Identifying Muscle Function-based Phenotypes Associated With Radiographic Progression of Secondary Hip Osteoarthritis.

OBJECTIVE: The purposes of our study were to (1) identify muscle function-based clinical phenotypes in patients with hip osteoarthritis (OA) and (2) determine the association between those phenotypes and radiographic progression of hip OA. DESIGN: Prospective cohort study. SETTING: Clinical biomechanics laboratory of a university. PARTICIPANTS: Fifty women patients with mild-to-moderate secondary hip OA (N=50) were recruited from the orthopedic department of a single institution. INTERVENTIONS: Not applicable. MAIN OUTCOME MEASURES: Two-step cluster analyses were performed to classify the patients, using hip flexion, extension, abduction, and external/internal rotation muscle strength (cluster analysis 1); relative hip muscle strength to total hip strength (ie, hip muscle strength balance; cluster analysis 2); and both hip muscle strength and muscle strength balance (cluster analysis 3) as variables. The association between the phenotype and hip OA progression over 12 months (indicated by joint space width [JSW] >0.5 mm) was investigated by logistic regression analyses. Hip joint morphology, hip pain, gait speed, physical activity, Harris hip score, and SF-36 scores were compared between the phenotypes. RESULTS: Radiographic progression of hip OA was observed in 42% of the patients. The patients were classified into 2 phenotypes in each of the 3 cluster analyses. The solution in cluster analyses 1 and 3 was similar, and high-function and low-function phenotypes were identified; however, no association was found between the phenotypes and hip OA progression. The phenotype 2-1 (high-risk phenotype) extracted in cluster analysis 2, which had relative muscle weakness in hip flexion and internal rotation, was associated with subsequent hip OA progression, even after adjusting for age and minimum JSW at baseline (adjusted odds ratio [95% confidence interval], 3.60 [1.07-12.05]; P=.039). CONCLUSION: As preliminary findings, the phenotype based on hip muscle strength balance, rather than hip muscle strength, may be associated with hip OA progression.

Effect of age on shear modulus, muscle thickness, echo intensity of the upper limb, lower limb, and trunk muscles in healthy women.

PURPOSE: This study aimed to examine the effect of age on the mechanical properties, muscle size, and muscle quality in the upper and lower limb and trunk muscles. METHODS: We evaluated the shear modulus (G), muscle thickness (MT), and echo intensity (EI) of the upper and lower limb and trunk muscles of 83 healthy women (21-83-year-old). The G values of some limb muscles were measured in relaxed and stretched positions. RESULTS: Regarding the effect of age on G at the distinct positions, the G of the upper limb muscles were not significantly correlated with age in the relaxed and stretched positions. In contrast, the G of the iliacus showed a significant negative correlation in both positions. Additionally, the G of the rectus femoris had a significant negative correlation only in the relaxed position. Regarding differences among body parts, the G of the lower limb and oblique abdominal muscles showed a significant negative correlation, but no correlation in the upper limb, rectus abdominis, and back muscles. Moreover, MT showed a significant negative correlation with age in the lower limb, abdominal, and erector spinae muscles, but no correlation was detected in the upper limb and lumbar multifidus muscles. EI had a significant positive correlation in all the muscles. CONCLUSION: The effect of age on G depended on body parts, and the G of the lower limb and oblique abdominal muscles negatively associated with age. Additionally, G in the relaxed position may be more susceptible to aging than G in the stretched position.

Synergistic Dominance Induced by Hip Extension Exercise Alters Biomechanics and Muscular Activity During Sprinting and Suggests a Potential Link to Hamstring Strain.

