Patellofemoral contact forces and knee gait mechanics 3 months after ACL reconstruction are associated with cartilage degradation 24 months after surgery.

OBJECTIVE: Evaluate patellofemoral cartilage health, as assessed by quantitative magnetic resonance imaging (qMRI) T2 relaxation times, 24-months after ACL reconstruction (ACLR) and determine if they were associated with patellofemoral contact forces and knee mechanics during gait 3 months after surgery. DESIGN: Thirty individuals completed motion analysis during overground walking at a self-selected speed 3 months after ACLR. An EMG-driven neuromusculoskeletal model was used to determine muscle forces, which were then used in a previously described model to estimate patellofemoral contact forces. Biomechanical variables of interest included peak patellofemoral contact force, peak knee flexion angle and moment, and walking speed. These same participants underwent a sagittal bilateral T2 mapping qMRI scan 24-months after surgery. T2 relaxation times were estimated for both patellar and trochlear cartilage. Paired t-tests were used to compare T2 relaxation times between limbs while Pearson correlations and linear regressions were utilized to assess the association between the biomechanical variables of interest and T2 relaxation times. RESULTS: Prolonged involved limb trochlear T2 relaxation times (vs uninvolved) were present 24-months after surgery, indicating worse cartilage health. No differences were detected in patellar cartilage. Significant negative associations were present within the involved limb for all the biomechanical variables of interest 3 months after ACLR and trochlear T2 relaxation times at 24-months. No associations were found in patellar cartilage or within the uninvolved limb. CONCLUSIONS: Altered involved limb trochlear cartilage health is present 24-months after ACLR and may be related to patellofemoral loading and other walking gait mechanics 3 months after surgery.

A more informed evaluation of medial compartment loading: the combined use of the knee adduction and flexor moments.

OBJECTIVE: To evaluate if the peak knee flexor moment (pKFM) provides unique and meaningful information about peak medial compartment loading above and beyond what is obtained from the peak knee adduction moment. METHODS: Standard video-based motion capture and EMG recordings were collected for 10 anterior cruciate ligament (ACL) reconstructed subjects walking at a self-selected speed. Knee joint moments were obtained using inverse dynamics and medial contact force was computed using an EMG-driven musculoskeletal model. Linear regression with the peak adductor moment entered first was implemented to isolate the unique contribution of the peak flexor moment to peak medial loading. RESULTS: Peak moments and medial contact force occurred during weight acceptance at approximately 23% of stance. The peak knee adduction moment (pKAM) was a significant predictor of peak medial loading (P = 0.004) accounting for approximately 63% of the variance. The pKFM was also a significant predictor (P = 0.009) accounting for an additional 22% of the variance. When entered together pKAM and pKFM accounted for more than 85% of the variance in peak medial compartment loading. CONCLUSION: The combined use of the peak knee flexor and adductor moments provides a significantly more accurate estimate of peak medial joint loading than the peak adduction moment alone. More accurate inferences of joint contact force will assist clinicians and researchers investigating relationships between joint loading and the onset and progression of knee osteoarthritis (OA).

Intent to quit among daily and non-daily college student smokers.

Given the high prevalence of young adult smoking, we examined (i) psychosocial factors and substance use among college students representing five smoking patterns and histories [non-smokers, quitters, native non-daily smokers (i.e. never daily smokers), converted non-daily smokers (i.e. former daily smokers) and daily smokers] and (ii) smoking category as it relates to readiness to quit among current smokers. Of the 4438 students at six Southeast colleges who completed an online survey, 69.7% (n = 3094) were non-smokers, 6.6% (n = 293) were quitters, 7.1% (n = 317) were native non-daily smokers, 6.4% (n = 283) were converted non-daily smokers and 10.2% (n = 451) were daily smokers. There were differences in sociodemographics, substance use (alcohol, marijuana, other tobacco products) in the past 30 days and psychosocial factors among these subgroups of students (P < 0.001). Among current smokers, there were differences in cigarettes smoked per day, recent quit attempts, self-identification as a smoker, self-efficacy and motivation to quit (P < 0.001). After controlling for important factors, converted non-daily smokers were more likely to be ready to quit in the next month versus native non-daily smokers (OR = 2.15, CI 1.32-3.49, P = 0.002). Understanding differences among young adults with different smoking patterns and histories is critical in developing interventions targeting psychosocial factors impacting cessation among this population.

Displacement response of juvenile arthritic wrists during grasp.

