was a small but consistent shortening of aponeuroses with increasing torque. Our findings suggest that aponeurosis length changes do not depend on force alone but depend critically on activation level and the type of contraction.

Morphological and mechanical properties of muscle and tendon in highly trained sprinters

The purpose of this study was to investigate muscle and tendon properties in highly trained sprinters and their relations to running performance. Fifteen sprinters and 15 untrained subjects participated in this study. Muscle thickness and tendon stiffness of knee extensors and plantar flexors were measured. Sprinter muscle thickness was significantly greater than that of the untrained subjects for plantar flexors, but not for knee extensors (except for the medial side). Sprinter tendon stiffness was significantly lower than that of the untrained subjects for knee extensors, but not for plantar flexors. The best official record of a 100-m race was significantly correlated to the muscle thickness of the medial side for knee extensors. In conclusion, the tendon structures of highly trained sprinters are more compliant than those of untrained subjects for knee extensors, but not for plantar flexors. Furthermore, a thicker medial side of knee extensors was associated with greater sprinting performance.

Can Achilles tendon moment arm be predicted from anthropometric measures in pre-pubescent children?

Muscle-tendon moment arm magnitudes are essential variables for accurately calculating muscle forces from joint moments. Their measurement requires specialist knowledge and expensive resources. Research has shown that the patellar tendon moment arm length is related to leg anthropometry in children. Here, we asked whether the Achilles tendon moment arm (MA(AT)) can be accurately predicted in pre-pubescent children from surface anthropometry. Age, standing height, mass, foot length, inter-malleolar ankle width, antero-posterior ankle depth, tibial length, lower leg circumference, and distances from the calcaneus to the distal head of the 1st metatarsal and medial malleolus were determined in 49 pre-pubescent children. MA(AT) was calculated at three different ankle positions (neutral, 10° plantarflexion, and 10° dorsiflexion) by differentiating tendon excursion, measured via ultrasonography, with respect to ankle angle change using seven different differentiation techniques. Backwards stepwise regression analyses were performed to identify predictors of MA(AT.) When all variables were included, the regression analysis accounted for a maximum of 49% of MA(AT) variance at the neutral ankle angle when a third-order polynomial was used to differentiate tendon excursion with respect to ankle angle. For this condition, foot length and the distance between calcaneus and 1st metatarsal were the only significant predictors, accounting for 47% of the variance (p<0.05). The absolute error associated with this regression model was 3.8±4.4 mm, which would result in significant error (mean=14.5%) when estimating muscle forces from joint moments. We conclude that MA(AT) cannot be accurately predicted from anthropometric measures in children.

Direct comparison of in vivo Achilles tendon moment arms obtained from ultrasound and MR scans

Accurate and reliable estimation of muscle moment arms is a prerequisite for the development of musculoskeletal models. Numerous techniques are available to estimate the Achilles tendon moment arm in vivo. The purposes of this study were 1) to compare in vivo Achilles tendon moment arms obtained using the center of rotation (COR) and tendon excursion (TE) methods and 2) to assess the reliability of each method. For the COR method, magnetic resonance (MR) images from nine participants were obtained at ankle angles of −15°, 0°, and +15° and analyzed using Reuleaux' method. For the TE method, the movement of the gastrocnemius medialis-Achilles tendon junction was recorded using ultrasonography as the ankle was passively rotated through its range of motion. The Achilles tendon moment arm was obtained by differentiation of tendon displacement with respect to ankle angular excursion using seven different differentiation techniques. Moment arms obtained using the COR method were significantly greater than those obtained using the TE method ( P < 0.01), but results from both methods were well correlated. The coefficient of determination between moment arms derived from the COR and TE methods was highest when tendon displacement was linearly differentiated over a ±10° interval ( R2 = 0.94). The between-measurement coefficient of variation was 3.9% for the COR method and 4.5–9.7% for the TE method, depending on the differentiation technique. The high reliabilities and strong relationship between methods demonstrate that both methods are robust against their limitations. The large absolute between-method differences (∼25–30%) in moment arms have significant implications for their use in musculoskeletal models.

Plasticity of human Achilles tendon mechanical and morphological properties in response to cyclic strain

The purpose of the current study in combination with our previous published data (Arampatzis et al., 2007) was to examine the effects of a controlled modulation of strain magnitude and strain frequency applied to the Achilles tendon on the plasticity of tendon mechanical and morphological properties. Eleven male adults (23.9 ± 2.2 yr) participated in the study. The participants exercised one leg at low magnitude tendon strain (2.97 ± 0.47%), and the other leg at high tendon strain magnitude (4.72 ± 1.08%) of similar frequency (0.5 Hz, 1s loading, 1s relaxation) and exercise volume (integral of the plantar flexion moment over time) for 14 weeks, 4 days per week, 5 sets per session. The exercise volume was similar to the intervention of our earlier study (0.17 Hz frequency; 3s loading, 3s relaxation) allowing a direct comparison of the results. Before and after the intervention ankle joint moment has been measured by a dynamometer, tendon-aponeurosis elongation by ultrasound and cross-sectional area of the Achilles tendon by magnet resonance images (MRI). We found a decrease in strain at a given tendon force, an increase in tendon-aponeurosis stiffness and tendon elastic modulus of the Achilles tendon only in the leg exercised at high strain magnitude. The cross-sectional area (CSA) of the Achilles tendon did not show any statistically significant (P > 0.05) differences to the pre-exercise values in both legs. The results indicate a superior improvement in tendon properties (stiffness, elastic modulus and CSA) at the low frequency (0.17 Hz) compared to the high strain frequency (0.5 Hz) protocol. These findings provide evidence that the strain magnitude applied to the Achilles tendon should exceed the value, which occurs during habitual activities to trigger adaptational effects and that higher tendon strain duration per contraction leads to superior tendon adaptational responses.

Mechanical properties of the achilles tendon aponeurosis are altered in athletes with achilles tendinopathy

BACKGROUND

Achilles tendinopathy is a considerable problem for active people. The degenerative processes associated with tendinopathy may be associated with changes in the inherent mechanical properties of the musculotendinous unit.