ABSTRACT: Iguchi, J, Hojo, T, Fujisawa, Y, Kuzuhara, K, Yanase, K, Hirono, T, Koyama, Y, Tateuchi, H, and Ichihashi, N. Synergistic dominance induced by hip extension exercise alters biomechanics and muscular activity during sprinting and suggests a potential link to hamstring strain. J Strength Cond Res 37(9): 1770-1776, 2023-Hamstring strain is likely to occur during the late swing phase or the first half of the stance phase in sprinting. During the late swing phase, the hamstrings and gluteus maximus (Gmax) contract eccentrically to decelerate the lower limb. We hypothesized that, when the Gmax becomes dysfunctional because of delayed onset muscle soreness (DOMS), the hamstring workload is increased (i.e., there is synergetic dominance), which could lead to an increased risk of strain. A total of healthy 15 male undergraduate or graduate students (age 23.1 ± 1.28 years) were recruited to perform exercises and maximal sprints. On day 1, before subjects performing DOMS-causing exercises, and on day 3, while subjects were experiencing DOMS in the Gmax, lower-limb biomechanical and muscle activity data were recorded using a motion analysis system and electromyography (EMG), respectively. Data were analyzed and compared between day 1 and day 3. Hip flexion angle on day 3 was significantly lower than that on day 1, but the opposite was true for the knee flexion angle (P < 0.05). Vastus medialis (VM), biceps femoris (BF), and Gmax muscle activities on day 3 were significantly higher than those on day 1 (P < 0.05). Peak propulsive forces on day 3 were significantly higher than those on day 1 (P < 0.05). Kinematic changes such as decreased hip flexion angle and EMG changes such as increased BF EMG activity on day 3 to compensate for the loss of function of the Gmax may potentially increase the risk of hamstring strain.

Age- and sex-related differences of muscle cross-sectional area in iliocapsularis: a cross-sectional study.

BACKGROUND: This study aimed to determine in how many individuals the iliocapsularis muscle (IC) could be identified on magnetic resonance imaging (MRI) and whether age and sex are associated with the cross-sectional area (CSA) of the IC. METHODS: Thirty-seven healthy younger adults and 40 healthy older adults were assigned to four groups: 1) 20 younger men; 2) 17 younger women; 3) 20 older men; and 4) 20 older women. The CSAs of the IC, IP, the rectus femoris (RF) and the quadriceps (QUAD) were quantified on an axial MRI. RESULTS: The number of individuals with the identified IC was n = 17 (85.0%) of 20 younger men, n = 15 (88.2%) of 17 younger women, n = 18 (90.0%) of 20 older men, and 19 (95.0%) of 20 older women. Our results showed the main effect of sex, but not age, in the CSA of the IC. The men-groups had larger CSA of the IC than the women-groups; however, no difference in CSA of the IC was found between the younger and older groups. Meanwhile, the main effects of age and sex were found for the IP, RF, and QUAD; thus, younger or men groups have larger CSAs of the three muscles than the older or women groups. The IC muscle can be discriminated in 85% - 95% of healthy individuals. CONCLUSION: Although sex and age are associated with the CSA of lower-limb muscles other than the IC, only sex is associated with the CSA of the IC.

The effect of Liquid ice after high-intensity exercise on muscle function compared to Block ice.

Cryotherapy is used to recover muscle damage after exercise and to treat acute sports injuries. Liquid ice (LI) can keep cold for a long time, and is assumed more effective than block ice (BI). From this, the aim of this study was to investigate the effects of LI on the change of passive stiffness (PS) as muscle function and to validate the effectiveness of LI compared to BI. We performed the experiment as part of a case series of verification of the effects of cryotherapy. 22 healthy men (target area: right leg) were randomized to two groups: LI group and BI group. PS was measured three times during experiment protocol, pre: before exercise; post; after treating each cryotherapy after exercise; 48h: 48 hours after pre. Statistical analysis compared the PS, the amount of change in PS, and the rate of change in PS between the two groups. The rate of change between pre and 48h in LI was significantly lower compared to that in BI (p = 0.03). There was no significant difference regarding other results between groups. It revealed that the difference of effect between LI and BI for PS of muscles after high-intensity exercises. These results could be helpful for the choice of intervention for reducing muscle stiffness after exercise and at sports field.

The relation between kinematic synergy to stabilize the center of mass during walking and future fall risks: a 1-year longitudinal study.

