A novel single-leg squat test with speed and accuracy requirements: Reliability and validity in anterior cruciate ligament reconstructed individuals.

BACKGROUND: Some traditional single-leg squat tests focused on number of repetitions may not demand precise control of lower limb dynamic alignment, especially in the frontal and transverse planes. The primary objective of this study was to evaluate test-retest reliability and construct validity of a novel single-leg squat test - the 'precision-squat test' (PST) - designed to assess performance under varying task demands that can impact the execution of lower limb movements. A secondary objective was to investigate whether musculoskeletal factors predict performance in the PST in healthy individuals. METHODS: Thirty healthy participants were assessed to verify test-retest reliability. To verify the test's construct validity, we compared the performance of 21 anterior cruciate ligament reconstructed (ACLR) individuals and 21 matched controls. Finally, 36 healthy individuals were assessed to verify the musculoskeletal factors related to PST performance. All participants performed the PST: they executed single-leg squats while moving a laser pointer (attached to the thigh) between two targets. We varied target size and distance between targets to manipulate the task difficulty. RESULTS: Reliability of the PST was excellent at all demand levels (intraclass correlation coefficient (ICC)(3,2) > 0.93). Squat time increased under test conditions involving higher task difficulty (P < 0.001) and in ACLR individuals compared with age-matched controls (P < 0.05). Regression analyses revealed that reduced knee extensors and hip external rotators torques are related to increased squat time (P < 0.05). CONCLUSIONS: PST is a valid and reliable tool to assess performance of healthy and ACLR individuals. In addition, hip and knee strength are associated with performance during the test.

Muscular performance characterization in athletes: a new perspective on isokinetic variables

Background: Isokinetic dynamometry allows the measurement of several variables related to muscular performance, many of which are seldom used, while others are redundantly applied to the characterization of muscle function. Objectives: The present study aimed to establish the particular features of muscle function that are captured by the variables currently included in isokinetic assessment and to determine which variables best represent these features in order to achieve a more objective interpretation of muscular performance. Method: This study included 235 male athletes. They performed isokinetic tests of concentric knee flexion and extension of the dominant leg at a velocity of 60º/s. An exploratory factor analysis was performed. Results: The findings demonstrated that isokinetic variables can characterize more than muscle torque production and pointed to the presence of 5 factors that enabled the characterization of muscular performance according to 5 different domains or constructs. Conclusions: The constructs can be described by torque generation capacity; variation of the torque generation capacity along repetitions; movement deceleration capacity; mechanical/physiological factors of torque generation; and acceleration capacity (torque development). Fewer than eight out of sixteen variables are enough to characterize these five constructs. Our results suggest that these variables and these 5 domains may lead to a more systematic and optimized interpretation of isokinetic assessments. .

Dynamic touch is affected in children with cerebral palsy.

Children with developmental disorders such as cerebral palsy have limited opportunities for effortful interactions with objects and tools. The goal of the study was to investigate whether children with cerebral palsy have deficits in their ability to perceive object length by dynamic touch when compared to typically developing children. Fourteen children with typical development and 12 children with cerebral palsy were asked to report the length of hand-held rods after wielding them out of sight. Multilevel regression models indicated that I1 (maximum principal moment of inertia) was a significant predictor of perceived length - LP (p<.0001). The effect of I1 on LP was significantly different among children (p=.001) and the presence of cerebral palsy (group factor) partially explained such variance (p=.002). In addition, accuracy and reliability of the length judgments made by children with cerebral palsy were significantly lower than the typically developing children (p<.05). Theoretical and clinical implications of these results were identified and discussed.

Muscular performance characterization in athletes: a new perspective on isokinetic variables.

BACKGROUND: Isokinetic dynamometry allows the measurement of several variables related to muscular performance, many of which are seldom used, while others are redundantly applied to the characterization of muscle function. OBJECTIVES: The present study aimed to establish the particular features of muscle function that are captured by the variables currently included in isokinetic assessment and to determine which variables best represent these features in order to achieve a more objective interpretation of muscular performance. METHOD: This study included 235 male athletes. They performed isokinetic tests of concentric knee flexion and extension of the dominant leg at a velocity of 60º/s. An exploratory factor analysis was performed. RESULTS: The findings demonstrated that isokinetic variables can characterize more than muscle torque production and pointed to the presence of 5 factors that enabled the characterization of muscular performance according to 5 different domains or constructs. CONCLUSIONS: The constructs can be described by torque generation capacity; variation of the torque generation capacity along repetitions; movement deceleration capacity; mechanical/physiological factors of torque generation; and acceleration capacity (torque development). Fewer than eight out of sixteen variables are enough to characterize these five constructs. Our results suggest that these variables and these 5 domains may lead to a more systematic and optimized interpretation of isokinetic assessments.