PURPOSE

The purpose of this study was to compare Achilles tendon-aponeurosis strain between male athletes with and without Achilles tendinopathy.

STUDY DESIGN

Cross-sectional study; Level of evidence, 3.

METHODS

Fifteen healthy men (age, 35 +/- 9 years; height, 1.78 +/- 0.05 m; mass, 79 +/- 11 kg) and 14 men with midportion Achilles tendinopathy (age, 40 +/- 8 years; height, 1.77 +/- 0.06 m; mass, 80 +/- 9 kg) who were all running over 20 km per week participated in the study. Each participant was tested in a single session that involved maximal isometric plantar flexion efforts being performed on a calf-raise apparatus while synchronous real-time ultrasonography of the triceps surae aponeurosis was recorded. Achilles tendon-aponeurosis strain (%) was calculated by dividing tendon displacement during plantar flexion by resting tendon length (intrarater reliability: intraclass correlation coefficient = .92).

RESULTS

Participants in the Achilles tendinopathy group (5.2% +/- 2.6%) had significantly (P = .039) higher Achilles tendon-aponeurosis strain compared with the control group (3.4% +/- 1.8%). In contrast, there were no significant between-group differences for maximal isometric plantar flexion force.

CONCLUSION

Achilles tendon-aponeurosis strain is higher in male athletes with tendinopathy than those without. The results of this study provide a rationale for current clinical approaches to management of Achilles tendinopathy, whereby repetitive mechanical loading may impart a positive benefit through reduced compliance of the musculotendinous unit.

Built for speed: musculoskeletal structure and sprinting ability

The musculoskeletal structure of the foot and ankle has the potential to influence human sprinting performance in complex ways. A large Achilles'tendon moment arm improves the mechanical advantage of the triceps surae but also produces larger shortening velocity during rapid plantarflexion, which detracts from the force-generating capacity of the plantarflexors. The lever arm of the ground reaction force that resists the muscular plantarflexor moment during propulsive push-off is constrained in part by the skeletal structure of the foot. In this study, we measured the plantarflexion moment arms of the Achilles' tendon, lateral gastrocnemius fascicle lengths and pennation angles, and anthropometric characteristics of the foot and lower leg in collegiate sprinters and height-matched non-sprinters. The Achilles' tendon moment arms of the sprinters were 25% smaller on average in sprinters than in non-sprinters (P&lt;0.001) whereas the sprinters' fascicles were 11%longer on average (P=0.024). The ratio of fascicle length to moment arm was 50% larger in sprinters (P&lt;0.001). Sprinters were found to have longer toes (P=0.032) and shorter lower legs (P=0.026)than non sprinters. A simple computer simulation of the sprint push-off demonstrated that shorter plantarflexor moment arms and longer toes, like those measured in sprinters, permit greater generation of forward impulse. Simulated propulsion was enhanced in both cases by increasing the `gear ratio'of the foot, thus maintaining plantarflexor fibre length and reducing peak fibre shortening velocity. Longer toes especially prolonged the time of contact, giving greater time for forward acceleration by propulsive ground reaction force.

Tendinopathy alters mechanical and material properties of the Achilles tendon

The purpose of this study was to investigate the in vivo material and mechanical properties of the human Achilles tendon in the presence of tendinopathy. Real-time ultrasound imaging and dynamometry were used to assess Achilles tendon stiffness, Young's modulus, stress, strain, and cross-sectional area (CSA) in 12 individuals with Achilles tendinopathy and 12 age- and gender-matched controls. The results of this study suggest that tendinopathy weakens the mechanical and material properties of the tendon. Tendinopathic tendons had greater CSA, lower tendon stiffness, and lower Young's modulus. These alterations in mechanical characteristics may put the Achilles tendon at a higher risk to sustain further injury and prolong the time to recovery. Results from this study may be used to design treatment strategies that specifically target these deficits, leading to faster and permanent recovery from tendinopathy.

Age-related deficit in dynamic stability control after forward falls is affected by muscle strength and tendon stiffness

The purpose of the work was to determine whether the age-related muscle weakness diminishes older adults' ability to use mechanisms responsible for maintaining dynamic stability after forward falls. Nine older and nine younger adults participated in this study. To analyse the capacities of the leg-extensor muscle-tendon units, all subjects performed isometric maximal voluntary plantarflexion and knee extension contractions on a dynamometer. The elongation of the gastrocnemius medialis and the vastus lateralis tendon and aponeuroses during isometric contraction was examined by ultrasonography. Recovery behaviour was determined after a sudden fall from two forward-inclined lean angles. Compared to older adults, younger adults had higher muscle strength and tendon stiffness. Younger adults created a higher margin of stability compared to older, independent of perturbation intensity. The main mechanism improving the margin of dynamic stability was the increase of the base of support. The results, further, demonstrated that the locomotion strategy employed before touchdown affects the stability of the stance phase and that muscle strength and tendon stiffness contributed significantly to stability control. We concluded that, to reduce the risk of falls, older individuals may benefit from muscle-tendon unit strengthening programs as well as from interventions exercising the mechanisms responsible for dynamic stability.

