Guangzhi Wang

In vivo load sharing among the quadriceps components

Knee extension is always performed with coordinated contractions of multiple quadriceps muscle components: however, how the load is shared among them under normal and pathological conditions is unclear. We hypothesized that: the absolute moment generated by each quadriceps component increases with the total knee extension moment; the relative contribution and its dependence on the total knee extension moment are different for different quadriceps components; and the centrally located large vastus intermedius (VI) is favored by the central nervous system at low levels of activation. Electrical stimulation was used to activate each quadriceps component selectively in six human subjects. The relationship between the knee extension moment generated by an individual quadriceps component and the corresponding compound muscular action potential (M‐wave) over various contraction levels was established for each quadriceps component. This relationship was used to calibrate the corresponding EMG signal and determine load sharing among quadriceps components during submaximal isometric voluntary knee extension. The VI contributed the most (51.8–39.6%) and vastus medialis the least (9.5–12.2%) to knee extension moment (P < 0.05). As the knee extension moment increased, the relative contribution of the VI decreased (P = 0.017) while the relation contribution of the vastus lateralis and medialis increased (P ≤ 0.012). The absolute moment generated by each quadriceps component always increased with the total knee extension moment (P < 0.002). Our in vivo approach determined subject‐ and condition‐specific load sharing among individual muscles and showed that the central nervous system utilized the centrally located, uniarticular VI in submaximal isometric knee extension.

In vivo and Noninvasive Three-Dimensional Patellar Tracking Induced by Individual Heads of Quadriceps

PURPOSE Unbalanced actions of the quadriceps components are closely linked to patellar mal-tracking and patellofemoral pain syndrome. However, it is not clear how individual quadriceps components pull and rotate the patella three dimensionally. The purpose of this study was to investigate in vivo and noninvasively patellar tracking induced by individual quadriceps components. METHODS Individual quadriceps component was activated selectively through electrical stimulation at the muscle motor point, and the resulting patellar tracking was measured in vivo and noninvasively in 18 knees of 12 subjects. The in vivo and noninvasively patellar tracking was corroborated with in vivo fluoroscopy and in vitro cadaver measurements. RESULTS Vastus medialis (VM) mainly pulled the patella first in the medial and second in the proximal directions and vastus lateralis (VL) pulled first in the proximal and second in the lateral directions. The oblique portion (VMO) of the VM pulled the patella mainly medially and the longus portion (VML) more proximally. Medial tilt was the major patellar rotation induced by VMO contraction at full knee extension. With the knee at the more flexed positions, the amplitude of patellar movement induced by comparable quadriceps contractions was reduced significantly compared to that at full knee extension, and VMO changed its main action from extending to flexing the patella. CONCLUSIONS The medial and lateral quadriceps components moved the patella in rather different directions, and rotated the patella differently about the mediolateral tilt and mediolateral rotation axes but similarly in extension. The approach can be used to investigate patellar tracking in vivo and noninvasively in both healthy subjects and patients with patellofemoral disorder and patellar malalignment.

Muscle strength in knee varus and valgus

PURPOSE The purpose of this study was to investigate the lower-limb muscle strength in knee varus-valgus and its dependence on knee varus-valgus position. The hypothesis was that humans could differentially contract the medial and lateral muscles crossing the knee and generate significant moments in knee valgus-varus. METHODS The subjects sat with the knee at full extension and secured from the medial, lateral, anterior, and posterior sides. Both hips were clamped from the lateral sides. The subjects adducted (abducted) the ipsilateral hip during the knee valgus (varus) maximal voluntary contraction with EMG signals recorded from muscles crossing the knee and knee joint moments measured using a six-axis force sensor. Frontal plane tibiofemoral movement was evaluated using a fluoroscope. RESULTS AND CONCLUSIONS The subjects differentially contracted the medial and lateral muscles, and fluoroscope images showed the corresponding tibiofemoral movement. The subjects showed considerable strength in knee varus and valgus. The active knee varus strength increased significantly with increasing knee valgus angle, and the valgus strength was significantly higher when the knee was in varus position (P < 0.039). Active valgus muscle strength at 5 degrees knee varus was significantly higher than the active varus strength at 5 degrees valgus (P = 0.002). The passive resistance moment increased linearly with increasing knee valgus and varus angles, and it accounted for 28% and 35% of the total (active plus passive) moment at the 5 degrees varus and 5 degrees valgus, respectively. The significant varus-valgus muscle strength demonstrated in this study may play important roles in performing various functional tasks, maintaining joint stability, and preventing potential injuries, whether the major load and motion at the knee is in the frontal plane or not.