OBJECTIVE: To analyze the displacement response of juvenile arthritic wrists during grasp in order to diagnose early ligamental laxity and facilitate early splinting. METHODS: X-rays of the wrists, made under standardized conditions, of 30 children with juvenile chronic arthritis (mean age 10.4 years, range 4.5-16.9) were analyzed after being digitalized. Osseous landmarks were identified, and coordinates were calculated from measured angles and lengths with an accuracy of 0.01'. Lunate and carpal-ulnar distance were obtained according to Youm, and ulnar variance according to Häfner. RESULTS: Overall, an increase in ulnar-lunate displacement and carpal narrowing and a decrease in ulnar variance were found. However, not all wrists responded to the same extent. Radial displacement of the lunate, though slight, was found in 2 wrists and the amount of ulnar displacement varied substantially (3.1% to 22.5%). The variance in amount of displacement could suggest that juvenile wrists do not respond to increased compressive forces to the same extent. CONCLUSION: The changes found are similar to those found in the healthy wrist. Furthermore, our findings suggest that the juvenile wrist acts in accordance with the generally accepted explanation for the development of malalignment in adult wrists. It seems that laxity of ligaments can be diagnosed early by the force grip maneuver during x-ray. It would have a significant impact on the moment of orthotic intervention as well as the design of the orthotic device. Further study along this line seems justified.

Muscle activity is different for humans performing static tasks which require force control and position control.

Muscle activation levels in humans were examined during two different static tasks which required the same joint angles and the same joint moments. In the isometric case, joint angles were fixed and subjects were required to match forces. In the isoinertial case, a constant load was imposed across the joint and the subject was required to match position. It was observed that for a specified posture and for specified load conditions, EMG activity varied depending on whether the limb was loaded isometrically or isoinertially. That is, different co-activation relationships were observed for position control versus force control tasks during otherwise similar conditions. These results imply that the neural command for static tasks depends on more than joint angles and load magnitude.

Human elbow joint torque is linearly encoded in electromyographic signals from multiple muscles.

When the central nervous system (CNS) develops a muscular activation pattern to accomplish a particular isometric task, it clearly uses information concerning the external task requirements. These task requirements serve as inputs to neural transformations that output muscular activations. However, the nature of the inputs is not exactly known. Electromyographic (EMG) signals from eight muscles spanning the human elbow, as well as the total joint torque, were collected during a submaximal isometric flexion/extension task at a single joint angle. The EMG data, without any torque information, were subjected to principal components analysis. We found that 98% of EMG data variation could be described by two principal components the first resembled the joint torque and the second resembled the sum of the EMG signals from all eight muscles. The findings suggest that the CNS encodes these two quantities during isometric tasks.

Muscle activation at the human knee during isometric flexion-extension and varus-valgus loads.

We examined the role of muscles in counteracting static loads in the transverse plane at the knee to determine if (a) knee muscles are activated to counteract isometric varus or valgus loads, (b) muscle activity during varus and valgus loads changes with the angle of knee flexion, and (c) the direction of a muscle's activation can be predicted by its moment arm orientations. For seven subjects, muscle activity was recorded during isometric tasks using surface and intramuscular electrodes from 10 muscles that span the knee. A six-degree-of-freedom load cell was rigidly attached to each subject's lower leg just above the ankle, and the subjects were instructed to push against the load cell so as to produce moments in the flexion-extension-varus-valgus plane at the knee. Moments in this plane were all of equal magnitude and varied in direction the full 360 degrees in 20 degrees increments. Most muscles were not activated to stabilize the knee against varus-valgus loads, but the sartorius, gracilis, and tensor fasciae latae showed substantial electromyographic activation in these directions. The load directions where muscles were principally active were observed to be dependent on joint angle for some muscles. In particular, the principal directions of activation for these three muscles changed as the angle of knee flexion changed. Similarly, a muscle's moment arm orientation was a good predictor of direction of activation for some muscles and a poor one for others. These results suggest that different muscles may play different roles in providing joint stability and that these roles are complex functions of muscle moment arm orientations, joint angles, external load directions, and possibly other undetermined parameters.

Selective muscle activation following rapid varus/valgus perturbations at the knee.

Knee muscles are generally divided into groups based on their function as flexors or extensors. In this study we sought to determine if muscles were selectively activated according to their potential roles as varus or valgus stabilizers following rapid loads to the knee. While subjects were supine, varus or valgus moments were applied to the knees of 10 human subjects using a servomotor-driven perturbation device. During the experiments, electromyograms (EMG) were recorded from seven muscles, four of which had medial moment arms relative to the knee center, and three of which had lateral moment arms. It was observed that, for all medial muscles, a statistically significant increase in muscle activation followed valgus loads as compared with varus loads. All lateral muscles except the vastus lateralis showed the opposite response (as expected). These results suggest that muscles can be reflexively activated independent of their roles as flexors or extensors to provide stability to the human knee during varus or valgus loads. The timing of the reflex is consistent with that arising from joint mechanoreceptors, although polysynaptic stretch reflex may also be involved.