Effects of estrogen on the mechanical behavior of the human Achilles tendon in vivo

The purpose of this study was to elucidate the effect of normal fluctuating [non-monophasic oral contraceptive pill (MOCP) users] and low, consistent (MOCP users) endogenous plasma estrogen levels on the strain behavior of the Achilles tendon in vivo. Twenty women (age 28.0 +/- 4.2 yr, height 1.67 +/- 0.07 m, mass 61.6 +/- 6.8 kg) who had been using the MOCP for at least 12 mo together with 20 matched women who were non-MOCP users (age 31.9 +/- 7.3 yr, height 1.63 +/- 0.05 m, mass 62.5 +/- 5.9 kg) participated in this study. Non-MOCP users were tested at the time of lowest (menstruation) and highest (approximately same as ovulation) estrogen, whereas MOCP users, who exhibited constant and attenuated endogenous estrogen levels, were tested at day 1 and day 14 of their cycle. At each test session, maximal isometric plantarflexion efforts were performed on a calf-raise apparatus while synchronous real-time ultrasonography of the triceps surae aponeurosis was recorded. Achilles tendon strain (%) was calculated by dividing tendon displacement during plantarflexion by resting tendon length. Repeated-measures ANOVA revealed a significant (P < 0.05) main effect of subject group with significantly lower Achilles strain (25.5%) in the MOCP users compared with the non-MOCP users. In conclusion, acute fluctuations in plasma estrogen across the menstrual cycle in non-MOCP users did not alter the strain behavior of the Achilles tendon. Conversely, long-term exposure to attenuated estrogen in MOCP users resulted in a decrease in Achilles tendon strain, which is thought to be attributed to the effects of endogenous estrogen on collagen synthesis. These findings have a number of important functional and clinical implications.

Effect of joint rotation correction when measuring elongation of the gastrocnemius medialis tendon and aponeurosis

It is well known that during maximal plantar flexion contractions the ankle joint rotation overestimates the actual elongation of the tendon and aponeurosis. The aim of this study was to examine the influence of the curve length changes of the Achilles tendon on the joint rotation corrected elongation and strain of the gastrocnemius medialis (GM) tendon and aponeurosis. Nine subjects (age: 29.4+/-5.7 years, body mass: 78.8+/-6.8 kg, body height: 178+/-4 cm) participated in the study. The subjects performed maximal voluntary isometric plantarflexion contractions in the prone position on a Biodex-dynamometer. Ultrasonography (Aloka SSD 4000) was used to visualize the muscle belly of the GM muscle-tendon unit. To calculate the curve length changes of the Achilles tendon its surface contour was reconstructed using a series of small reflective skin markers having a diameter of 2.5mm. The elongation of the GM tendon and aponeurosis was calculated (a) as the difference of the measured and the passive (due to joint rotation) displacement of the tendon and aponeurosis and (b) as the difference of the measured displacement and the length changes of the reconstructed Achilles tendon surface contour. The absolute difference between the elongation obtained by both methods were 1.2+/-0.4mm. These differences were due to the higher changes in length obtained by the reconstruction of the tendon curved surface contour as compared to the changes observed in the passive displacement of the digitised point at the aponeurosis. Without correcting for angle joint rotation, the measured elongation clearly overestimates the actual elongation of the GM tendon and aponeurosis. After the passive displacement correction the calculated elongation still overestimates the actual elongation of the GM tendon and aponeurosis. However, this overestimation has a negligible effect on the examined in vivo strain ( approximately 0.3%) of the tendon and aponeurosis.

An Algorithm for Automated Analysis of Ultrasound Images to Measure Tendon Excursion in Vivo

The accuracy of an algorithm for the automated tracking of tendon excursion from ultrasound images was tested in three experiments. Because the automated method could not be tested against direct measurements of tendon excursion in vivo, an indirect validation procedure was employed. In one experiment, a wire “phantom” was moved a known distance across the ultrasound probe and the automated tracking results were compared with the known distance. The excursion of the musculotendinous junction of the gastrocnemius during frontal and sagittal plane movement of the ankle was assessed in a single cadaver specimen both by manual tracking and with a cable extensometer sutured to the gastrocnemius muscle. A third experiment involved estimation of Achilles tendon excursion in vivo with both manual and automated tracking. Root mean squared (RMS) error was calculated between pairs of measurements after each test. Mean RMS errors of less than 1 mm were observed for the phantom experiments. For the in vitro experiment, mean RMS errors of 8–9% of the total tendon excursion were observed. Mean RMS errors of 6–8% of the total tendon excursion were found in vivo. The results indicate that the proposed algorithm accurately tracks Achilles tendon excursion, but further testing is necessary to determine its general applicability.

Mechanical and morphological properties of the triceps surae muscle–tendon unit in old and young adults and their interaction with a submaximal fatiguing contraction

The purposes of this study were to examine (a) whether the morphological properties of the muscle gastrocnemius medialis (GM) contribute to the known enhanced muscle fatigue resistance during submaximal sustained isometric plantar flexion contraction of old compared to young adults and (b) whether a submaximal fatiguing contraction differently affects the mechanical properties of the GM tendon and aponeurosis of old and young adults. Fourteen old and 12 young male subjects performed maximal voluntary isometric plantar flexions (MVC) on a dynamometer before and after a submaximal fatiguing task (40% MVC). Moments and EMG signals from the gastrocnemius medialis and lateralis, soleus and tibialis anterior muscles were measured. The elongation of the GM tendon and aponeurosis and the morphological properties of its contractile element were examined by means of ultrasonography. The old adults showed lower maximal ankle joint moment, stiffness and fascicle length in both tested conditions. The submaximal fatiguing contraction did not affect the force-strain relationship of the GM tendon and aponeurosis of either young or old adults. The time to task failure was longer for the old adults and was strongly correlated with the fascicle length (r(2)=0.50, P<0.001). This provides evidence on that the lower ratio of the active muscle volume to muscle force for the old adults might be an additional mechanism contributing to the known age related increase in muscle fatigue resistance.

Mechanical properties during healing of Achilles tendon ruptures to predict final outcome: a pilot Roentgen stereophotogrammetric analysis in 10 patients

Background

There are presently few methods described for in vivo monitoring of the mechanics of healing human tendon ruptures, and no methods for prediction of clinical outcome. We tested if Roentgen stereophotogrammetric analysis (RSA) can be used to follow the restoration of mechanical properties during healing of ruptured Achilles tendons, and if early measurements can predict clinical results.

Methods

Achilles tendon repair was studied with RSA in 10 patients with a total rupture. Tantalum beads were implanted in conjunction with surgical repair. The patients were evaluated at 6, 12 and 18 weeks, and after 1 year. RSA was performed with two different mechanical loadings, and the strain induced by increasing load was measured. The transverse area was determined by ultrasound. CT scan at 12 weeks confirmed that the tantalum beads were located within the tendons. Functional testing was done after 1 year. A heel raise index was chosen as primary clinical outcome variable.

