Factors affecting postoperative flexion in total knee arthroplasty

In the review of 67 cases with total condylar (TC) prostheses, 59 with TC posterior stabilizers, 70 with TC prostheses modified with flat posterior tibial plateau, and 35 with porous-coated arthroplasty (PCA) prostheses, multiple cross-examinations of various factors for postoperative flexion were performed. Follow up was 2 to 9 years. In nearly all cases, no further improvement of flexion was noted after 1 year following surgery. The most influential factor for good postoperative flexion was intense physical therapy, leading to good suprapatellar pouch reconstitution. Residual flexion contracture was more frequent when the posterior cruciate ligament (PCL) was retained in the cases with significant preoperative flexion contracture. However, retention of PCL or preoperative ACL condition did not bear any significance to the ultimate flexion.

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 antagonists were studied as a function of limb velocity to assess their contribution to joint stiffness and laxity. Normalized antagonist coactivation patterns developed from surface EMG recordings from the hamstrings and quadriceps during maximal effort isokinetic extension and flexion, respectively, demonstrated characteristic variations as the joint velocity increased from 15 deg/sec up to 240 deg/sec. The two-tailed t-test (P less than 0.01) was performed on the data obtained from eight normal knees. The results indicate that both hamstrings and quadriceps demonstrate a significant increase (greater than 100%) in their antagonist coactivation pattern during the final 40 degrees of fast extension and flexion movements, respectively, as limb velocity increases. A minor decrease in antagonist activity of the hamstrings (24%) and quadriceps (8%) was evident during the initial phase of the extension and flexion movements, respectively, 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 stiffness 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.

Electromyogram power spectra frequencies associated with motor unit recruitment strategies

The isolated contributions of motor unit recruitment and firing rate variations to the median frequency of the electromyogram's power density spectrum were determined. Orderly stimulation of the cat gastrocnemius motor units via nerve electrodes gave rise to linearly increasing median frequency regardless of the action potential firing rate of the active motor units. Increase in the discharge rate of all the motor units resulted in nearly constant median frequency. It was concluded that the increasing average conduction velocity during motor unit recruitment is the major contributor to variations in the electromyogram median frequency. The possibility of using the median frequency as the index to identify the recruitment control strategies employed by various muscles during increasing force contraction is suggested.

The RGO Generation II: muscle stimulation powered orthosis as a practical walking system for thoracic paraplegics

The RGO Generation II reciprocating gait orthosis was jointly developed by Louisiana State University Medical Center and Durr-Fillauer Medical, Inc, to overcome four problems encountered with the existing model: 1) The high energy cost of locomotion; 2) the great arm strength required for patients to stand up from the seated position without assistance; 3) difficulty (especially for patients with hamstring contracture) in remaining standing owing to failure of the knee latch to lock except in full extension; and 4) problems in balancing when ambulating on an incline. The RGO Generation II employs concurrent electrostimulation of the rectus femoris and hamstrings to assist in rising and balancing and a ratchet-type latching device to improve safety and stability in standing. Alternating stimulation of the rectus femoris and contralateral hamstrings are used for locomotion. Testing in six patients with thoracic paraplegia demonstrated an average 30+% reduction in energy expenditure at a walking speed of .05 m/s and a 15+% reduction at .37 m/s; improved mobility and better balance on inclines; and unassisted rising in all patients. Walking range was increased from an average of 100 m to an average of 800 m. More research is needed to provide stair-climbing ability and to further reduce energy expenditure.

Intracapsular hip fractures: case reports and discussion

Two cases of intracapsular hip fracture illustrate some of the treatment options available for Garden Stage IV fractures. In Case 1, a prosthesis was chosen because of the patient's physiologic age and poor general health. In Case 2, a compression screw augmented with a single-pin fixation to prevent rotation was used. This patient, although only 3 years younger than the first patient, was in good health. Although there was some posterior comminution, an attempt at fixation, allowing time for bone healing, was appropriate. Arthroplasty may be needed in several years if bone healing fails.

The role of the hamstrings in the rehabilitation of the anterior cruciate ligament-deficient knee in athletes

There is ample evidence for the synergy between the knee musculature and ligaments, and the overall maintenance of joint integrity. In fact it could be reasonably concluded that the joint's antagonist muscle acts as a 'regulator' compensating for various internal and external disturbances of the intended movement. Undoubtedly, well conditioned muscles provide substantial improvements in joint stiffness and laxity, and reduce the risk of ligamentous injuries in the elite athlete. More research is needed to delineate the details of the questions which this new approach raises.

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 joint's 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 muscle's 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 joint's 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.

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

The objective of this study was to quantify the coactivation 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 muscle 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 antagonist exerts nearly constant opposing torque throughout joint range of motion. Comparison of data recorded from normal healthy subjects with that of high performance athletes with hypertrophied quadriceps demonstrated strong inhibitory effects on the hamstrings coactivations. Athletes who routinely exercise their hamstrings, 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 distribution, and regulating the joint's mechanical impedance. The reduced coactivation pattern of the unexercised antagonist to a hypertrophied muscle increases the risk of ligamentous damage, as well as demonstrates the adaptive properties of the antagonist muscle in response to exercise. It was also concluded that reduced risk of knee injuries in high performance athletes with muscular imbalance could result from complementary 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.

Biomechanics of muscle overuse injuries: a theoretical approach

This article reviews the highlights of biomechanical principles associated with the proper function of the muscle, and extends the discussion to the limits set by the biomechanical concepts that, when exceeded, result in tissue injuries. The principles governing proper muscle functions and its injuries are also used to provide some guidelines for formation of exercise regimens that can minimize potential injuries.

Historical update and new developments on the EMG-force relationships of skeletal muscles

The EMG-force relationships of skeletal muscles are of significant interest to the orthopedic-biomechanics researcher and have been a topic of interest and controversy for the last three decades. We present the developments reported in the literature, from the early 1950s, relevant to this topic, outlining the details of the controversy and providing additional new information resulting from recent studies. New data clearly illustrate that the EMG-force relationships are dependent on the firing rate and recruitment control strategy used by a muscle. Muscles that use motor unit recruitment to obtain the initial 50% of their maximal force, and use firing rate increase to complement the remaining 50%, have a nearly linear EMG-force relationship. Muscles that use recruitment to obtain 60% and up to 100% of their maximal force demonstrate progressive increase in non-linearity of their EMG-force curves. A clear warning is issued against the indiscriminate use of EMG as a representative of muscle force. The recruitment pattern of the muscle under consideration should first be obtained in such a way that the appropriate EMG-force curve is selected.