R. D'Ambrosia

Muscular coactivation: The role of the antagonist musculature in maintaining knee stability

The objective of this study was to quantify the coacti vation patterns of the knee flexor and extensor muscles as part of continued efforts to identify the role of the antagonist muscles in maintaining joint stability. The simultaneous EMG from the flexor and extensor muscles of the knee were recorded during maximal effort, slow isokinetic contractions (15 deg/sec) on the plane parallel to the ground to eliminate the effect of gravity. The processed EMG from the antagonist mus cle was normalized with respect to its EMG as agonist at maximal effort for each joint angle. The plots of normalized antagonist EMG versus joint angle for each muscle group were shown to relate inversely to their moment arm variations over the joint range of motion. Additional calculations demonstrated that the antago nist exerts nearly constant opposing torque throughout joint range of motion. Comparison of data recorded from normal healthy subjects with that of high perform ance athletes with hypertrophied quadriceps demon strated strong inhibitory effects on the hamstrings coac tivations. Athletes who routinely exercise their ham strings, however, had a coactivation response similar to that of normal subjects. We concluded that coactivation of the antagonist is necessary to aid the ligaments in maintaining joint stability, equalizing the articular surface pressure dis tribution, and regulating the joints mechanical imped ance. The reduced coactivation pattern of the unexer cised antagonist to a hypertrophied muscle increases the risk of ligamentous damage, as well as demon strates the adaptive properties of the antagonist muscle in response to exercise. It was also concluded that reduced risk of knee injuries in high performance ath letes with muscular imbalance could result from com plementary resistive exercise of the antagonist muscle.

The synergistic action of the anterior cruciate ligament and thigh muscles in maintaining joint stability

The synergistic action of the ACL and the thigh muscles in maintaining joint stability was studied experimentally. The EMG from the quadriceps and hamstring muscle groups was recorded and analyzed in three separate experimental procedures in which the knee was stressed. The test revealed that direct stress of the ACL has a moderate inhibitory effect on the quadriceps, but simultaneously it directly excites the hamstrings. Similar responses were also obtained in patients with ACL damage during loaded knee extension with tibia subluxation, indicating that an alternative reflex arc unrelated to ACL receptors was available to maintain joint integrity. The antagonist muscles (hamstrings) were clearly demonstrated to assume the role of joint stabilizers in the patient who has a deficient ACL. The importance of an appropriate muscle-conditioning rehabilitation program in such a patient is substantiated.

Anterior-posterior and rotational displacement of the tibia elicited by quadriceps contraction

The anterior-posterior displacement and rotation of the tibia elicited by isolated loading of the quadriceps mus cle was determined as a function of joint angle and muscle load using a computerized radiographic tech nique. Data collected from 12 fresh-frozen cadaveric knees demonstrated that quadriceps contraction can result in significant (<7 mm) anterior displacement of the tibia in the range of 0° to 80° of flexion, and a mild (<2 mm) posterior displacement in the range of 80° to 120° of flexion. Peak anterior displacement of 6.3 mm was observed at 30° of flexion under a 12 kg load in the quadriceps, while a constant 1.5 mm posterior displacement was observed throughout flexion angles exceeding 80°. It was further shown that the magnitude of the anterior displacement increased nonlinearly as the quadriceps force increased. Loading of the quadri ceps also resulted in internal rotation of the tibia in the range of 0° to 90° of flexion, and in external rotation of the tibia in the range of 90° to 120°. Peak internal rotation of 7° was observed at 15° of flexion and a peak external rotation of 1 ° was detected at 120° of flexion. Larger quadriceps load resulted in larger rota tion. We concluded that quadriceps contraction during knee extension has direct impact on anterior displace ment and rotation of the tibia and therefore on anterior cruciate ligament stress, increasing it as the muscles force is increased during knee extension. It is sug gested that partial quadriceps atrophy in knees with anterior cruciate ligament deficiency may be explained as a protective response. This would bring into question the practice of quadriceps exercise after ligament inju ries and repair, as well as current orthotics concepts.