Results

The strain was median 0.90, 0.32 and 0.14 percent per 100 N tendon force at 6 weeks, 18 weeks and one year respectively. The error of measurement was 0.04 percent units at 18 weeks. There was a large variation between patients, which appears to reflect biological variation. From 6 to 18 weeks, there was a negative correlation between increase in transverse area and increase in material properties, suggesting that healing is regulated at the organ level, to maximize stiffness. Modulus of elasticity during this time correlated with a heel raise index at one year (Rho = 0.76; p = 0.02).

Conclusion

We conclude that the RSA method might have potential for comparing different treatments of Achilles tendon ruptures.

The passive, human calf muscles in relation to standing: the short range stiffness lies in the contractile component

Using short duration perturbations, previous attempts to measure the intrinsic ankle stiffness during human standing have revealed a substantial stabilizing contribution (65-90% normalized to load stiffness 'mgh'). Others regard this method as unsuitable for the low-frequency conditions of quiet standing and believe the passive contribution to be small (10-15%). This latter view, consistent with a linear Hill-type model, argues that during standing, the contractile portion of the muscle is much less stiff than the tendon. Here, for upright subjects, we settle this issue by measuring the stiffness of the contractile portion of the passive calf muscles using low-frequency ankle rotations. Using ultrasound we tracked the changes in muscle contractile length and partitioned the ankle rotation into contractile and extra-contractile (series elastic) portions. Small ankle rotations of 0.15 and 0.4 deg show a contractile to series elastic stiffness ratio (K(ce)/K(se)) of 12 +/- 9 and 6.3 +/- 10, respectively, with both elements displaying predominantly elastic behaviour. Larger, 7 deg rotations reveal the range of this ratio. It declines in a non-linear way from a high value (K(ce)/K(se) = 18 +/- 11) to a low value (K(ce)/K(se) = 1 +/- 0.4) as rotation increases from 0.1 to 7 deg. There is a marked transition at around 0.5 deg. The series elastic stiffness (K(se)/mgh) remains largely constant (77 +/- 13%) demonstrating the contractile component origin of passive, short range stiffness. The linear Hill-type model does not describe the range-related stiffness relevant to the progression from quiet standing to perturbed balance and movement and can lead to inaccurate predictions regarding human balance.

Adaptational responses of the human Achilles tendon by modulation of the applied cyclic strain magnitude

Tendons are able to remodel their mechanical and morphological properties in response to mechanical loading. However, there is little information about the effects of controlled modulation in cyclic strain magnitude applied to the tendon on the adaptation of tendon's properties in vivo. The present study investigated whether the magnitude of the mechanical load induced as cyclic strain applied to the Achilles tendon may have a threshold in order to trigger adaptation effects on tendon mechanical and morphological properties. Twenty-one adults (experimental group, N=11; control group, N=10) participated in the study. The participants of the experimental group exercised one leg at low-magnitude tendon strain (2.85+/-0.99%) and the other leg at high-magnitude tendon strain (4.55+/-1.38%) of similar frequency and volume. After 14 weeks of exercise intervention we found a decrease in strain at a given tendon force, an increase in tendon-aponeurosis stiffness and tendon elastic modulus and a region-specific hypertrophy of the Achilles tendon only in the leg exercised at high strain magnitude. These findings provide evidence of the existence of a threshold or set-point at the applied strain magnitude at which the transduction of the mechanical stimulus may influence the tensional homeostasis of the tendons. The results further show that the mechanical load exerted on the Achilles tendon during the low-strain-magnitude exercise is not a sufficient stimulus for triggering further adaptation effects on the Achilles tendon than the stimulus provided by the mechanical load applied during daily activities.

Muscle - tendon unit mechanical and morphological properties and sprint performance

The objective of this study was to determine whether sprint performance is related to the mechanical (elongation - force relationship of the tendon and aponeurosis, muscle strength) and morphological (fascicle length, pennation angle, muscle thickness) properties of the quadriceps femoris and triceps surae muscle - tendon units. Two groups of sprinters (slow, n = 11; fast, n = 17) performed maximal isometric knee extension and plantar flexion contractions on a dynamometer at 11 different muscle - tendon unit lengths. Elongation of the tendon and aponeurosis of the gastrocnemius medialis and the vastus lateralis was measured using ultrasonography. We observed no significant differences in maximal joint moments at the ankle and knee joints or morphological properties of the gastrocnemius medialis and vastus lateralis between groups (P > 0.05). The fast group exhibited greater elongation of the vastus lateralis tendon and aponeurosis at a given tendon force, and greater maximal elongation of the vastus lateralis tendon and aponeurosis during maximum voluntary contraction (P < 0.05). Furthermore, maximal elongation of the vastus lateralis tendon and aponeurosis showed a significant correlation with 100-m sprint times (r = -0.567, P = 0.003). For the elongation - force relationship at the gastrocnemius medialis tendon and aponeurosis, the two groups recorded similar values. It is suggested that the greater elongation of the vastus lateralis tendon and aponeurosis of the fast group benefits energy storage and return as well as the shortening velocity of the muscle - tendon unit.

Effects of joint angle and age on ankle dorsi- and plantar-flexor strength

This study aimed at examining the effects of joint angle and age on the maximal voluntary contraction (MVC) torque, for the agonist and antagonist muscle groups around the ankle, i.e., the dorsi- and plantar-flexors. To this aim, neural and muscular factors were investigated in two groups of healthy men: 11 young (mean age, 24 years) and 18 older (mean age, 78 years). Plantar-flexion (PF) and dorsiflexion (DF) isometric MVC torques were measured in three different ankle joint angles and surface electromyographic activities of the triceps surae and of the tibialis anterior muscles were recorded. The main findings were that the DF-to-PF MVC torque ratio varied with joint angle and age, indicating that aging affected at different rates the two muscle groups: this ratio was always higher in older adults because of the PF strength decline with aging. Furthermore, the DF MVC torque-angle relationship appeared to be especially explained by neural factors, whereas the relationship in PF seemed to be mainly due to muscular parameters. These relationships would not be a discriminating factor between the two age groups. As a consequence, measurements at one ankle joint angle, whatever the angle, are thus enough to examine the differences within age groups and to perform a rapid assessment of the imbalance at the ankle joint.

