Quantification of patellar tendon strain and opportunities for personalized tendon loading during back squats.

Tendon strain during exercise is a critical regulatory factor in tendon adaptive responses and there are indications for an optimal range of strain that promotes tendon adaptation. Back squats are used to improve patellar tendon properties in sport and clinical settings. To date, the operating patellar tendon strain during back squats is unknown and current recommendations for individual exercise loading are based on the one repetition maximum (1RM). Here, we quantified patellar tendon strain during loaded back squats at 40, 60 and 80% of the 1RM and during maximum isometric knee extension contractions (MVC) using ultrasonography. Kinematics, ground reaction forces and muscle electromyographic activity were also recorded. Additionally, maximum tendon strain during the MVC and the percentage of 1RM were used as explanatory variables to estimate the individual patellar tendon strain during the squats. Strain increased with increasing 1RM loading (4.7 to 8.2%), indicating that already medium-loading back squats may provide a sufficient stimulus for tendon adaptation. The individual variability was, however, too high to generalize these findings. Yet, there was a high agreement between the individually estimated and measured patellar tendon strain (R2 = 0.858) during back squats. We argue that this approach may provide new opportunities for personalized tendon exercise.

Differences in motor response to stability perturbations limit fall-resisting skill transfer.

This study investigated transfer of improvements in stability recovery performance to novel perturbations. Thirty adults (20-53 yr) were assigned equally to three treadmill walking groups: groups exposed to eight trip perturbations of either low or high magnitude and a third control group that walked unperturbed. Following treadmill walking, participants were exposed to stability loss from a forward-inclined position (lean-and-release) and an overground trip. Lower limb joint kinematics for the swing phase of recovery steps was compared for the three tasks using statistical parametric mapping and recovery performance was analysed by margin of stability and base of support. The perturbation groups improved stability (greater margin of stability) over the eight gait perturbations. There was no group effect for stability recovery in lean-and-release. For the overground trip, both perturbation groups showed similar enhanced stability recovery (margin of stability and base of support) compared to controls. Differences in joint angle kinematics between treadmill-perturbation and lean-and-release were more prolonged and greater than between the two gait perturbation tasks. This study indicates that: (i) practising stability control enhances human resilience to novel perturbations; (ii) enhancement is not necessarily dependent on perturbation magnitude; (iii) differences in motor response patterns between tasks may limit transfer.

Stability recovery performance in adults over a wide age range: A multicentre reliability analysis using different lean-and-release test protocols.

The ability to effectively increase the base of support is crucial to prevent from falling due to stability disturbances and has been commonly assessed using the forward-directed lean-and-release test. With this multicentre study we examined whether the assessment of stability recovery performance using two different forward lean-and-release test protocols is reliable in adults over a wide age range. Ninety-seven healthy adults (age from 21 to 80 years) were randomly assigned to one out of two lean angle protocols: gradual increase to maximal forward-lean angle (maximal lean angle; n = 43; seven participants were excluded due to marker artefacts) or predefined lean angle (single lean angle; n = 26; 21 participants needed to be excluded due to multiple stepping after release or marker artefacts). Both protocols were repeated after 0.5 h and 48 h to investigate intra- and inter-session reliability. Stability recovery performance was examined using the margin of stability at release (MoSRL) and touchdown (MoSTD) and increase in base of support (BoSTD). Intraclass correlation coefficients (confidence intervals at 95%) for the maximal lean angle and for the single lean angle were respectively 0.93 (0.89-0.96) and 0.94 (0.89-0.97) in MoSRL, 0.85 (0.77-0.91) and 0.67 (0.48-0.82) in MoSTD and 0.88 (0.81-0.93) and 0.80 (0.66-0.90) in BoSTD, with equivalence being revealed for each parameter between all three measurements (p < 0.01). We concluded that the assessment of stability recovery performance parameters in adults over a wide age range with the means of the forward lean-and-release test is reliable, independent of the used lean angle protocol.

Operating length and velocity of human vastus lateralis muscle during walking and running.