The effect of joint velocity on the contribution of the antagonist musculature to knee stiffness and laxity

The electromyographic (EMG) coactivation patterns of the knee flexors and extensors when acting as antag onists were studied as a function of limb velocity to assess their contribution to joint stiffness and laxity. Normalized antagonist coactivation patterns devel oped from surface EMG recordings from the hamstrings and quadriceps during maximal effort isokinetic exten sion and flexion, respectively, demonstrated character istic variations as the joint velocity increased from 15 deg/sec up to 240 deg/sec. The two-tailed t-test (P < 0.1) was performed on the data obtained from eight normal knees. The results indicate that both hamstrings and quadriceps demonstrate a significant increase (>100%) in their antagonist coactivation pattern during the final 40° of fast extension and flexion movements, respectively, as limb velocity increases. A minor de crease in antagonist activity of the hamstrings (24%) and quadriceps (8%) was evident during the initial phase of the extension and flexion movements, respec tively, as joint velocity increased. We concluded that as limb velocity is increased, there is a substantial reflexive (unintentional) increase in the contribution of the antagonist musculature to joint stiff ness and reduction of laxity. The results also suggest that strength training of the hamstrings (rather than quadriceps) should be considered as a modality for conservative treatment of ACL deficiencies, as well as an adjunct to surgical reconstruction. Such training can also reduce the risk of high performance athletes in a reflexive manner by increasing joint stiffness.

Muscular co-contraction and control of knee stability

A computerized radiographic technique was used to determine the effect of hamstring antagonist co-contraction on the stability of the joint during isometric knee extension. Data collected from 12 cadaver knees showed that significant anterior displacement and internal rotation of the tibia occurred during isolated quadriceps loading, whereas significant reduction in anterior displacement and rotation occurred upon simultaneous low-level loading of the hamstrings in the range of motion of 15°-80° flexion. Hamstrings co-contraction was ineffective in the range of 0°-15° of flexion. Larger hamstrings loads resulted in more pronounced reduction in the anterior displacement and rotation of the tibia. We concluded that hamstring co-contraction has significant effect on maintaining knee stability, providing synergistic action to the anterior cruciate ligament (ACL) by preventing excessive anterior displacement and internal rotation of the tibia. We also concluded that hamstring strength training is essential therapy in conservative treatment of ACL-deficient knees, as an adjunct therapy to ligament repair procedures and as preventive therapy in high-performance athletes subject to potential risk of ligamentous injuries.

Electromyogram coactivation patterns of the elbow antagonist muscles during slow isokinetic movement.

Electromyograms from the flexor and extensor muscles of normal human elbows were simultaneously recorded during maximal-effort isokinetic movement at 15 degrees/s over the joints full range of motion. The antagonist electromyogram was normalized with respect to its electromyogram when acting as agonist at maximal effort and plotted as a function of joint angle. The coactivation patterns were nearly inversely related to each muscles moment arm variations with joint angle, suggesting that the antagonist may have generated constant opposing torque throughout the movement. Female subjects had a statistically significant higher coactivation level of the flexors and extensors compared with that of males, reflecting the increase in joint efficiency associated with daily muscular activity which is manifested by reduction in antagonist activity. The functional role of antagonist coactivation in augmenting ligament stabilizing functions, equalizing the pressure distribution over the articular surface, and regulating the joints mechanical impedance are discussed. The source of such coactivation appears to be due to proprioceptive and joint kinesthetic afferent input in addition to possible direct common drive.