Musculoskeletal adaptations in chronic spinal cord injury: effects of long-term soleus electrical stimulation training

OBJECTIVE

The purpose of this study was to determine whether long-term electrical stimulation training of the paralyzed soleus could change this muscle's physiological properties (torque, fatigue index, potentiation index, torque-time integral) and increase tibia bone mineral density.

METHODS

Four men with chronic (>2 years) complete spinal cord injury (SCI; American Spinal Injury Association classification A) trained 1 soleus muscle using an isometric plantar flexion electrical stimulation protocol. The untrained limb served as a within-subject control. The protocol involved ~ 30 minutes of training each day, 5 days a week, for a period of 6 to 11 months. Mean compliance over 11 months of training was 91% for 3 subjects. A fourth subject achieved high compliance after only 5 months of training. Mean estimated compressive loads delivered to the tibia were approximately 110% of body weight. Over the 11 months of training, the muscle plantar flexion torque, fatigue index, potentiation index, and torque-time integral were evaluated periodically. Bone mineral density (dual-energy x-ray absorptiometry) was evaluated before and after the training program.

RESULTS

The trained limb fatigue index, potentiation index, and torque-time integral showed rapid and robust training effects (P<.05). Soleus electrical stimulation training yielded no changes to the proximal tibia bone mineral density, as measured by dual-energy x-ray absorptiometry. The subject with low compliance experienced fatigue index and torque-time integral improvements only when his compliance surpassed 80%. In contrast, his potentiation index showed adaptations even when compliance was low.

CONCLUSIONS

These findings highlight the persistent adaptive capabilities of chronically paralyzed muscle but suggest that preventing musculoskeletal adaptations after SCI may be more effective than reversing changes in the chronic condition.

Aging and running experience affects the gearing in the musculoskeletal system of the lower extremities while walking

The aims of this study were to investigate whether older adults modify their walking mechanics to compensate for the degeneration in their muscle-tendon units (MTUs), and to examine whether running has a beneficial effect on walking mechanics in younger and older adults. The investigation was conducted on 30 older and 19 younger adults divided into two subgroups: runners versus non-active. In previous studies we documented that older adults had lower leg-extensor muscle strength and tendon stiffness compared to younger. Runners and non-active subjects had similar MTU capacities. In this study we analysed walking kinematics and kinetics (1.6 m/s) from the same subjects. Older adults showed a lower gear ratio (ratio between moment arm of the ground reaction force and moment arm of muscle) at the triceps surae MTU during the initial and mid part of ground contact, lower average horizontal forces and lower average ankle joint moment during ground contact compared to younger (p<0.05). Compared to non-active subjects, runners had a lower gear ratio at the quadriceps femoris MTU during the initial and final part of ground contact, lower average horizontal forces and lower maximal knee joint moment during ground contact independent of the subject's age (p<0.05). We concluded that the older adults modify the gearing at the ankle joint in order to adjust the task effort to the reduced triceps surae muscle strength. It appears, further, that runners walked more effectively from a mechanical standpoint compared to non-active subjects, which suggests that runners may be able to transfer motor adaptation from running to walking even in old age.

The Adaptations to Strength Training

High-resistance strength training (HRST) is one of the most widely practiced forms of physical activity, which is used to enhance athletic performance, augment musculo-skeletal health and alter body aesthetics. Chronic exposure to this type of activity produces marked increases in muscular strength, which are attributed to a range of neurological and morphological adaptations. This review assesses the evidence for these adaptations, their interplay and contribution to enhanced strength and the methodologies employed. The primary morphological adaptations involve an increase in the cross-sectional area of the whole muscle and individual muscle fibres, which is due to an increase in myofibrillar size and number. Satellite cells are activated in the very early stages of training; their proliferation and later fusion with existing fibres appears to be intimately involved in the hypertrophy response. Other possible morphological adaptations include hyperplasia, changes in fibre type, muscle architecture, myofilament density and the structure of connective tissue and tendons. Indirect evidence for neurological adaptations, which encompasses learning and coordination, comes from the specificity of the training adaptation, transfer of unilateral training to the contralateral limb and imagined contractions. The apparent rise in whole-muscle specific tension has been primarily used as evidence for neurological adaptations; however, morphological factors (e.g. preferential hypertrophy of type 2 fibres, increased angle of fibre pennation, increase in radiological density) are also likely to contribute to this phenomenon. Changes in inter-muscular coordination appear critical. Adaptations in agonist muscle activation, as assessed by electromyography, tetanic stimulation and the twitch interpolation technique, suggest small, but significant increases. Enhanced firing frequency and spinal reflexes most likely explain this improvement, although there is contrary evidence suggesting no change in cortical or corticospinal excitability. The gains in strength with HRST are undoubtedly due to a wide combination of neurological and morphological factors. Whilst the neurological factors may make their greatest contribution during the early stages of a training programme, hypertrophic processes also commence at the onset of training.

Changes in fascicle length from rest to maximal voluntary contraction affect the assessment of voluntary activation

The purpose of this study was to investigate the effect of the differences between the actual fascicle length during a voluntary contraction and the fascicle length at rest of the triceps surae muscle on the determination of the voluntary activation (VA) by using the interpolated twitch technique. Twelve participants performed isometric voluntary maximal (MVC) and submaximal (20%, 40%, 60% and 80% MVC) contractions at two different ankle angles (75 degrees and 90 degrees ) under application of the interpolated twitch technique. Two ultrasound probes were used to determine the fascicle length of soleus, gastrocnemius medialis and gastrocnemius lateralis muscles. Further, the MVCs and the twitches were repeated for six more ankle angles (85 degrees , 95 degrees , 100 degrees , 105 degrees , 110 degrees and 115 degrees ). The VA of the triceps surae muscle were calculated (a) using the rest twitch force (RTF) measured during the same trial as the interpolated twitch force (ITF; traditional method) and (b) using the RTF at an ankle angle where the fascicle length showed similar values between ITF and RTF (fascicle length consideration method). The continuous changes in fascicle length from rest to MVC affect the accuracy of the assessment of the VA. The traditional method overestimates the assessment of the VA on average 4% to 12%, especially at 90 degrees ankle angle (i.e. short muscle length). The reason for this influence is the unequal force-length potential of the muscle at twitch application by the measure of ITF and RTF. These findings provide evidence that the fascicle length consideration method permits a more precise prediction (an improvement of 4-12%) of the voluntary contraction compared to the traditional method.