According to the force-length-velocity relationships, the muscle force potential during locomotion is determined by the operating fibre length and velocity. We measured fascicle and muscle-tendon unit length and velocity as well as the activity of the human vastus lateralis muscle (VL) during walking and running. Furthermore, we determined the VL force-length relationship experimentally and calculated the force-length and force-velocity potentials (i.e. fraction of maximum force according to the force-length-velocity curves) for both gaits. During the active state of the stance phase, fascicles showed significantly (p < 0.05) smaller length changes (walking: 9.2 ± 4.7% of optimal length (L0); running: 9.0 ± 8.4%L0) and lower velocities (0.46 ± 0.36 L0/s; 0.03 ± 0.83 L0/s) compared to the muscle-tendon unit (walking: 19.7 ± 5.3%L0, -0.94 ± 0.32 L0/s; running: 34.5 ± 5.8%L0, -2.59 ± 0.41 L0/s). The VL fascicles operated close to optimum length (L0 = 9.4 ± 0.11 cm) in both walking (8.6 ± 0.14 cm) and running (10.1 ± 0.19 cm), resulting in high force-length (walking: 0.92 ± 0.08; running: 0.91 ± 0.14) and force-velocity (0.91 ± 0.08; 0.97 ± 0.13) potentials. For the first time we demonstrated that, in contrast to the current general conception, the VL fascicles operate almost isometrically and close to L0 during the active state of the stance phase of walking and running. The findings further verify an important contribution of the series-elastic element to VL fascicle dynamics.

Exercises of dynamic stability under unstable conditions increase muscle strength and balance ability in the elderly.

The purpose of this study was to assess the effectiveness of a specific exercise intervention of mechanisms to control dynamic postural stability under unstable conditions in old adults. Forty-seven old adults (65-80 years) were assigned to 2 experimental groups (muscle strength group, n = 15; perturbation-based group, n = 16) and a control group (n = 16). The strength group performed resistance exercises for leg and trunk muscles, while the perturbation-based group exercised mechanisms of dynamic stability under unstable conditions. The training duration was 14 weeks, with training sessions twice a week for 1.5 hours. Muscle strength, balance ability, and balance recovery performance were investigated before and after the interventions using maximal isometric plantar flexion and knee extension contractions, the approach of the center of pressure to the anterior limits of stability and simulated forward falls. Both interventions increased balance recovery performance in simulated forward falls (81%, d = 1.50 and 80%, d = 1.08 in the muscle strength and perturbation-based group, respectively), while the control group did not show any changes. Plantar flexor strength increased 20% (d = 0.72) in the muscle strength and 23% (d = 1.03) in the perturbation-based group, while muscle strength of the knee extensors increased only in the muscle strength group (8%, d = 0.76). On the other hand, only the perturbation-based group showed a significant improvement of standing balance ability (38%, d = 1.61). We conclude that a perturbation-based training program focusing on exercising mechanisms of dynamic stability in unstable conditions has the potential to enhance muscle strength as well as sensory information processing within the motor system during sudden and static balance tasks and, as a consequence, reduce the risk of falls in old adults.

Muscle and tendon adaptation in adolescent athletes: A longitudinal study.

There is evidence that a non-uniform adaptation of muscle and tendon in young athletes results in increased tendon stress during mid-adolescence. The present longitudinal study investigated the development of the morphological and mechanical properties of muscle and tendon of volleyball athletes in a time period of 2 years from mid-adolescence to late adolescence. Eighteen elite volleyball athletes participated in magnetic resonance imaging and ultrasound-dynamometry sessions to determine quadriceps femoris muscle strength, vastus lateralis, medialis and intermedius morphology, and patellar tendon mechanical and morphological properties in mid-adolescence (16 ± 1 years) and late adolescence (18 ± 1 years). Muscle strength, anatomical cross-sectional area (CSA), and volume showed significant (P < 0.05) but moderate increases of 13%, 6%, and 6%, respectively. The increase of patellar tendon CSA (P < 0.05) was substantially greater (27%) and went in line with increased stiffness (P < 0.05; 25%) and reduced stress (P < 0.05; 9%). During late adolescence, a pronounced hypertrophy of the patellar tendon led to a mechanical strengthening of the tendon in relation to the functional and morphological development of the muscle. These adaptive processes may compensate the unfavorable relation of muscle strength and tendon loading capacity in mid-adolescence and might have implications on athletic performance and tendon injury risk.

Increased unilateral tendon stiffness and its effect on gait 2-6 years after Achilles tendon rupture.