The isometric length-force models of nine different skeletal muscles

The length-force relations of nine different skeletal muscles in the hindlimb of the cat were determined experimentally, with electrical stimulation of the sciatic nerve as the activation mode. It was shown that the active-, passive-, and total-force patterns varied widely among the muscles. The tibialis posterior (TP), medial and lateral gastrocnemius (MG, LG) and flexor digitorum longus (FDL) had a symmetric active-force curve, whereas the tibialis anterior (TA), peroneus brevis (PB), peroneus longus (PL), extensor digitorum longus (EDL), and soleus (SOL) had an asymmetric curve which exhibits about 25% of the maximal isometric force at extreme lengths. The SOL, EDL, and LG had a low-level passive force which appeared at short muscle length, whereas all other muscles exhibited initial passive force just before the optimal length. The total force was rising quasi-linearly for the SOL, whereas the other muscles exhibited an intermediate plateau about the optimal length. The LG and FDL had a substantial but temporary intermediate dip in the total force as the muscle was elongated past the optimal length. The elongation range of the various muscles also varied, ranging from +/- 15 to +/- 30% of the optimal length. The elongation range was symmetric for the FDL, LG, MG, TP, SOL, and EDL, and asymmetric for the PL, PB, and TA, being -12 to + 17%, -12 to + 17%, and -35 to + 12%, respectively. Two different models which incorporate muscle architecture were successfully fitted to the experimental data of the muscles except for the MG and TA. The architecture of these two muscles is highly nonhomogeneous and contains compartments with two pennation patterns or two different optimal lengths. New models, which add spatially and temporally the individual characteristics of each compartment of the muscles, were constructed for these two muscles. The new models demonstrated high correlation to the experimental data obtained from the MG and TA. It was concluded that the length-force relation varies widely among various skeletal muscles and is probably dependent on the primary function of the muscle in the context of integrated movement; this is a manifestation of architectural factors such as fiber pennation pattern and angle, cross-sectional area, ratio of muscle to tendon length, distribution of the fiber length within the muscle and compartmental pennation.

Identification of Fibronectin in Preparations of Osteoarthritic Human Cartilage

Several high molecular weight proteins were observed in dissociative extracts of osteoarthritic, but not of normal, human cartilage. By gel electrophoresis, by DEAE-cellulose and gelatin-agarose chromatography, and immunologically, they were found to be identical to fibronectin. Incorporation of tritiated proline into these proteins indicated that this material was not a synovial fluid contaminant. Interactions with the proteoglycans suggested that, in articular cartilage, the role of fibronectin may be more closely associated with proteoglycans than with collagen. The appearance of fibronectin in the diseased cartilage suggests that this may be a feature of the chondrocytes repair response to the loss of extracellular matrix.

EMG-force relations of a single skeletal muscle acting across a joint: Dependence on joint angle.

The electromyogram (EMG)-force relations of a single skeletal muscle (soleus) acting on a joint set isometrically at various flexion angles was studied using electrical nerve stimulation as the experimental method. The EMG-force relationships were linear at extremes of joint extension and became progressively nonlinear as the flexion angle increased. Joint angles at extreme extension showed the least passive and active force whereas their corresponding EMG versus force relations were linear. At extremes of joint flexion large passive force was accompanied by minimal active force. The largest active forces were recorded at midrange of the joints excursion (90 degrees ). The source of the increasing nonlinearity was traced to large variability in the active and passive muscle forces as a function of joint angle, excluding all but minor variability in the EMG with joint position. We concluded that length-dependent variations of active and passive forces of the muscle (length-tension relations) compounded with the variation of its moment arm when acting across the joint require major consideration in biomechanical studies in which EMG is used to represent muscle force indirectly.

Isotonic length/force models of nine different skeletal muscles.

The isotonic length/force relationships of nine skeletal muscles in the cats hindlimb were determined using electrical stimulation of the sciatic nerve branches. Large variability in the active, passive, total force patterns and elongation ranges was found. The lateral gastrocnemius (LG), medial gastrocnemius (MG), peroneus longus (PL), flexor digitorum longus (FDL), tibialis posterior (TP) and soleus (Sol) showed symmetric active force curces, whereas those of the extensor digitorum longus (EDL), tibialis anterior (TA) and peroneus brevis (PB) were asymmetric. The total force curves of the EDL, LG, MG, FDL, TP and Sol increased quasilinearly through the elongation range, whereas the PL and PB increased in a nonlinear fashion. The TA had an intermediate plateau. The ranges were generally asymmetric, with a longer shortening range than lengthening past the optimum length. A simple model of the active force was fitted to all except the MG, EDL and TA, which are complex, with at least two compartments. These were successfully fitted with a two-compartment model. The variabilities encountered in the various isotonic length/force curves confirm the need to represent muscles according to their architecture to account for the variety of properties exhibited, which reflect their adaptations to their functions.