In vivo changes in the human patellar tendon moment arm length with different modes and intensities of muscle contraction

The purpose of this study was to examine the effect of different muscle contraction modes and intensities on patellar tendon moment arm length (d(PT)). Five men performed isokinetic concentric, eccentric and passive knee extensions at an angular velocity of 60 deg/s and six men performed gradually increasing to maximum effort isometric muscle contractions at 90( composite function) and 20( composite function) of knee flexion. During the tests, lateral X-ray fluoroscopy imaging was used to scan the knee joint. The d(PT) differences between the passive state and the isokinetic concentric and extension were quantified at 15( composite function) intervals of knee joint flexion angle. Furthermore, the changes of the d(PT) as a function of the isometric muscle contraction intensities were determined during the isometric knee extension at 90( composite function) and 20( composite function) of knee joint flexion. Muscle contraction-induced changes in knee joint flexion angle during the isometric muscle contraction were also taken into account for the d(PT) measurements. During the two isometric knee extensions, d(PT) increased from rest to maximum voluntary muscle contraction (MVC) by 14-15%. However, when changes in knee joint flexion angle induced by the muscle contraction were taken into account, d(PT) during MVC increased by 6-26% compared with rest. Moreover, d(PT) increased during concentric and eccentric knee extension by 3-15%, depending on knee flexion angle, compared with passive knee extension. These findings have important implications for estimating musculoskeletal loads using modelling under static and dynamic conditions.

Influence of the muscle-tendon unit's mechanical and morphological properties on running economy

The purpose of this study was to test the hypothesis that runners having different running economies show differences in the mechanical and morphological properties of their muscle-tendon units (MTU) in the lower extremities. Twenty eight long-distance runners (body mass: 76.8+/-6.7 kg, height: 182+/-6 cm, age: 28.1+/-4.5 years) participated in the study. The subjects ran on a treadmill at three velocities (3.0, 3.5 and 4.0 m s(-1)) for 15 min each. The V(O(2)) consumption was measured by spirometry. At all three examined velocities the kinematics of the left leg were captured whilst running on the treadmill using a high-speed digital video camera operating at 250 Hz. Furthermore the runners performed isometric maximal voluntary plantarflexion and knee extension contractions at eleven different MTU lengths with their left leg on a dynamometer. The distal aponeuroses of the gastrocnemius medialis (GM) and vastus lateralis (VL) were visualised by ultrasound during plantarflexion and knee extension, respectively. The morphological properties of the GM and VL (fascicle length, angle of pennation, and thickness) were determined at three different lengths for each MTU. A cluster analysis was used to classify the subjects into three groups according to their V(O(2)) consumption at all three velocities (high running economy, N=10; moderate running economy, N=12; low running economy, N=6). Neither the kinematic parameters nor the morphological properties of the GM and VL showed significant differences between groups. The most economical runners showed a higher contractile strength and a higher normalised tendon stiffness (relationship between tendon force and tendon strain) in the triceps surae MTU and a higher compliance of the quadriceps tendon and aponeurosis at low level tendon forces. It is suggested that at low level forces the more compliant quadriceps tendon and aponeurosis will increase the force potential of the muscle while running and therefore the volume of active muscle at a given force generation will decrease.

Influences of tendon stiffness, joint stiffness, and electromyographic activity on jump performances using single joint

The present study aimed to examine the influences of tendon stiffness, joint stiffness, and electromyographic activity on jump performances consisting of a single-joint movement. Twenty-four men performed three kinds of unilateral maximal jump using only the ankle joint (squat jump: SJ; countermovement jump: CMJ; drop jump: DJ) on the sledge apparatus. The relative differences in the jump height of CMJ and DJ compared to SJ were defined as pre-stretch augmentation. During jumping exercises, electromyographic activities (mEMG) were recorded from the plantar flexors. Ankle joint stiffness was calculated as the change in the joint torque divided by the change in ankle joint angle during the eccentric phase of DJ. Achilles tendon stiffness was measured using ultrasonography during isometric plantar flexion. No significant correlations were found between joint stiffness and pre-stretch augmentation in both CMJ and DJ. On the contrary, tendon stiffness was significantly correlated with pre-stretch augmentation in both CMJ (r = -0.471) and DJ (r = -0.502). The relative mEMG value of CMJ (to that of SJ) during the concentric phase was significantly correlated with pre-stretch augmentation (r = 0.481), although this relationship was not found in DJ. These results suggested that (1) the greater jump height in CMJ could be explained by both the tendon elasticity and the increased activation level of muscle, (2) tendon elasticity played a more significant role in the enhancement of jump height during DJ, and (3) joint stiffness was not related to either pre-stretch augmentation or tendon stiffness.

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.