Achilles tendon rupture (ATR) alters tissue composition, which may affect long-term tendon mechanics and ankle function during movement. However, a relationship between Achilles tendon (AT) properties and ankle joint function during gait remains unclear. The primary hypotheses were that (a) post-ATR tendon stiffness and length differ from the noninjured contralateral side and that (b) intra-patient asymmetries in AT properties correlate to ankle function asymmetries during gait, determined by ankle angles and moments. Ultrasonography and dynamometry were used to assess AT tendon stiffness, strain, elongation, and rest length in both limbs of 20 ATR patients 2-6 years after repair. Three-dimensional ankle angles and moments were determined using gait analysis. Injured tendons exhibited increased stiffness, rest length, and altered kinematics, with higher dorsiflexion and eversion, and lower plantarflexion and inversion. Intra-patient tendon stiffness and tendon length ratios were negatively correlated to intra-patient ratios of the maximum plantarflexion moment and maximum dorsiflexion angle, respectively. These results suggest that after surgical ATR repair, higher AT stiffness, but not a longer AT, may contribute to deficits in plantarflexion moment generation. These data further support the claim that post-ATR tendon regeneration results in the production of a tissue that is functionally different than noninjured tendon.

Asymmetry of Achilles tendon mechanical and morphological properties between both legs.

Although symmetry of Achilles tendon (AT) properties between legs is commonly assumed in research and clinical settings, different loading profiles of both legs in daily life (i.e., foot dominance) may affect the tendon properties in a side-depended manner. Therefore, AT properties were examined with regard to symmetry between legs. Thirty-six male healthy adults (28 ± 4 years), who were physically active but not involved in sports featuring dissimilar leg load participated. Mechanical and morphological AT properties of the non-dominant and dominant leg were measured by means of ultrasound, magnetic resonance imaging and dynamometry. The AT of the dominant leg featured a significant higher Young's modulus and length (P < 0.05) but a tendency toward lower maximum strain (P = 0.068) compared with the non-dominant leg. The tendon cross-sectional area and stiffness were not significantly different between sides. The absolute asymmetry index of the investigated parameters ranged from 3% to 31% indicating poor AT side symmetry. These findings provide evidence of distinct differences of AT properties between both legs in a population without any sport-specific side-depended leg loading. The observed asymmetry may be a result of different loading profiles of both legs during daily activities (i.e., foot dominance) and challenges the general assumption of symmetrical AT properties between legs.

Muscle shape consistency and muscle volume prediction of thigh muscles.

The present study investigated the applicability of a muscle volume prediction method using only the muscle length (L(M)), the maximum anatomical cross-sectional area (ACSA(max)), and a muscle-specific shape factor (p) on the quadriceps vastii. L(M), ACSA(max), muscle volume, and p were obtained from magnetic resonance images of the vastus intermedius (VI), lateralis (VL), and medialis (VM) of female (n = 20) and male (n = 17) volleyball athletes. The average p was used to predict muscle volumes (V(p)) using the equation V(p) = p × ACSA(max) × L(M). Although there were significant differences in the muscle dimensions between male and female athletes, p was similar and on average 0.582, 0.658, 0.543 for the VI, VL, and VM, respectively. The position of ACSA(max) showed low variability and was at 57%, 60%, and 81% of the thigh length for VI, VL, and VM. Further, there were no significant differences between measured and predicted muscle volumes with root mean square differences of 5-8%. These results suggest that the muscle shape of the quadriceps vastii is independent of muscle dimensions or sex and that the prediction method could be sensitive enough to detect changes in muscle volume related to degeneration, atrophy, or hypertrophy.

Evidence of imbalanced adaptation between muscle and tendon in adolescent athletes.

Adolescence may be regarded as a critical phase of tissue plasticity in young growing athletes, as the adaptation process of muscle-tendon unit is affected by both environmental mechanical stimuli and maturation. The present study investigated potential imbalances of knee extensor muscle strength and patellar tendon properties in adolescent compared with middle-aged athletes featuring long-term musculotendinous adaptations. Nineteen adolescent elite volleyball athletes [(A), 15.9 ± 0.6 years] and 18 middle-aged competitively active former elite volleyball athletes [(MA), 46.9 ± 0.6 years] participated in magnetic resonance imaging and ultrasound-dynamometry sessions to determine quadriceps femoris muscle strength, vastus lateralis morphology and patellar tendon mechanical and morphological properties. There was no significant age effect on the physiological cross-sectional area of the vastus lateralis and maximum knee extension moment (P > 0.05) during voluntary isometric contractions. However, the patellar tendon cross-sectional area was significantly smaller (A: 107.4 ± 27.5 mm(2) ; MA: 121.7 ± 39.8 mm(2) ) and the tendon stress during the maximal contractions was significantly higher in adolescent compared with the middle-aged athletes (A: 50.0 ± 10.1 MPa; MA: 40.0 ± 9.5 MPa). These findings provide evidence of an imbalanced development of muscle strength and tendon mechanical and morphological properties in adolescent athletes, which may have implications for the risk of tendon overuse injuries.