Assessment of gait in toddlers with normal motor development and in hemiplegic children with mild motor impairment: a validity study.

BACKGROUND: The optimization of gait performance is an important goal in the rehabilitation of children with cerebral palsy (CP) who present a prognosis associated with locomotion. Gait analysis using videos captured by digital cameras requires validation. OBJECTIVE: To evaluate the validity of a method that involves the analysis of videos captured using a digital camera for quantifying the temporal parameters of gait in toddlers with normal motor development and children with CP. METHOD: Eleven toddlers with normal motor development and eight children with spastic hemiplegia who were able to walk without assistive devices were asked to walk through a space contained in the visual field of two instruments: a digital camera and a three-dimensional motion analysis system, Qualisys Pro-Reflex. The duration of the stance and swing phases of gait and of the entire gait cycle were calculated by analyzing videos captured by a digital camera and compared to those obtained by Qualisys Pro-Reflex, which is considered a highly accurate system. RESULTS: The Intraclass Correlation Coefficient (ICC) demonstrated excellent agreement (ICC>0.90) between the two procedures for all measurements, except for the swing phase of the normal toddlers (ICC=0.35). The standard error of measurement was less than 0.02 seconds for all measures. CONCLUSIONS: The results reveal similarities between the two instruments, suggesting that digital cameras can be valid instruments for quantifying two temporal parameters of gait. This congruence is of clinical and scientific relevance and validates the use of digital cameras as a resource for helping the assessment and documentation of the therapeutic effects of interventions targeted at the gait of children with CP.

Assessment of gait in toddlers with normal motor development and in hemiplegic children with mild motor impairment: a validity study / Avaliacao da marcha de lactentes com desenvolvimento motor normal e criancas hemiplegicas com comprometimento motor leve: estudo de validade

BACKGROUND: The optimization of gait performance is an important goal in the rehabilitation of children with cerebral palsy (CP) who present a prognosis associated with locomotion. Gait analysis using videos captured by digital cameras requires validation. OBJECTIVE: To evaluate the validity of a method that involves the analysis of videos captured using a digital camera for quantifying the temporal parameters of gait in toddlers with normal motor development and children with CP. METHOD: Eleven toddlers with normal motor development and eight children with spastic hemiplegia who were able to walk without assistive devices were asked to walk through a space contained in the visual field of two instruments: a digital camera and a three-dimensional motion analysis system, Qualisys Pro-Reflex. The duration of the stance and swing phases of gait and of the entire gait cycle were calculated by analyzing videos captured by a digital camera and compared to those obtained by Qualisys Pro-Reflex, which is considered a highly accurate system. RESULTS: The Intraclass Correlation Coefficient (ICC) demonstrated excellent agreement (ICC>0.90) between the two procedures for all measurements, except for the swing phase of the normal toddlers (ICC=0.35). The standard error of measurement was less than 0.02 seconds for all measures. CONCLUSIONS: The results reveal similarities between the two instruments, suggesting that digital cameras can be valid instruments for quantifying two temporal parameters of gait. This congruence is of clinical and scientific relevance and validates the use of digital cameras as a resource for helping the assessment and documentation of the therapeutic effects of interventions targeted at the gait of children with CP. .