BACKGROUND: Incorrect body weight shifting is a frequent cause of falls, and the control of the whole-body center of mass (CoM) by segmental coordination is essential during walking. Uncontrolled manifold (UCM) analysis is a method of examining the relation between variance in segmental coordination and CoM stability. However, no prospective cohort study has thoroughly investigated how variance in segmental configurations to stabilize the CoM relates to future falls. This study explored whether variance to stabilize the CoM was related to future falls. METHODS: At the baseline visit, 30 community-dwelling older adults walked 20 times on a 6-m walkway. Using kinematic data collected during walking by a three-dimensional motion capture system, UCM analysis was performed to investigate how segmental configuration contributes to CoM stability in the frontal plane. One year after the baseline visit, we evaluated whether the subjects experienced falls. Twelve subjects had experienced falls, and 16 had not. Comparisons of variance between older adults with and without falls were conducted by covariate analysis. RESULTS: No significant differences in variance were found in the mediolateral direction, whereas in the vertical direction, older adults with fall experiences had a greater variance, reflecting an unstable CoM, than those with no fall experiences. CONCLUSIONS: We verified that the high variance in segmental configurations that destabilize the CoM in the vertical direction was related to future falls. The variables of UCM analysis can be useful for evaluating fall risk.

Effects of trunk lean and foot lift exercises in sitting position on abdominal muscle activity and the contribution rate of transversus abdominis.

PURPOSE: Abdominal hollowing exercise has been recommended to improve trunk stability. Trunk lean and foot lift exercises while sitting may easily promote abdominal muscle activity even in people who cannot perform abdominal hollowing consciously. The purpose of the present study was to examine the changes in abdominal muscle activity and contribution rate of the transversus abdominis muscle (TrA) when leaning the trunk and lifting the foot during sitting. METHODS: The muscle stiffnesses (indicators of muscle activity) of the right rectus abdominis, external oblique, internal oblique, and TrA of 14 healthy men were measured during abdominal hollowing and the following nine sitting tasks: reference posture, 15° and maximal posterior trunk lean, 20° and maximal ipsilateral and contralateral trunk lean, and ipsilateral and contralateral foot lift. The TrA contribution rate was calculated by dividing the TrA stiffness by the sum of the abdominal muscles' stiffnesses. RESULTS: The TrA stiffness was significantly higher in abdominal hollowing than in reference posture, posterior and ipsilateral trunk lean, and ipsilateral foot lift, but not higher than in contralateral trunk lean and contralateral foot lift. There was no significant difference in the TrA contribution rates between abdominal hollowing and ipsilateral or contralateral foot lift. CONCLUSION: The contralateral trunk lean or contralateral foot lift could enhance TrA activity for people who cannot perform abdominal hollowing consciously. The contralateral foot lift could particularly be beneficial to obtain selective activity of TrA.

Influence of simulated hip muscle weakness on hip joint forces during deep squatting.

This study aimed to determine the effects of simulated hip muscle weakness on changes in hip joint forces during deep squat motion. Ten healthy individuals performed squat motion at three different positions (0° foot angle [N-squat], 10° toe-in [IN-squat], and 30° toe-out [OUT-squat]). A scaled musculoskeletal model for each participant was used to calculate the muscle and hip joint forces. For each hip muscle, models of full strength, mild muscle weakness (15% decrease), and severe muscle weakness (30% decrease) were created. The muscles affecting the hip joint forces were identified, and the rate of change in the joint forces was compared among the three squat conditions. The anterior hip joint force was increased in the muscle weakness models of the inferior gluteus maximus (iGlutMax) and iGlutMax+deep external rotator (ExtRot) muscles. With 30% muscle weakness of these muscles, statistically significant differences in the rate of increase in the anterior joint force were observed in the following order: IN-squat (iGlutMax, 29.5%; iGlutMax+ExtRot, 41.4%), N-squat (iGlutMax, 18.3%; iGlutMax+ExtRot, 27.8%), and OUT-squat (iGlutMax, 5.6%; iGlutMax+ExtRot, 9.3%). OUT-squat may be recommended to minimize the increase in hip joint forces if accompanied by hip muscle weakness.