Diffusion tensor imaging of skeletal muscle--correlation of fractional anisotropy to muscle power.

PURPOSE: Recent DTI studies demonstrated the possibility of fiber geometry visualization in skeletal muscle. We tested for an association between muscle power and standard DTI parameters, e. g. fractional anisotropy. MATERIALS AND METHODS: Maximal muscle power (Lmax) of the soleus muscle was determined in 11 healthy subjects. Subsequently DTI was performed and standard parameters (fractional anisotropy - FA, mean diffusivity - MD, parallel diffusivity - PD, radial diffusivity - RD) were extracted in an ROI of the soleus muscle. RESULTS: We found a signficant association of Lmax with FA (neg. correlation: r = -0.85, p = 0.0015) and RD (pos. correlation r = 0.80, p = 0.047). There was no signficant association of MD or PD. CONCLUSION: Maximum muscle power is an indirect measure of fiber type distribution. The correlation between muscle power and DTI parameters can be explained by differences in fiber diameter and differences in the intracellular microstructure of type-1 and type-2 fibers. DTI should be evaluated as a tool for non-invasive quantification of fiber type distribution in skeletal muscle.

Repeatability and reproducibility of OSSCA, a functional approach for assessing the kinematics of the lower limb.

Marker-based gait analysis of the lower limb that uses assumptions of generic anatomical morphology can be susceptible to errors, particularly in subjects with high levels of soft tissue coverage. We hypothesize that a functional approach for assessing skeletal kinematics, based on the application of techniques to reduce soft tissue artefact and functionally identify joint centres and axes, can more reliably (repeatably and reproducibly) assess the skeletal kinematics than a standard generic regression approach. Six healthy adults each performed 100 repetitions of a standardized motion, measured on four different days and by five different observers. Using OSSCA, a combination of functional approaches to reduce soft tissue artefact and identify joint centres and axes, the lengths of the femora and tibiae were determined to assess the inter-day and inter-observer reliability, and compared against a standard generic regression approach. The results indicate that the OSSCA was repeatable and reproducible (ICC lowest bound 0.87), but also provided an improvement over the regression approach (ICC lowest bound 0.69). Furthermore, the analysis of variance revealed a statistically significant variance for the factor "observers" (p<0.01; low-reproducibility) when using the regression approach for determining the femoral lengths. Here, this non-invasive, rapid and robust approach has been demonstrated to allow the repeatable and reproducible identification of skeletal landmarks, which is insensitive to marker placement and measurement session. The reliability of the OSSCA thus allows its application in clinical studies for reducing the uncertainty of approach-induced systematic errors.

Motor task and muscle strength followed different adaptation patterns after anterior cruciate ligament reconstruction.

AIM: The aims of this study were: 1) to investigate the adaptations in walking, running and muscle strength after anterior cruciate ligament (ACL) reconstruction and 2) to examine the interaction between muscle strength and walking or running kinematics at different time intervals following an ACL reconstruction. METHODS: Eleven patients 3-6 months postsurgery, 11 patients 6-12 months postsurgery and 13 patients 12-24 months post surgery were filmed using two cameras (50 Hz) whilst walking (1 m/s) and running (2.5 m/s) on a treadmill. In addition, the strength of the muscles surrounding the knee, hip and ankle joints was tested using an isokinetic protocol (60 degrees/s). RESULTS: The comparison between the injured and the non-injured limbs revealed significant lower knee extension and flexion angles during the stance phase of the injured limb (P<0.05). This was found in the 3-6 month postsurgery period when walking and running and in the 6-12 month postsurgery period when mainly running. The analysis of the muscle strength revealed lower (P<0.05) maximal joint moments for the knee extensors, the knee flexors and the hip flexors of the injured limb during all the observed post surgery periods. CONCLUSION: The results of this study indicate that the adaptation of the motor task and the muscle strength follows different time patterns. This suggests that a decrease in muscle strength capabilities can be tolerated up to a certain extent by the patients when walking and during sub maximal running. However, when the decrease in muscle strength exceeds a certain threshold the biological system, being flexible, changes its locomotion strategy.