CONTEXTUALIZAÇÃO : A otimização da marcha é objetivo relevante na reabilitação de crianças com prognóstico de locomoção na paralisia cerebral (PC). A análise da marcha com vídeos capturados por câmeras digitais necessita ser validada. OBJETIVO : Avaliar a validade de um método que envolve a inspeção de vídeos capturados por câmera digital para quantificação de variáveis temporais da marcha de lactentes com desenvolvimento motor normal e crianças com PC. MÉTODO : Onze lactentes com desenvolvimento motor normal e oito crianças com PC do tipo hemiplegia espástica capazes de deambular sem dispositivos de auxílio de marcha foram solicitadas a caminhar em um espaço contido no campo visual de dois instrumentos: câmera digital e sistema tridimensional de análise de movimento Qualisys Pro-Reflex. As medidas de duração das fases de apoio e balanço e o tempo total do ciclo da marcha foram calculados a partir da análise de vídeos de câmera digital e comparados às medidas obtidas pelo Qualisys Pro-Reflex, considerado sistema de alta acurácia. RESULTADOS: O Coeficiente de Correlação Intraclasse (CCI) demonstrou concordância excelente (CCI>0,90) nas medidas dos dois grupos, exceto para a fase de balanço dos lactentes (CCI=0,35). O erro padrão das medidas foi menor que 0,02 segundos para todas as mensurações. CONCLUSÕES : Resultados revelam semelhança entre os dois instrumentos, sugerindo que a câmera digital pode ser instrumento válido para quantificação de dois parâmetros temporais da marcha. Tal congruência tem relevância clínica e científica, preconizando uso da câmera ...

Impact of leg length and body mass on the stride length and gait speed of infants with normal motor development: a longitudinal study.

BACKGROUND: Gait acquisition is supported by changes in the neuromusculoskeletal system of the child. Changes in the dimensions of the body structures resulting from the growth of the child partly explain gait improvement in the first year of life. OBJECTIVES: To evaluate whether changes in body mass and leg length modulate the effect of independent gait practice (experience) on gait speed and stride length. METHOD: Thirty-two infants with normal development were monitored monthly from the acquisition of independent gait until six months post-acquisition. Longitudinal evaluations included measurements of the body mass and leg length of each child. Temporospatial variables of gait (speed and stride length) were documented using the Qualisys Pro-reflex(r) system. The data were analyzed using multilevel regression models, with a significance level of α=0.05. RESULTS: An effect of the practice time on speed (p<0.0001) and stride length (p<0.0001) was observed. The change in leg length had a marginal effect on the rate of gait speed change: children whose leg growth was faster showed a higher rate of speed change (p=0.07). No other effects of anthropometric parameters were observed. CONCLUSIONS: The results suggest that the practice time promotes the improvement of the gait pattern of infants in the first year of life. However, the effects of the leg length and body weight of infants on the benefit of practice time remain undefined.

Impact of leg length and body mass on the stride length and gait speed of infants with normal motor development: A longitudinal study / Impact of leg length and body mass on the stride length and gait speed of infants with normal motor development: A longitudinal study

BACKGROUND: Gait acquisition is supported by changes in the neuromusculoskeletal system of the child. Changes in the dimensions of the body structures resulting from the growth of the child partly explain gait improvement in the first year of life. OBJECTIVES: To evaluate whether changes in body mass and leg length modulate the effect of independent gait practice (experience) on gait speed and stride length. METHOD: Thirty-two infants with normal development were monitored monthly from the acquisition of independent gait until six months post-acquisition. Longitudinal evaluations included measurements of the body mass and leg length of each child. Temporospatial variables of gait (speed and stride length) were documented using the Qualisys Pro-reflex(r) system. The data were analyzed using multilevel regression models, with a significance level of α=0.05. RESULTS: An effect of the practice time on speed (p<0.0001) and stride length (p<0.0001) was observed. The change in leg length had a marginal effect on the rate of gait speed change: children whose leg growth was faster showed a higher rate of speed change (p=0.07). No other effects of anthropometric parameters were observed. CONCLUSIONS: The results suggest that the practice time promotes the improvement of the gait pattern of infants in the first year of life. However, the effects of the leg length and body weight of infants on the benefit of practice time remain undefined. .

CONTEXTUALIZAÇÃO: A aquisição da marcha é suportada por mudanças no sistema neuromusculoesquelético da criança. a literatura aponta que modificações nas dimensões das estruturas corporais resultantes do crescimento da criança explicam, em parte, o aprimoramento da marcha no primeiro ano de vida. OBJETIVOS: Avaliar se mudanças na massa corporal e comprimento da perna modulam o efeito da prática da marcha independente nos seguintes parâmetros da marcha: velocidade e comprimento da passada. MÉTODO: Trinta e dois lactentes com desenvolvimento normal foram acompanhados mensalmente a partir da aquisição da marcha independente até seis meses pós-aquisição. Avaliações longitudinais incluíram mensurações da massa corporal e do comprimento da perna de cada criança. As variáveis temporoespaciais da marcha (velocidade e comprimento da passada) foram documentadas utilizando-se o sistema Qualisys Pro-Reflex (r) . Os dados foram analisados por modelos de regressão multinível, com nível de significância α=0,05. RESULTADOS: Encontrou-se um efeito do tempo de prática nas medidas de velocidade (p<0,0001) e comprimento da passada (p<0,0001). a mudança no comprimento da perna teve um efeito marginal na taxa de mudança da velocidade de marcha: crianças cujo crescimento da perna foi mais acelerado apresentaram uma taxa de mudança de velocidade maior (p=0,07). Nenhum outro efeito dos parâmetros antropométricos foi observado. CONCLUSÕES: Os resultados sugerem que o tempo de prática promove o aprimoramento do padrão de marcha de lactentes no primeiro ano de vida. Por sua vez, os efeitos do comprimento da perna e da massa corporal ...

