Strategies of muscular support of varus and valgus isometric loads at the human knee

In this paper we studied how subjects activate their muscles in response to static varus and valgus loads at the knee. The muscles' contributions to the external moments were estimated using an EMG driven biomechanical model of the knee. The individual muscle activation and loading patterns were examined to identify the strategies that the nervous system uses to support varus and valgus knee moments. It was found that the (1) co-contraction of the hamstrings and quadriceps, and (2) activation of the gracilis and tensor fascia lata increased with the increasing magnitude of the varus and valgus moments. These 2 activation patterns provided positive support of valgus and varus loads at the knee The sartorius appears to be activated to provide positive support of valgus loads at the knee, whereas during varus moments this muscle increases the varus load on the knee, i.e. provides negative support. Generally, the hamstrings and quadriceps co-contraction contributed to most of the muscular support of the varus and valgus moments. In addition, co-contraction supported 11-14% of the external moment in pure varus and pure valgus respectively. It appears that there are activation strategies with the specific purpose to support varus and valgus moments, albeit small, which suggest dual goals of the neuromotor system during the support of varus and valgus moments.     

Rotational flexibility of the human knee due to varus/valgus and axial moments in vivo

Knee ligamentous injuries persist in the sport of Alpine skiing. To better understand the load mechanisms which lead to injury, pure varus/valgus and pure axial moments were applied both singly and in combination to the right knees of six human test subjects. The corresponding relative knee rotations in three degrees of freedom were measured. Knee flexion angles for each test subject were 15 and 60 degrees for the individual moments and 60 degrees for the combination moments. For both knee flexion angles the hip flexion angle was 0 degrees. Leg muscles were quiescent and axial force was minimal during all tests. Tables of data include sample statistics for each of four flexibility parameters in each loading direction. Data were analyzed statistically to test for significant differences in flexibility parameters between the test conditions. In flexing the knee from 15 to 60 degrees, the resulting knee rotations under single moments depended upon flexion angle with varus, valgus, and internal rotations increasing significantly. Also, rotations were different depending on load direction; varus rotation was significantly different and greater than valgus rotation at both flexion angles. Also external rotation was significantly different and greater than internal at 15 degrees flexion, but not at 60 degrees flexion. Coupled rotations under single moments were also observed. Applying pure varus/valgus moments resulted in coupled external/internal rotations which were inconsistent and hence not significant. Applying pure axial moments resulted in consistent and hence significant varus/valgus rotations; an external axial moment induced varus rotation and an internal axial moment induced valgus rotation. For combination moments, varus/valgus rotations decreased significantly from those rotations at similar load levels in the single moment studies. Also, a varus moment significantly increased external rotation and a valgus moment significantly decreased internal rotation. These differences indicate significant interaction between corresponding load combinations. These results suggest that load interaction is a potentially important phenomenon in knee injury mechanics.     

Muscular adaptations to resistance exercise in the elderly

Neuropathic, metabolic, hormonal, nutritional and immunological factors contribute to the development of sarcopenia. This loss of muscle mass associated with ageing, is a main cause of muscle weakness, but the loss of muscle strength typically exceeds that of muscle size, with a resulting decrease in force per unit of muscle cross-sectional area. Recent evidence suggests that, in addition to a reduction in neural drive and in fibre specific tension, changes in muscle architecture contribute significantly to the loss of muscle force through alterations in muscle mechanical properties. Older muscle, however, maintains a high degree of plasticity in response to increased loading since considerable hypertrophy and a reversal of the alterations in muscle architecture associated with ageing are observed with resistive training.     

Effect of muscular activity on valgus/varus laxity and stiffness of the knee

Quantitative changes in valgus/varus knee stability with different levels of muscular activity were determined for five subjects. A specially designed machine was used to measure resistance to angulation in the frontal plane. This device held the thigh stationary, the knee straight, an cycled the leg from side to side at a constant rate between present moment limits. Resistance to this forced valgus/varus motion was measured simultaneously with torque about the knee in the sagittal plane. Muscle activity was monitored by electromyography (EMG). Direct comparison of moment-rotation characteristics allowed changes in stability to be quantified as a function of extension and flexion torque. Extension torques less than 20% of the maximum increased varus stability more than valgus stability. Flexion torques of the same relative magnitude increased valgus stability more than varus stability. Comparison with the literature suggested that prevention of opening of the lateral side of the joint under varus loading was responsible for increased varus stability with increasing torque, both with extension and flexion torques.     