Assessment of muscle volume and physiological cross-sectional area of the human triceps surae muscle in vivo.

The purpose of the present study was to investigate whether it is possible to predict the individual muscle volumes within the triceps surae (TS) muscle group by means of easily measurable parameters based on a theoretical consideration. A further objective was to verify the use of the available literature data to assess the contribution of each muscle of the group to the entire TS volume or physiological cross-sectional-area (PCSA). Therefore, magnetic resonance images of the right calf of 13 male subjects were acquired and each muscle of the TS was reconstructed. Muscle length (l(m)), the maximum anatomical cross-sectional-area (ACSA(max)) and muscle volume were obtained from the 3D models. To assess the PCSA, fascicle length was determined by ultrasonography. In general, muscle volume can be expressed as a fraction of the product of ACSA(max) and l(m). The size of the fraction depends on muscle shape and its coefficient of variance among the examined population was considerable low (soleus 6%, gastrocnemius 4% and gastrocnemius lateralis 7%) in the present study. The product of ACSA(max) and l(m) was, therefore, suitable to assess muscle volume (root mean squares, RMS 4-7%). Further, the soleus, gastrocnemius medialis and gastrocnemius lateralis accounted on average for about 52+/-3%, 32+/-2% and 16+/-2% of the total TS volume and 62+/-5%, 26+/-3% and 12+/-2% of the entire TS PCSA, respectively. The coefficient of variance of the relative portions were 5-10% for muscle volume and 8-17% for the PCSA.

[Graft type-related versus graft type-unrelated ACL treatment: influence on adaptation of muscle strength and motor skills]. / Ersatzplastikabhängige vs. nicht ersatzplastikabhängige ACL-Behandlung. Einfluss auf die Adaptation der Muskelkräfte und den motorischen Aufgaben.

INTRODUCTION: The purposes of this study were to identify adaptational phenomena in running and walking characteristics as well as in muscle strength after an ACL injury and graft-type related ACL treatment in order to determine the effect of the different treatments on the regeneration of muscle capacity and functional motor skills for a given task. METHOD: The investigation was conducted on 11 patients with patella reconstructions (PT: patella tendon), 12 patients with semitendinosus reconstruction (ST: semitendinosus tendon) and 10 patients without reconstruction. Besides a clinical examination, the subject's walking (1 m/s) and running (2.5 m/s) characteristics were recorded by 2 cameras operating at 50 Hz. During the same test session, we determined the force potential of the muscles surrounding the hip, knee and ankle joints during isokinetic contractions. RESULTS: The comparison between injured and non-injured limbs revealed significant changes (p<0.05) only for the injured limb. For both types of reconstruction treatment the injured limb showed a significantly lower maximal knee extension joint moment as well as a lower angular displacement of the knee joint during the stance phase while walking and running (p<0.05). Furthermore, patients with PT reconstruction showed lower maximal knee flexion and hip flexion joint moments, and a lower angular displacement of the knee joint in flexion during the stance phase while running (p<0.05). The ratios of the maximal isokinetic knee extension moments between the injured and non injured limbs were lower for the reconstruction treatment compared to the treatment without reconstruction and further lower for the PT reconstructed compared to the ST reconstructed patients (p<0.05). CONCLUSIONS: The findings of the present study indicate that, using a similar regeneration programme, the muscle capacities and functional motor skills show faster regeneration processes for patients without reconstruction compared to a reconstruction treatment after an ACL injury. Concerning the reconstruction treatment, PT reconstructions seem to affect the degeneration process of the muscle capacities of the treated limb more than ST reconstructions.

[Conservative versus operative treatment after ACL-rupture: influence on the muscle strength capability of the lower extremity]. / Konservative vs. operative Behandlungsmethode nach einer ACL-Ruptur: Einfluss auf die muskulären Kraftfähigkeiten der unteren Extremität.