Relação entre rigidez articular passiva e torque concêntrico dos rotadores laterais do quadril / Relationship between joint passive stiffness and hip lateral rotator concentric torque

CONTEXTUALIZAÇÃO: Rigidez passiva adequada do quadril pode impedir movimentos excessivos dos membros inferiores no plano transverso durante a realização de atividades funcionais. O fortalecimento muscular dos rotadores laterais do quadril poderia ser utilizado na tentativa de aumentar a rigidez dessa articulação. No entanto, a relação entre rigidez passiva e força dos músculos do quadril não está documentada na literatura. OBJETIVO: Investigar a associação entre rigidez passiva do quadril durante o movimento de rotação medial e torque concêntrico dos rotadores laterais dessa articulação em indivíduos saudáveis. MÉTODO: Foram avaliados 26 indivíduos com média de idade de 24,42±2,77 anos. Para quantificação da rigidez passiva do quadril, o torque passivo de resistência durante a rotação medial dessa articulação foi mensurado por um dinamômetro isocinético. A rigidez foi determinada como a inclinação média da curva de torque passivo obtida nos primeiros 20° do movimento. Eletromiografia foi utilizada para verificar o repouso dos músculos do quadril durante esse procedimento. O dinamômetro isocinético também foi utilizado para avaliação do pico de torque e trabalho máximo dos rotadores laterais do quadril em uma amplitude de 55° de rotação. RESULTADOS: Regressões lineares demonstraram coeficientes de correlação r=0,70 (R²=0,50/p<0,001) e r=0,77 (R²=0,59/p<0,001) entre rigidez do quadril e as medidas de pico de torque e trabalho muscular dos rotadores laterais, respectivamente. CONCLUSÕES: Existe associação de moderada a boa entre rigidez passiva do quadril e torque concêntrico dos rotadores laterais dessa articulação. A associação demonstrada sugere que o fortalecimento dos rotadores laterais pode ser eficaz em aumentar a rigidez do quadril.

BACKGROUND: Adequate passive stiffness of the hip joint can prevent the occurrence of excessive transverse plane lower limb movement during functional activities. Strength training of the hip lateral rotator muscles can be used to increase the stiffness of this joint. However, the relationship between hip joint passive stiffness and muscle strength remains undocumented in the literature. OBJECTIVE: To investigate the association between hip passive stiffness measured during medial rotation and hip lateral rotator concentric torque in healthy young adults. METHOD: Twenty-six individuals with mean age of 24.42±2.77 years participated in the present study. To quantify hip stiffness, the passive resistance torque during medial rotation was measured using an isokinetic dynamometer. Stiffness was determined by the mean slope of the passive torque curve obtained in the first 20° of motion. Electromyography was used to ensure inactivity of the hip muscles during this procedure. The isokinetic dynamometer was also used for assessment of hip lateral rotator peak torque and work in a range of motion of 55° of rotation. RESULTS: Linear regressions demonstrated correlation coefficients of r=0.70 (R²=0.50/p<0.001) and r=0.77 (R²=0.59/p<0.001) between hip passive stiffness and the measures of lateral rotator peak torque and work, respectively. Conclusions: There is a moderate to good association between hip passive stiffness and lateral rotator concentric torque. This association suggests that lateral rotator strength training can increase hip stiffness.

Relationship between joint passive stiffness and hip lateral rotator concentric torque.