Age-related changes in muscle thickness and echo intensity of trunk muscles in healthy women: comparison of 20-60s age groups.

PURPOSE: The objective of this study was to investigate the age-related changes in muscle thickness and muscle echo intensity of trunk in subjects including wide range of age groups. METHODS: The subjects were 112 healthy women (age range 20-60s). The rectus abdominis, external oblique, internal oblique, transversus abdominis, erector spinae, and lumbar multifidus muscles were examined. To confirm the differences among the age groups, the linear mixed effect models were performed. RESULTS: There were significant decreases in muscle thickness of the rectus abdominis and external oblique muscles in the 50s and 60s age groups compared to those in the 20s age group, and a significant decrease in muscle thickness of the erector spinae muscle in the 60s age group compared to those in the 20s age group. However, there was no significant difference among the age groups in muscle thickness of other trunk muscles. There were significant increases in echo intensity of the abdominal muscles in other age groups compared to those in the 20s age group, and significant increases in echo intensity of the back muscles in the age groups over 40 compared to those in the 20s group. CONCLUSION: Our study revealed that muscle quality may be more affected by age than muscle quantity and the effects of aging differ among muscles.

Gait kinematics of the hip, pelvis, and trunk associated with external hip adduction moment in patients with secondary hip osteoarthritis: toward determination of the key point in gait modification.

BACKGROUND: A larger daily cumulative hip loading, which is the product of the external hip adduction moment (HAM) impulse during gait and the number of steps per day has been identified as a factor associated with the progression of secondary hip osteoarthritis (OA). The cause of the increased HAM impulse in patients with hip OA has not been identified. The purpose of this study was to identify the gait parameters associated with HAM impulse during gait in patients with secondary hip OA. METHODS: Fifty-five patients (age 22-65 years) with mild-to-moderate secondary hip OA participated in this cross-sectional study. The HAM impulse during gait was measured using a three-dimensional gait analysis system. To identify the gait parameters associated with HAM impulse, hierarchical multiple regression analysis was performed. The first model (basic model) included body weight and stance phase duration. The second models included gait parameters (gait speed; ground reaction force [GRF] in frontal plane; and hip, pelvic, and trunk angle in frontal plane) and hip pain in addition to the basic model. RESULTS: Body weight and stance phase duration explained 61% of the variance in HAM impulse. In the second model, which took into account body weight and stance phase duration, hip adduction angle (9.4%), pelvic tilt (6.5%), and trunk lean (3.2%) in addition to GRF explained the variance in the HAM impulse. Whereas larger hip adduction angle and pelvic tilt toward the swing limb were associated with a larger HAM impulse, larger trunk lean toward the stance limb was associated with smaller HAM impulse. CONCLUSION: In patients with excessive hip adduction and pelvic tilt toward the swing limb during gait, gait modification may contribute to the reduction of hip joint loading.

Greater Lumbar Extension During Dolphin Kick and Psoas Major Tightness in Swimmers With Low Back Pain.

CONTEXT: In competitive swimming, many swimmers experience low back pain (LBP). Lumbar hyperextension may cause LBP, and tight hip-flexor muscle may cause lumbar extension during swimming. OBJECTIVE: The purpose of this study was to clarify the features of the elastic moduli of the muscles and the lumbar extension when swimmers with LBP perform a dolphin kick (DK). DESIGN: Cross-sectional study. SETTING: Single center. OTHER PARTICIPANTS: Eleven male college swimmers were enrolled as the LBP group (who have LBP when swimming and during a lumbar extension), and 21 male college swimmers were recruited as the control group (no LBP). INTERVENTIONS: The elastic moduli of the psoas major, iliacus, teres major, latissimus dorsi, pectoralis major, and pectoralis minor were measured through ultrasonic shear wave elastography. The lumbar and hip extension angles during a DK were measured using a video camera. The passive hip extension and shoulder-flexion range of motion (ROM) were measured using a goniometer. MAIN OUTCOME MEASURES: Muscle elastic moduli and lumbar extension angles during DK. RESULTS: The characteristics, muscle elastic moduli, DK motion, and ROM were compared between the 2 groups. LBP group demonstrated significantly higher elastic modulus of the psoas major and lower modulus of pectoralis minor compared with the control group. Also, LBP group showed greater lumbar extension during a DK and less hip extension ROM than the control group. CONCLUSIONS: The higher elastic modulus of the psoas major and greater lumbar extension during a DK may be related to the LBP in swimmers.