PURPOSE: In this study two groups of ACL-rupture patients having had a conservative or an operative treatment and a similar rehabilitation were compared regarding their muscle strength capabilities, and the clinical outcome. METHODS: Thirty-three patients (mean age 31 years) were selected according to the Lysholm-scores. Twelve patients were conservatively treated (Group I) and were examined between six and 16 months post trauma. Group II consisted of 21 operatively treated patients which were examined between 6 and 16 months post surgery. All patients underwent a clinical exam following the OAK evaluation form and were tested on a Cybex 6000 isokinetic machine. The subjects had to perform concentric contractions of the extensor and flexor muscles of the knee, hip and ankle at 60 and 120 degrees /s. In addition the knee musculature was isometrically tested at knee angles of 0 and 45 degrees . RESULTS: The clinical examination revealed significant (P < 0.05) differences between groups. The operatively treated patients scored better at the tests for ligamentous stability, whereas the testing of the muscular capabilities revealed significant differences (P < 0.05) favouring the conservatively treated patients in the isokinetic measures, (maximal concentric torque M (max,con)) of the knee extensors, knee flexors, and ankle plantarflexors. CONCLUSION: Whereas the clinical examination only revealed the ligamentous stability to be better results in the operatively treated patients, the conservatively treated patients showed better results in the isokinetik strength measures of the knee extensors, knee flexors, and ankle plantarflexors. The better muscular profiles found for the conservatively treated patients suggest that a better regeneration of the corresponding muscular capabilities after an ACL rupture is possible, provided a specific and controlled rehabilitation program is performed. This seems to be more difficult in operatively treated patients even six to 16 months after the surgical intervention.

Strain and elongation of the human gastrocnemius tendon and aponeurosis during maximal plantarflexion effort.

Regarding the strain and elongation distribution along the tendon and aponeurosis the literature is reporting different findings. Therefore, the purpose of this study was to examine in vivo the elongation and the strain of the human gastrocnemius medialis tendon and aponeurosis simultaneously at the same trial during maximal voluntary plantarflexion efforts. Twelve subjects participated in the study. The subjects performed isometric maximal voluntary contractions of their left leg on a Biodex-dynamometer. The kinematics of the leg were recorded using the Vicon 624 system with 8 cameras operating at 120 Hz. Two ultrasound probes were used to visualise the tendon (myotendinous junction region) and the distal aponeurosis of the gastrocnemius medialis respectively. The main findings were: (a) the absolute elongation of the gastrocnemius medialis tendon was different to that of the aponeurosis, (b) the strain of the gastrocnemius medialis tendon did not differ from the strain of the aponeurosis, (c) during the "isometric" plantarflexion the ankle angle exhibited significant changes, and (d) the non-rigidity of the dynamometer arm-foot system and the coactivity of the tibialis anterior both have a significant influence on the moment exerted at the ankle joint. Thus the strain of the human gastrocnemius medialis tendon and aponeurosis estimated in vivo using two-dimensional ultrasonography is uniform. To calculate the elongation of the whole tendon it is necessary to multiply the strain calculated for the examined part of the tendon by the total length of the tendon.

[Development of muscle strength in knee, hip and ankle joints after ACL reconstruction]. / Entwicklung der muskulären Kraftfähigkeiten im Knie-, Hüft- und Fussgelenk nach einer ACL-Rekonstruktion.

INTRODUCTION: We searched for answers for following questions: How does the knee muscle strength change in dependence of post OP time after the same ACL-rehabilitation's program? Which changes in muscle strength occur in hip and ankle muscles after ACL? METHODS: In this study 69 patients with ACL-Reconstruction participated, mean 31 years. The Lysholm- and OAK-scores of all were good respectively very good. 28 patients were 3 - 6 months post OP (Gr. I), 20 were 6 - 12 months post OP (Gr. II), 21 was 12 - 24 months post OP (Gr. II). All patients were tested on the isokinetic Cybex 6000, for dynamic concentric strength and endurance. The evaluation parameters chosen were: max.moment at angle speed of 60 degrees /sec and endurance index at 120 degrees /sec, RESULTS: The analysis of the max. moment showed significant (P < 0.05) changes in both knee extension and flexion in all groups, in the hip flexors and extensors in group I, and hip flexors in group II and III. In the ankle joint we only found significant changes in the plantarflexors of group I. Looking at the endurance index we observed that many muscle groups of the injured leg presented higher values, however the difference is only significant in the knee flexors and extensors of group I and in the hip flexors of group III. CONCLUSION: Our study showed strength loss in both knee extensors and knee flexors in all patients who had undergone reconstruction surgery after the same ACL-rehabilitation's program. We can state that there is strength loss in hip and ankle muscles in 3 - 6 months post OP patients. A reduction of muscle strength at the hip flexor from all patients was seen after 12 months post-OP. It is suggested that the strength loss in the hip flexors could be due to bi-articular muscles surrounding the knee.This results has to be consider on the planning of strength training after ACL injuries.