Muscular adaptations to combinations of high- and low-intensity resistance exercises

Acute and long-term effects of resistance-training regimens with varied combinations of high- and low-intensity exercises were studied. Acute changes in the serum growth hormone (GH) concentration were initially measured after 3 types of regimens for knee extension exercise: a medium intensity (approximately 10 repetition maximum [RM]) short interset rest period (30 s) with progressively decreasing load ("hypertrophy type"); 5 sets of a high-intensity (90% of 1RM) and low-repetition exercise ("strength type"); and a single set of low-intensity and high-repetition exercise added immediately after the strength-type regimen ("combi-type"). Postexercise increases in serum GH concentration showed a significant regimen dependence: hypertrophy-type > combi-type > strength-type (p < 0.05, n = 8). Next, the long-term effects of periodized training protocols with the above regimens on muscular function were investigated. Male subjects (n = 16) were assigned to either hypertrophy/combi (HC) or hypertrophy/ strength (HS) groups and performed leg press and extension exercises twice a week for 10 weeks. During the first 6 weeks, both groups used the hypertrophy-type regimen to gain muscular size. During the subsequent 4 weeks, HC and HS groups performed combi-type and strength-type regimens, respectively. Muscular strength, endurance, and cross sectional area (CSA) were examined after 2, 6, and 10 weeks. After the initial 6 weeks, no significant difference was seen in the percentage changes of all variables between the groups. After the subsequent 4 weeks, however, 1RM of leg press, maximal isokinetic strength, and muscular endurance of leg extension showed significantly (p < 0.05) larger increases in the HC group than in the HS group. In addition, increases in CSA after this period also tended to be larger in the HC group than in the HS group (p = 0.08). The results suggest that a combination of high- and low-intensity regimens is effective for optimizing the strength adaptation of muscle in a periodized training program.     

The influence of knee varus and valgus on quadriceps muscle activity changes induced by stretching and kneeling

This study aimed to determine the influence of knee varus (VARUS) and valgus (VALGUS) on the differences in individual quadriceps muscle (QM) activity during knee extension maximum voluntary isometric contractions (MVICs) and sit/stand transitions and on the changes in individual QM activity during sit/stand transitions after QM stretching and kneeling. Ten young healthy males each with VARUS and VALGUS were included. The electromyography signals of the vastus medialis (VM), vastus lateralis, and rectus femoris were recorded during sit/stand transitions before and after rest, stretching, and kneeling and during knee extension MVICs after rest. The individual muscle-to-total muscle activity ratio was assessed. The VARUS group exhibited a significantly higher VM muscle activity ratio in the sit-to-stand and stand-to-sit tasks than in knee extension MVICs (p = 0.004 and p = 0.044, respectively) and a tendency that the VM muscle activity ratio increased in the sit-to-stand task after stretching (p = 0.051), whereas the VALGUS group exhibited no significance. Individuals with VARUS required high VM muscle activity ratios during sit/stand transitions. Future studies should be conducted to determine whether habitual sit-to-stand exercises after QM stretching are effective in preventing knee medial osteoarthritis development in individuals with VARUS.     

Reaction of the Cardiovascular System to Local Muscular Activity at Different Ages

Five age groups corresponding to the main stages of human ontogenesis (from 13 to 90 years) were examined during local muscle work with a hand dynamometer. Differences in the adaptive cardiac reactions to static or dynamic loading were revealed. Age-related differences in cardiac reactions were observed, testifying to a beneficial heart functioning during loading at 30–35 years of age and cardiac strain at 18–20 and 60–74 years of age.     

Development of an image-based technique to examine joint congruency at the elbow

Identifying joint contact in articular joints is important for both the biomechanical investigation of joint mechanics and the study of osteoarthritis. The purpose of this study is to develop a proximity mapping technique to non-invasively determine joint congruency, as a surrogate of joint contact. To illustrate the capabilities of this algorithm, a cadaveric upper extremity was positioned at varying degrees of elbow flexion. This technique was validated using a gold standard experimental casting technique. The pattern of the cast showed an excellent agreement with the generated proximity map using the inter-bone distance algorithm. The results from this study agree with the results of previous studies examining joint contact at the elbow both in the location and in the tracking of the joint contact throughout elbow flexion. Ultimately, this technique will lead to an increased understanding of the effect of malalignment and instability of the joint on contact mechanics.     

Effects of resistance training on elbow flexors of highly competitive bodybuilders.

The influence of gender on muscular adaptation of the elbow flexors to 24 wk of heavy resistance training was studied in five male bodybuilders (MB) and five female bodybuilders (FB) who were highly competitive. Muscle cross-sectional area (CSA), fiber area, and fiber number were determined from the biceps brachii, and voluntary elbow flexor torque was obtained at velocities of contraction between 0 and 300 degrees/s. Biceps and flexor CSA was 75.8 and 81% greater, respectively, in MB than in FB, but muscle CSA was not significantly altered by the training program in either group. Because estimated fiber number and the volume density of nonmuscle tissue were similar in MB and FB, most of the gender difference in muscle CSA appeared to be due to greater absolute mean fiber areas in MB (10.51 and 10.68 x 10(3) microns 2 pre- and posttraining, respectively) than in FB (5.33 and 5.96 x 10(3) microns 2 pre- and posttraining, respectively). In neither MB nor FB did fiber type achieve further hypertrophy during the 24-wk training program. These data suggest that the extent of any change in muscle mass or muscle fiber characteristics is minimal after a bodybuilder of either gender has attained a high degree of muscle mass and a highly competitive status.     