Dynamic and static properties of the human knee joint in axial rotation

Joint laxity, muscle strength, elastic stiffness, viscosity and limb inertia about the internal-external rotation axis of the human knee joint were studied in vivo for normal and anterior cruciate ligament (ACL) injured subjects. A joint driving device was developed to perturb the knee joint about the axial rotation axis in various patterns while the subject kept the knee relaxed or contracted knee muscles at certain preset levels about the axial rotation axis. Compared to the flexion-extension axis, the knee joint showed much lower (but still significant) axial rotation muscle strength, substantially smaller limb inertia, lower viscosity and comparable joint stiffness. Active muscle contraction increased the joint stiffness and viscosity in axial rotation substantially. It also reduced knee axial laxity considerably. Co-contraction was more likely to occur in axial rotation muscle contraction than in flexion-extension, which tended to make active axial rotation joint stiffness higher. An ACL injured knee tended to show larger axial rotation laxity and lower joint stiffness, but muscle contraction reduced its differences from uninjured knees. The passive joint stiffness in axial rotation was the lowest among the three rotational axes.

Dynamics of patellar tendon reflex in spastic multiple sclerosis patients

Patellar tendon reflexes were studied in fourteen normal subjects and ten multiple sclerosis (MS) patients. An instrumented hammer was used to tap the patellar tendon and record the tapping force, while the quadriceps EMG and knee extension torque were recorded isometrically as the reflex responses. The relationship between tapping force and reflex torque was characterized using several system measures: the tendon reflex gain (G/sub tr/), contraction rate (R/sub c/), half-relaxation rate (R/sub hr/), contraction time (t/sub c/), half-relaxation time (t/sub hr/), and reflex loop delay (t/sub d/). G/sub tr/, R/sub c/, and R/sub hr/ of the MS patients were significantly higher than their counterparts in normal controls (p<0.001 for all cases). t/sub d/ of the MS patients was significantly shorter than that of the normal group (p<0.005). G/sub tr/, R/sub c/, and R/sub hr/ values correlated more closely with clinical measures of spasticity (Ashworth scale and tendon reflex scale) than did pure output measures (peak reflex torque and quadriceps EMG signals). Tendon reflexes were more repeatable in the MS population than in normal controls. The higher correlation to clinical measures and lower variation of G/sub tr/, R/sub c/, and R/sub hr/ indicate the potential values of these dynamic system measures in clinical practice as a more accurate and consistent approach towards quantification of the tendon jerk and spasticity.

Passive and active mechanical properties of the human knee joint in abduction-adduction

Biomechanical properties about the abduction-adduction axis of the human knee joint were studied in vivo, using a newly developed joint driving device. Abduction-adduction torque-angle relationship reflecting knee abduction-adduction laxity and stiffness was determined quasi-statically at full extension for both relaxed knee and for knees actively producing abduction-adduction moment. Knee joint stiffness, viscosity and limb inertia about the abduction-adduction axis were estimated through a dynamic experiment for both passive and actively contracting knees. It was found that human knees have significant abduction-adduction strength, which can be used to reduce abduction-adduction laxity and increase stiffness and is potentially significant in maintaining joint stability and control joint motion. The knee joint showed much higher stiffness and viscosity in abduction-adduction than in flexion-extension for the same level of background muscle torque.

Identification of time-varying joint dynamics using wavelets

A wavelet-based method was investigated to identify time-varying properties of joint dynamics. Wavelet decomposition was used to expand each time-varying coefficient of an autoregressive with exogenous input (ARX) model into a finite set of basis sequences, and singular value decomposition was used to obtain more robust parameter estimates of the expansion. With a set of well-selected basis, the time-varying ARX coefficients could be well approximated by a combination of a small number of basis sequences, which simplified the identification of the time-varying parameters. The estimated time-varying ARX parameters were converted to a second-order continuous-time system characterizing joint dynamics with joint stiffness, viscosity and limb inertia. Simulation based on a time-varying joint dynamics model showed that the method tracked the time-varying system parameter closely.