Gait- and Posture-Related Factors Associated With Changes in Hip Pain and Physical Function in Patients With Secondary Hip Osteoarthritis: A Prospective Cohort Study.

OBJECTIVE: To identify gait- and posture-related factors associated with changes in hip pain and physical function in patients with hip osteoarthritis (OA). DESIGN: Prospective cohort study. SETTING: Clinical biomechanics laboratory of a university. PARTICIPANTS: Consecutive sampling of female patients with mild-to-moderate secondary hip OA (N=30). MAIN OUTCOME MEASURES: Hip pain (visual analog scale) and physical function (physical component summary of the Medical Outcomes Study 36-Item Short-Form Health Survey) were measured at baseline and 12 months later. With changes in hip pain and physical function as dependent variables, linear regression analyses were performed with gait- and posture-related factors as independent variables with and without adjustment for age, joint space width, and hip pain or physical function at baseline. Posture-related factors included angles of thoracic kyphosis, lumbar lordosis, sacral inclination, spinal inclination, and spinal mobility. Gait-related factors were walking speed, steps per day, joint angles, external hip joint moment impulses, and daily cumulative hip moments. RESULTS: Multiple linear regression analyses showed that limited hip extension (adjusted standardized B coefficient [95% confidence interval]: -0.52 [-0.88 to -0.17]) and limited external rotation angles (-0.51 [-0.85 to -0.18]) during walking were associated with the worsening of hip pain. An increased thoracic kyphosis (-0.54 [-0.99 to -0.09]), less sacral anterior tilt (0.40 [0.01-0.79]), reduced thoracic spine mobility (0.59 [0.23-0.94]), less steps per day (0.53 [0.13-0.92]), and a slower walking speed (0.45 [0.04-0.86]) were associated with deterioration in physical function. CONCLUSIONS: Gait- and posture-related factors should be considered when assessing risk and designing preventive interventions for the clinical progression of secondary hip OA.

Effects of the trunk position on muscle stiffness that reflects elongation of the lumbar erector spinae and multifidus muscles: an ultrasonic shear wave elastography study.

PURPOSE: The present study aimed to clarify the effects of the trunk position on muscle stiffness that reflects elongation of the lumbar erector spinae and lumbar multifidus muscles using ultrasonic shear wave elastography (SWE). METHODS: The study included ten healthy men. The shear elastic modulus of the left lumbar erector spinae and lumbar multifidus muscles were evaluated using ultrasonic SWE. Measurement postures for the left lumbar erector spinae muscle were (1) prone position (Rest), (2) sitting position with the trunk flexed (Flexion), (3) the Flexion position adding right trunk lateral flexion (Flexion-Lateral Flexion), and (4) the Flexion position adding right trunk rotation (Flexion-Rotation 1). The left lumbar multifidus muscle were measured in positions (1)-(3), and (5) the Flexion position adding left trunk rotation (Flexion-Rotation 2). RESULTS: The shear elastic modulus of the lumbar erector spinae muscle in the Flexion-Lateral Flexion position was significantly higher than that in the Rest, Flexion, or Flexion-Rotation 1 positions. Shear elastic modulus of the lumbar multifidus muscle was similar in the Flexion, Flexion-Lateral Flexion, and Flexion-Rotation 2 positions, but significantly lower in the Rest position. CONCLUSIONS: The results of the present study suggest that the lumbar erector spinae muscle is stretched effectively in the position adding trunk contralateral lateral flexion to flexion. The results also indicate that the lumbar multifidus muscle, which does not appear to be affected by adding trunk contralateral lateral flexion or ipsilateral rotation to flexion, is stretched effectively in the trunk flexion position.