Effects of isometric training at different knee angles on the muscle-tendon complex in vivo

The purpose of this study was to investigate the influences of isometric training at different joint angles on the muscle size and function of the human muscle-tendon complex in vivo. Furthermore, we tried to gain a better understanding of the mechanisms involved in angle specificity after isometric training from the aspect of neuromuscular adaptation and the changes in the properties of the muscle-tendon complex. Nine males completed 12-week unilateral training program (70% of maximal voluntary contraction (MVC) x 15 s x six sets) on the knee extensors at 50 degrees (shorter muscle length: ST) and 100 degrees (longer muscle length: LT). The internal muscle force (mechanical stress) is higher at 100 degrees than at 50 degrees because of the difference in the moment arm length, although there were no difference in the relative torque level, contraction and relaxation times, and repetition between ST and LT. Before and after training, isometric strength at eight angles and muscle volume were determined. Tendon elongation of knee extensors was measured by ultrasonography. There was no significant difference in the rate of increment of muscle volume between the protocols. Tendon stiffness increased significantly for LT, but not for ST. Although significant gain was limited to angles at or near the training angle for ST, increases in MVC at all angles were found for LT. These results suggest that only mechanical stress (internal muscle force imposed on muscle and tendon) contributes to adaptation in the tendon stiffness, although metabolic (relative torque level, etc.) and mechanical stress relate to muscle hypertrophy. Furthermore, increment of tendon stiffness for LT might contribute to increase torque output at smaller angles other than the training angle.

Age-related degeneration in leg-extensor muscle–tendon units decreases recovery performance after a forward fall: compensation with running experience

The goals of this study were to investigate whether the lower muscle-tendon units (MTUs) capacities in older affect their ability to recover balance with a single-step after a fall, and to examine whether running experience enhances and protects this motor skill in young and old adults. The investigation was conducted on 30 older and 19 younger divided into two subgroups: runners versus non-active. In previous studies we documented that the older had lower leg extensor muscle strength and tendon stiffness while running had no effect on MTUs capacities. The current study examined recovery mechanics of the same individuals after an induced forward fall. Younger were better able to recover balance with a single-step compared to older (P < 0.001); this ability was associated with a more effective body configuration at touchdown (more posterior COM position relative to the recovery foot, P <0.001). MTUs capacities classified 88.6% of the subjects into single- or multiple-steppers. Runners showed a superior ability to recover balance with a single-step (P < 0.001) compared to non-active subjects due to a more effective mechanical response during the stance phase (greater knee joint flexion, P <0.05). We concluded that the age-related degeneration of the MTUs significantly diminished the older adults' ability to restore balance with a single-step. Running seems to enhance and protect this motor skill. We suggested that runners, due to their running experience, could update the internal representation of mechanisms responsible for the control of dynamic stability during a forward fall and, thus, were able to restore balance more often with a single-step compared to the non-active subjects.

Adaptational phenomena and mechanical responses during running: effect of surface, aging and task experience

The goals of the study were to identify adaptational phenomena in running mechanics over a variety of surfaces due to age related changes in the muscle-tendon units (MTUs) capacities, to examine whether running experience is associated with adaptational effects on running mechanics over a variety of surfaces even at old age, and to investigate whether surface condition affects running mechanics. The investigation was executed on 30 old and 19 young including 29 runners and 20 non-active subjects. In a previous study we documented that the older had lower MTUs capacities. In the present study running mechanics were analysed as the same subjects ran at 2.7 m/s over three surfaces having different compliance. Surface condition did not affect centre of mass trajectory, duty factor or joint kinetics (P > 0.01). Older react to the reduced MTUs capacity by increasing duty factor and benefiting from a mechanical advantage for the triceps surae MTU and a lower rate of force generation on all surfaces (P < 0.01). Runners displayed lower average horizontal forces and a higher mechanical advantage for the quadriceps femoris MTU for all surfaces (P < 0.01). The results provided strong evidence on that running strategy remained essentially unchanged over a variety of surfaces. Adaptive improvements in running mechanics due to task experience were present for all surfaces and did not depend on age. We further concluded that older adults were able to recalibrate their running strategy to adjust the task effort to the reduced MTUs capacities in a feedforward control manner for a variety of mechanical environments.

Human patellar tendon moment arm length: measurement considerations and clinical implications for joint loading assessment

Detailed understanding of the knee joint loading requires the calculation of muscle and joint forces in different conditions. In these applications the patellar tendon moment arm length is essential for the accurate estimation of the tibiofemoral joint loading. In this article, different methods that have been used to determine the patellar tendon moment arm length under in vivo and in vitro conditions are reviewed. The limitations and advantages associated with each of the methods are evaluated together with their applications in the different loading conditions that the musculoskeletal system is subjected to. The three main measurement methods that this review considers are the geometric method, the tendon excursion method and the direct load method. A comparison of relevant quantitative results is presented to asses the impact of the errors of each method on the quantification of the patellar tendon moment arm and the implications for joint loading assessment in clinical applications.

Effects of neuromuscular electrical stimulation parameters on specific tension

This study examined the effects of altering surface neuromuscular electrical stimulation (SNMES) parameters on the specific tension of the quadriceps femoris muscle. Seven able-bodied subjects had magnetic resonance images taken of both thighs prior to and immediately after four SNMES protocols to determine the activated muscle cross-sectional area (CSA). The four protocols were: (1) research (RES, 100 Hz, 450 micros, and amplitude set to evoke 75% of maximal voluntary isometric torque, MVIT); (2) pulse duration (PD, 100 Hz, 150 micros, same current as in RES); (3) frequency (FREQ, 25 Hz, 450 micros, and same current as in RES); (4) amplitude (AMP, 100 Hz, 450 mus, and current set to evoke the average of the initial torques of PD and FREQ, 45 +/- 9% of MVIT). Reducing the amplitude of the current from 75 to 45% of MVIT did not alter specific tension, 25 +/- 8 N/cm2, suggesting that the amplitude probably affects torque and the area of activated muscle proportionally. Shortening the pulse duration from 450 to 150 micros caused specific tension to drop from 25 +/- 6 to 20 +/- 6 N/cm2 (P < 0.05), indicating that pulse duration increased torque and the activated CSA disproportionally. Alternatively, reducing the frequency from 100 to 25 Hz decreased specific tension from 25 +/- 6 to 17 +/- 4 N/cm2 (P < 0.05), suggesting that the frequency increased torque without affecting the activated CSA. Clinicians who administer SNMES should be aware of the magnitude of adaptations to a given amplitude, pulse duration, and frequency.