Effect of varus/valgus malalignment on bone strains in the proximal tibia after TKR: an explicit finite element study

Malalignment is the main cause of tibial component loosening. Implants that migrate rapidly in the first two post-operative years are likely to present aseptic loosening. It has been suggested that cancellous bone stresses can be correlated with tibial component migration. A recent study has shown that patient-specific finite element (FE) models have the power to predict the short-term behavior of tibial trays. The stresses generated within the implanted tibia are dependent on the kinematics of the joint; however, previous studies have ignored the kinematics and only applied static loads. Using explicit FE, it is possible to simultaneously predict the kinematics and stresses during a gait cycle. The aim of this study was to examine the cancellous bone strains during the stance phase of the gait cycle, for varying degrees of varus/valgus eccentric loading using explicit FE. A patient-specific model of a proximal tibia was created from CT scan images, including heterogeneous bone properties. The proximal tibia was implanted with a commercial total knee replacement (TKR) model. The stance phase of gait was simulated and the applied loads and boundary conditions were based on those used for the Stanmore knee simulator. Eccentric loading was simulated. As well as examining the tibial bone strains (minimum and maximum principal strain), the kinematics of the bone-implant construct are also reported. The maximum anterior-posterior displacements and internal-external rotations were produced by the model with 20 mm offset. The peak minimum and maximum principal strain values increased as the load was shifted laterally, reaching a maximum magnitude for -20 mm offset. This suggests that when in varus, the load transferred to the bone is shifted medially, and as the bone supporting this load is stiffer, the resulting peak bone strains are lower than when the load is shifted laterally (valgus). For this particular patient, the TKR design analyzed produced the highest cancellous bone strains when in valgus. This study has provided an insight in the variations produced in bone strain distribution when the axial load is applied eccentrically. To the authors' knowledge, this is the first time that the bone strain distribution of a proximal implanted tibia has been examined, also accounting for the kinematics of the tibio-femoral joint as part of the simulation. This approach gives greater insight into the overall performance of TKR.     

Squat jump training at maximal power loads vs. heavy loads: effect on sprint ability

Training at a load maximizing power output (Pmax) is an intuitively appealing strategy for enhancement of performance that has received little research attention. In this study we identified each subject's Pmax for an isoinertial resistance training exercise used for testing and training, and then we related the changes in strength to changes in sprint performance. The subjects were 18 well-trained rugby league players randomized to two equal-volume training groups for a 7-week period of squat jump training with heavy loads (80% 1RM) or with individually determined Pmax loads (20.0-43.5% 1RM). Performance measures were 1RM strength, maximal power at 55% of pretraining 1RM, and sprint times for 10 and 30 m. Percent changes were standardized to make magnitude-based inferences. Relationships between changes in these variables were expressed as correlations. Sprint times for 10 m showed improvements in the 80% 1RM group (-2.9 +/- 3.2%) and Pmax group (-1.3 +/- 2.2%), and there were similar improvements in 30-m sprint time (-1.9 +/- 2.8 and -1.2 +/- 2.0%, respectively). Differences in the improvements in sprint time between groups were unclear, but improvement in 1RM strength in the 80% 1RM group (15 +/- 9%) was possibly substantially greater than in the Pmax group (11 +/- 8%). Small-moderate negative correlations between change in 1RM and change in sprint time (r approximately -0.30) in the combined groups provided the only evidence of adaptive associations between strength and power outputs, and sprint performance. In conclusion, it seems that training at the load that maximizes individual peak power output for this exercise with a sample of professional team sport athletes was no more effective for improving sprint ability than training at heavy loads, and the changes in power output were not usefully related to changes in sprint ability.     

Tennis elbow and the cervical spine.

The exact cause of tennis elbow, a common condition, is still obscure. While the condition may well be entirely due to a local disorder at the elbow, the results of a study of 50 patients whose condition was resistant to 4 weeks of treatment directed to the elbow suggest that the underlying condition may have been (at least in these patients) a reflex localization of pain from radiculopathy at the cervical spine. Clinical, radiologic and electromyographic findings supported this suggestion. The pain was demonstrated to be muscular tenderness, which was maximal and specific at motor points. Treatment directed to the cervical spine appeared to give relief in the majority of patients. The more resistant the condition, the more severe were the radiologic and electromyographic findings in the cervical spine.