BACKGROUND: Adequate passive stiffness of the hip joint can prevent the occurrence of excessive transverse plane lower limb movement during functional activities. Strength training of the hip lateral rotator muscles can be used to increase the stiffness of this joint. However, the relationship between hip joint passive stiffness and muscle strength remains undocumented in the literature. OBJECTIVE: To investigate the association between hip passive stiffness measured during medial rotation and hip lateral rotator concentric torque in healthy young adults. METHOD: Twenty-six individuals with mean age of 24.42±2.77 years participated in the present study. To quantify hip stiffness, the passive resistance torque during medial rotation was measured using an isokinetic dynamometer. Stiffness was determined by the mean slope of the passive torque curve obtained in the first 20° of motion. Electromyography was used to ensure inactivity of the hip muscles during this procedure. The isokinetic dynamometer was also used for assessment of hip lateral rotator peak torque and work in a range of motion of 55° of rotation. RESULTS: Linear regressions demonstrated correlation coefficients of r=0.70 (R²=0.50/p<0.001) and r=0.77 (R²=0.59/p<0.001) between hip passive stiffness and the measures of lateral rotator peak torque and work, respectively. CONCLUSIONS: There is a moderate to good association between hip passive stiffness and lateral rotator concentric torque. This association suggests that lateral rotator strength training can increase hip stiffness.

Stretching versus strength training in lengthened position in subjects with tight hamstring muscles: a randomized controlled trial.

Stretching is used to modify muscle length. However, its effects seem to be temporary. There is evidence in animal models that strengthening in a lengthened position may induce long lasting changes in muscle length. The objective of this study was to compare changes in hamstrings flexibility, peak torque angle and stretch tolerance after two training programs: stretching and strengthening in a lengthened position. Forty-five subjects with tight hamstrings were randomly assigned into three groups: control, stretching and strength training in lengthened position. The interventions were performed three times a week for eight weeks. The subjects were assessed before and after the end of the programs. Data provided by an isokinetic dynamometer were used to assess hamstrings flexibility, peak torque angle, and stretch tolerance. The data analysis demonstrated that strengthening in lengthened position changed peak torque angle in the direction of knee extension (p=0.001). No change in flexibility was observed (p=0.449). Both experimental groups showed an increase in stretch tolerance (p=0.001). The results demonstrated that strengthening in a lengthened position produced a shift of the torque-angle curve, which suggests an increase in muscle length. Conversely, stretching did not produce modification of torque-angle curve and flexibility; its effects appear restricted to increases in stretch tolerance.

Alterations of stiffness and resting position of the elbow joint following flexors resistance training.

Skeletal muscles adapt their length and stiffness according to the functional demands to which they are regularly submitted. The modification of muscle stiffness and length induced by resistance training can alter joint stiffness and, theoretically, change joint resting position. Thirty subjects, randomly assigned to two groups, were submitted to a resistance training of the elbow flexor muscles of the non-dominant arm. This training was performed in the inner range in group 1 and throughout the complete range of motion in group 2. The dominant arm of each subject was considered the control. A biomechanical model of a hybrid mass-spring pendulum was used to estimate the elbow joint stiffness and the elbow resting position was assessed using a standard goniometer. There was a significant increase in joint stiffness of the experimental arm after complete range of motion training, which was not observed after the training performed in inner range. The resting position of the experimental arm was modified to a significantly greater flexion angle in both groups. No change was observed after resistance training in the control arm. The results showed that modifications in joint stiffness seem to depend on the volume of work imposed to skeletal muscles. In addition, both models of resistance training changed, through different mechanisms, the elbow joint resting position. These findings suggest that posture and joint stability may be modified by specific strengthening protocols.

Caracterização da performance muscular em atletas profissionais de futebol / Characterization of professional soccer players' muscle performance

INTRODUÇÃO E OBJETIVOS: Estudos que relacionam parâmetros de performance muscular com risco de lesão e desempenho em diferentes esportes têm sido freqüentemente reportados na literatura. Entretanto, há carência de dados que caracterizem a performance muscular em atletas profissionais do futebol brasileiro. Portanto, o objetivo deste estudo foi realizar uma análise descritiva dos parâmetros relacionados à performance muscular dessa população. MÉTODOS: A amostra consistiu de 117 atletas pertencentes a clubes de elite do futebol mineiro. Para avaliação da performance muscular foi utilizado um dinamômetro isocinético e os testes consistiram de contrações concêntricas máximas das musculaturas avaliadas. No quadril, as musculaturas adutora e abdutora foram avaliadas nas velocidades 30º/s, 60º/s e 120º/s. No joelho, foram avaliados os músculos flexores e extensores a 60º/s, 180º/s e 300º/s. No tornozelo, os músculos dorsoflexores, flexores plantares, inversores e eversores foram avaliados nas velocidades 30º/s, 60º/s e 180º/s. Para análise dos dados foram utilizados estatística descritiva e testes t pareados para avaliar diferenças entre pernas. RESULTADOS: Os resultados deste estudo caracterizam o perfil de atletas profissionais do futebol relativo à capacidade de produção de torque, trabalho máximo e potência média. Além disso, foram observadas diferenças significativas entre pernas em algumas variáveis. CONCLUSÃO: Os dados normativos estabelecidos podem ser utilizados como valores de referência na prevenção, treinamento e reabilitação dos atletas, além de servirem de referência para futuros estudos que tenham como objetivo relacionar os parâmetros de performance muscular à incidência de lesões no futebol.