Influence of the mechanical properties of the muscle-tendon unit on force generation in runners with different running economy

In earlier studies, we found more economical runners having a more compliant quadriceps femoris (QF) tendon at low force levels, and a higher contractile strength and stiffness at the triceps surae (TS). To better understand how these differences influence force generation economy and energy recovery, we simulated contractions using a Hill-type muscle model and the previously determined muscle properties as input parameters. For eight different activation levels, we simulated isovelocity concentric contractions preceded by an isovelocity stretch. The length changes and contraction velocities imposed to the muscle-tendon units (MTU) corresponded to those happening whilst running. The main results of the simulations were: (a) a more compliant tendon at low force levels (QF) led to an advantage in force-generation due to a decrease in shortening velocity of the CE, (b) a higher contractile strength and higher stiffness at the TS led to a disadvantage in force-generation at high activation levels and to an advantage at low activation levels. In addition at the high economy runners both MTUs showed an advantageous energy release during shortening, which at the QF was mainly due to a higher elongation of the SEE and at the TS mainly to the higher contractile strength. Especially at low activation levels both MTUs showed an advantageous force generation per activation and a higher energy release as compared to the low economy runners.

Weakness in end-range plantar flexion after Achilles tendon repair

BACKGROUND

Separation of tendon ends after Achilles tendon repair may affect the tendon repair process and lead to postoperative end-range plantarflexion weakness.

HYPOTHESIS

Patients will have disproportionate end-range plantarflexion weakness after Achilles tendon repair.

STUDY DESIGN

Descriptive laboratory study.

METHODS

Four-strand core suture repairs of Achilles tendon were performed on 1 female and 19 male patients. Postoperatively, patients were nonweightbearing with the ankle immobilized for 4 weeks. Plantarflexion torque, dorsiflexion range of motion, passive joint stiffness, toe walking, and standing single-legged heel rise (on an incline, decline, and level surface) were assessed after surgery (mean, 1.8 years postoperative; range, 6 months-9 years). Maximum isometric plantarflexion torque was measured at 20 degrees and 10 degrees of dorsiflexion, neutral, and 10 degrees and 20 degrees of plantar flexion. Percentage strength deficit (relative to noninvolved leg) was computed at each angle. Passive dorsiflexion range of motion was measured goniometrically. Passive joint stiffness was computed from increase in passive torque between 10 degrees and 20 degrees of dorsiflexion, before isometric contractions.

RESULTS

Significant plantarflexion weakness was evident on the involved side at 20 degrees and 10 degrees of plantar flexion (34% and 20% deficits, respectively; P <.001), with no torque deficits evident at other angles (6% at neutral, 3% at 10 degrees of dorsiflexion, 0% at 20 degrees of dorsiflexion). Dorsiflexion range of motion was not different between involved and noninvolved sides (P = .7). Passive joint stiffness was 34% lower on the involved side (P <.01). All patients could perform an incline heel rise; 14 patients could not perform a decline heel rise (P <.01).

CONCLUSION

Disproportionate weakness in end-range plantar flexion, decreased passive stiffness in dorsiflexion, and inability to perform a decline heel rise are evident after Achilles tendon repair. Possible causes include anatomical lengthening, increased tendon compliance, and insufficient rehabilitation after Achilles tendon repair.

CLINICAL RELEVANCE

Impairments will have functional implications for activities (eg, descending stairs and landing from a jump). Weakness in end-range plantar flexion may be an unrecognized problem after Achilles tendon repair.

Can the patellar tendon moment arm be predicted from anthropometric measurements?

The purpose of this study was to examine the relations between patellar tendon moment arm length and several relevant anthropometric characteristics of 22 healthy men. The patellar tendon moment arm length was measured using magnetic resonance imaging with two different methods: (1) measurement of patellar tendon moment arm length (d(PT)) with respect to the tibiofemoral contact point (d(PTCP)) and (2) measurement of d(PT) with respect to the intersection point of the anterior and posterior cruciate ligament (d(PTIP)). Pearson correlation coefficients and a stepwise linear regression analysis were used to examine the relationships between the d(PT) and anthropometric measurements taken. Furthermore, a Student's t-test was used to determine differences between the d(PTCP) and d(PTIP) values. Only knee circumference was a significant d(PTCP) predictor (P < 0.05) but with a very low R2 (0.139). None of the anthropometric parameters examined was found to be a significant d(PTIP) predictor. The correlation coefficients ranged from -0.04 to 0.42. The d(PTIP) values were significantly higher (by 0.84-1.89 cm) than the d(PTCP) values (P < 0.05). These results are in disagreement with previous in vitro findings that d(PT) variance may be explained by knee joint size differences. Hence, existing imaging-based methodologies remain necessary for accurate quantification of the patellar tendon moment arm.

Electrically induced muscle contractions influence bone density decline after spinal cord injury

STUDY DESIGN

Longitudinal repeated-measures; within-subject control.

OBJECTIVE

We examined the extent to which an isometric plantar flexion training protocol attenuates bone loss longitudinally after SCI.

SUMMARY OF BACKGROUND DATA

After spinal cord injury (SCI), bone mineral density (BMD) of paralyzed extremities rapidly declines, likely because of loss of mechanical loading of bone via muscle contractions.

METHODS

Six individuals with complete paralysis began a 3-year unilateral plantar flexor muscle activation program within 4.5 months after SCI. The opposite limb served as a control. Compliance with recommended dose was > 80%. Tibia compressive force was > 140% of body weight.

RESULTS

Bilateral hip and untrained tibia BMD declined significantly over the course of the training. Lumbar spine BMD showed minimal change. Percent decline in BMD (from the baseline condition) for the trained tibia (approximately 10%) was significantly less than the untrained tibia (approximately 25%) (P < 0.05). Trained limb percent decline in BMD remained steady over the first 1.5 years of the study (P < 0.05).

CONCLUSIONS

Compressive loads of approximately 1 to 2 times body weight, induced by muscle contractions, partially prevent the loss of BMD after SCI. Future studies should establish dose-response curves for attenuation of bone loss after SCI.