Effect of different knee flexion angles with a constant hip and knee torque on the muscle forces and neuromuscular activities of hamstrings and gluteus maximus muscles.

PURPOSE: This study examined the effect of different knee flexion angles with a constant hip and knee torque on the muscle force and neuromuscular activity of the hamstrings and gluteus maximus. METHODS: Twenty healthy males lay in prone position and held their lower limb with hip flexion at 45° and knee flexion at either 10° or 80°. At these angles, the hip and knee torques are identical. Under three load conditions: passive (referred to as Unloaded), active (Loaded), and active with 3-kg weight added to the shank (Loaded + 3 kg), the muscle stiffness (i.e., an indicator of muscle force) and neuromuscular activity of the hamstrings and gluteus maximus were measured using shear wave elastography and surface electromyography. RESULTS: The muscle stiffness and neuromuscular activity of the hamstrings and gluteus maximus increased significantly with the load. Muscle stiffness in the hamstrings was significantly lower at knee flexion of 80° than at 10° for Unloaded, but not for either Loaded or Loaded + 3 kg. The neuromuscular activity of the hamstrings was significantly greater at knee flexion of 80° than at 10° for both Loaded and Loaded + 3 kg. The muscle stiffness or neuromuscular activity of the gluteus maximus showed no significant differences between knee angles. CONCLUSIONS: When the passive force in the hamstrings decreases with knee flexion, sufficient muscle force to maintain the hip and knee torques against an external load is generated by preferentially increasing the neuromuscular activity of the hamstrings, rather than increasing the synergetic muscle force.

Electromyographic analysis of training to selectively strengthen the lumbar multifidus muscle: effects of different lifting directions and weight loading of the extremities during quadruped upper and lower extremity lifts.

OBJECTIVE: The lumbar multifidus muscle (LMF) is a lower back muscle that contributes to spinal stability. Several electromyographic analyses have evaluated LMF activity during various types of training. The present study examined the activity of the back muscles during quadruped upper and lower extremity lifts (QULELs) with different lifting direction and weight loading of extremities. METHODS: Seventeen healthy men were included as subjects. The exercise conditions comprised raising the upper extremity of one side and the lower extremity of the opposite side in a quadruped position with different lifting direction and weight loading. The various combinations of lifts were modifications of conventional QULEL, in which the upper extremity is raised to 180° shoulder flexion and the lower extremity to 0° hip extension. The effects of different lifting directions and weight loading on LMF and lumbar erector spinae (LES) muscle activities were measured using surface electromyography. RESULTS: The LMF activity and the LMF/LES activity ratio on the side of lower extremity lifting were higher during QULEL with the upper and lower extremities in abduction than during conventional QULEL. The LMF/LES activity ratio was lower during QULEL with weight loading on the upper and lower extremities than during conventional QULEL. CONCLUSION: The results of the present study suggest that QULEL with shoulder and hip abduction is more effective to selectively strengthen LMF on the side, where the lower extremity is lifted. Loading weight onto both the lifted upper and lower extremities during QULEL is disadvantageous as a selective LMF training method because the LMF/LES activity ratio is low.

Effects of high-velocity resistance training on muscle function, muscle properties, and physical performance in individuals with hip osteoarthritis: a randomized controlled trial.