INTRODUCTION AND OBJECTIVE: The association of muscular performance with risk of injury and functional performance in different sports has been reported in numerous studies in the literature. However, there is a paucity of data that characterizes the muscular performance in Brazilian professional soccer athletes. Therefore, the objective of this study was to make a descriptive analysis of parameters related to the muscular performance of this population. METHODS: The sample of this study was composed of 117 athletes belonging to professional soccer clubs in Minas Gerais State. In order to evaluate the muscular performance of the athletes, an isokinetic dynamometer was used and the tests involved maximum voluntary contractions of the selected muscles. Hip abductors and adductors were assessed at the speeds of 30º/s, 60º/s and 120º/s; knee flexors and extensors at 60º/s, 180º/s and 300º/s; and ankle dorsiflexors, plantarflexors, invertors and evertors at 30º/s, 60º/s and 180º/s. Descriptive statistics were used to present the normative data and paired t-tests were used to identify significant differences between legs considering the parameters evaluated in this study. RESULTS: This study generated normative data to characterize the profile of Brazilian professional soccer players relative to their capacity of producing torque, muscle work and power. Significant differences were observed between legs considering some of the study's variables. CONCLUSION: The established normative data can be used as reference values in the prevention, training and rehabilitation of the athletes. In addition, these data may be used as reference for future studies with the objective of testing the association between muscular performance and incidence of injury in soccer practice.

Proprioception in individuals with ACL-deficient knee and good muscular and functional performance.

The objective of this study was to verify whether proprioception is affected in individuals with ACL-deficient knees and good functional and muscular performances. Eleven subjects with ACL injury and 11 controls participated in the study. Functional performance was assessed using the Cincinnati Knee Rating System (CKRS), hop index, and figure-eight ratio. An isokinetic test was done to evaluate muscular performance. Proprioception was evaluated through position, sense and threshold tests. Analyses of variance were used for data analysis. The injured subjects scored significantly lower in the CKRS (p = 0.001). No statistically significant differences were found in the hop index, in the figure-eight ratio, or in peak torque. There were no statistically significant differences in proprioception bettween groups and between legs. These results indicated that the individuals evaluated in this study with ACL injury and good functional and muscular performance did not have proprioceptive deficits, suggesting that the ligament mechanoreceptors, in some cases, might not contribute relevantly to proprioception.

Analyses of dynamic co-contraction level in individuals with anterior cruciate ligament injury.

BACKGROUND: A complete understanding of neural mechanisms by which ligament receptors may contribute to joint stability is not well established. It has been suggested that these receptors may be involved in a neuromuscular process related to the modulation of dynamic co-contraction, as a means of guaranteeing functional joint stability. HYPOTHESIS: Individuals with ACL injury have diminished dynamic co-contraction. STUDY DESIGN: Exploratory, cross-sectional design. METHODS: Ten subjects with unilateral ACL injury treated conservatively, and ten subjects without history of injury participated in the study. The co-contraction level was assessed through EMG recordings of the vastus lateralis and biceps femoris before and after a perturbation imposed on the subjects during a walking task. RESULTS: Subjects with ACL injury presented significantly lower co-contraction level pre-perturbation (p = 0.045) and post-perturbation (p = 0.046) than those in the control group. CONCLUSIONS: The bilateral decrease in muscular co-contraction presented by individuals with ACL injury suggests that ligament and joint receptors may be responsible for a bilateral dynamic increase in muscle and joint stiffness that could result in a greater joint stability. CLINICAL RELEVANCE: This study analyzed a neuromuscular mechanism that might contribute to the functional stability of the knee joint.