OBJECTIVE: To investigate the effects of high-velocity resistance training on muscle function, muscle properties, and physical performance in patients with hip osteoarthritis by comparison with those of low-velocity resistance training. DESIGN: Single-blind randomized controlled trial. SETTING: Home-based exercise programmes. SUBJECTS: A total of 46 women with hip osteoarthritis were randomly assigned to the high-velocity (n = 23) or low-velocity (n = 23) training group. INTERVENTIONS: Both groups underwent an eight-week daily home-based resistance training programme using an elastic band. Exercises involved hip abduction, extension, and flexion and knee extension. Participants in the high-velocity group performed the concentric phase of each repetition as rapidly as possible and returned to the initial position eccentrically in 3 s. Participants in the low-velocity group performed both the concentric and eccentric phases in 3 s. MAIN MEASURES: The following outcome measures were evaluated: isometric muscle strength, muscle power, muscle thickness, muscle echo intensity, maximum walking speed, Timed Up and Go test, 3-minute walking test, Harris Hip Score, and hip pain. RESULTS: Decreases in the time for performing the Timed Up and Go test (mean changes: high-velocity group -0.46 s, low-velocity group -0.23 s) and echo intensity of the gluteus maximus (mean changes: high-velocity group -6.8, low-velocity group -1.0) were significantly greater in the high-velocity group than in the low-velocity group. No significant difference was observed in changes of other outcome measures between the groups. CONCLUSION: This study revealed that high-velocity training for patients with hip osteoarthritis has partially a greater effect on muscle properties and physical performance than low-velocity training.

Differences in muscle activation patterns during step recovery in elderly women with and without a history of falls.

BACKGROUND AND AIMS: This study aimed at comparing the patterns of muscle activation used in stepping to regain balance during a forward fall between subjects with and without a history of falling and at identifying the causes of functional deficits in recovery stepping. METHODS: Elderly women with and without a history of falling (fallers: n = 12, mean age ± SD = 82.8 ± 4.5 years; non-fallers: n = 17, age = 81.4 ± 3.4 years) participated in the study. The subjects were suspended in a forward-leaning position by a lean-control cable with a load of 15 % of body weight and instructed to regain standing balance upon release by taking a single step forward. Electromyography (EMG) data were obtained from five lower extremity muscles on the stepping side, and the muscle activation patterns were compared between fallers and non-fallers. RESULTS: Fallers had a shorter step length and slower step velocity than non-fallers. The EMG time-to-peak for the gastrocnemius muscle, which provides push-off prior to foot lift-off, was slower for fallers than for non-fallers, whereas the EMG onset times of the biceps femoris and gastrocnemius muscles were similar between the groups. The fallers exhibited significantly delayed muscle deactivation of the upper leg and increased co-contraction between the rectus femoris and biceps femoris during the stepping phase than did the non-fallers. CONCLUSIONS: These findings suggest that the muscle activation pattern during the regain balance may reflect an inability to step forward rapidly in elderly women with a history of falls.

The effect of sex and fatigue on lower limb kinematics, kinetics, and muscle activity during unanticipated side-step cutting.

PURPOSE: To determine how sex and fatigue affect lower limb kinematics, kinetics, and muscle activity during unanticipated side-step cutting. METHODS: Twenty-three physically active subjects (men 11, women 12) performed 10 successful trials of cutting manoeuvres each to either side under unanticipated conditions in response to 2 light emitting diodes before and after fatigue conditions. Data were analysed and compared regarding sex and fatigue conditions using two-way repeated measures analysis of variance. RESULTS: After fatigue-inducing exercise, women demonstrated larger impulses of ground reaction force (IGRF) during the first 50 ms (2.4 ± 0.8 vs. 2.1 ± 0.9, P < 0.05) than did men. Significant primary effects of sex indicated that women showed a smaller hip flexion angle at initial contact (40.4 ± 6.9° vs. 49.7 ± 9.1°, P < 0.05) and at maximum flexion angle (41.3 ± 7.7° vs. 51.4 ± 9.0°, P < 0.05) compared with men. Significant primary effects of fatigue were observed in the gluteus maximus muscle during 50 ms before initial contact (+21.5 ± 48.3 %, P < 0.05) and in the semimembranosus muscle during 50 ms before initial contact (-6.2 ± 20.1 %, P < 0.05) and the first 50 ms of side-step cutting (-7.9 ± 26.6 %, P < 0.05). CONCLUSIONS: Our results suggest that sex differences, especially larger IGRF in a fatigue state combined with less hip flexion angle, lead to women having a higher risk for anterior cruciate ligament (ACL) injury. These findings may contribute to understanding the underlying mechanism of injury and development of preventive exercises against ACL injury.