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.

The isometric functional capacity of muscles that cross the elbow.

We hypothesized that muscles crossing the elbow have fundamental differences in their capacity for excursion, force generation, and moment generation due to differences in their architecture, moment arm, and the combination of their architecture and moment arm. Muscle fascicle length, sarcomere length, pennation angle, mass, and tendon displacement with elbow flexion were measured for the major elbow muscles in 10 upper extremity specimens. Optimal fascicle length, physiological cross-sectional area (PCSA), moment arm, operating range on the force-length curve, and moment-generating capacity were estimated from these data. Brachioradialis and pronator teres had the longest (17.7cm) and shortest (5.5cm) fascicles, respectively. Triceps brachii (combined heads) and brachioradialis had the greatest (14.9cm(2)) and smallest (1.2cm(2)) PCSAs, respectively. Despite a comparable fascicle length, long head of biceps brachii operates over a broader range of the force-length curve (length change=56% of optimal length, 12.8cm) than the long head of triceps brachii (length change=28% of optimal length, 12. 7cm) because of its larger moment arm (4.7cm vs. 2.3cm). Although brachioradialis has a small PCSA, it has a relatively large moment-generating capacity (6.8cm(3)) due to its large moment arm (average peak=7.7cm). These results emphasize the need to consider the interplay of architecture and moment arm when evaluating the functional capabilities of a muscle.

Variation of muscle moment arms with elbow and forearm position.

We hypothesized that the moment arms of muscles crossing the elbow vary substantially with forearm and elbow position and that these variations could be represented using a three-dimensional computer model. Flexion/extension and pronation/supination moment arms of the brachioradialis, biceps, brachialis, pronator teres, and triceps were calculated from measurements of tendon displacement and joint angle in two anatomic specimens and were estimated using a computer model of the elbow joint. The anatomical measurements revealed that the flexion/extension moment arms varied by at least 30% over a 95 degrees range of motion. The changes in flexion/extension moment arm magnitudes with elbow flexion angle were represented well by the computer model. The anatomical studies and the computer model demonstrate that the biceps flexion moment arm peaks in a more extended elbow position and has a larger peak when the forearm is supinated. Also, the peak biceps supination moment arm decreases as the elbow is extended. These results emphasize the need to account for the variation of muscle moment arms with elbow flexion and forearm position.

Maximum isometric moments generated by the wrist muscles in flexion-extension and radial-ulnar deviation.

Maximum isometric and passive moments about the wrist were measured for a range of flexion-extension and radial-ulnar deviation angles in 10 healthy adult males. Each subject was seated in a test apparatus with his shoulder abducted 90 degrees, elbow flexed 90 degrees, and body and forearm constrained. Peak flexion moments ranged from 5.2 to 18.7 N m (mean = 12.2, SD = 3.7), while peak extension moments ranged from 3.4 to 9.4 N m (mean = 7.1, SD = 2.1). The average flexion moment peaked at 40 degrees of flexion, whereas the average extension moment was relatively constant from 30 degrees flexion to 70 degrees extension. Peak moments generated by the radial and ulnar deviators ranged from 7.9 to 15.3 N m (mean = 11.0, SD = 2.0) and 5.9 to 11.9 N m (mean = 9.5, SD = 2.2), respectively. Passive moments in flexion-extension were near zero in the central 150 degrees of motion, but increased at the end of the range of motion. The average passive moment was 0.5 N m in 90 degrees flexion and 1.2 N m in 90 degrees extension. Average passive moments about the radial-ulnar deviation axis were near zero with the wrist radially deviated and at neutral, but increased to 0.9 N m in full ulnar deviation.

How muscle architecture and moment arms affect wrist flexion-extension moments.

The purpose of this investigation was to determine how the moment arms and architecture of the wrist muscles influence their isometric moment-generating characteristics. A three-dimensional computer graphic model was developed that estimates the moment arms, maximum isometric forces, and maximum isometric flexion-extension moments generated by 15 muscles about the wrist over a range of wrist flexion angles. In combination with measurements of muscle strength, we used this model to answer three questions: (1) why is peak wrist flexion moment greater than peak extension moment, (2) why does flexion moment vary more with wrist flexion angle than does extension moment, and (3) why does flexion moment peak with the wrist in a flexed position? Analysis of the model revealed that the peak flexion moment is greater than the peak extension moment primarily because of the larger (110%) summed physiologic cross-sectional area of the flexors. The larger variation of flexion moment with flexion angle is caused mainly by greater variation of the moment arms of the major wrist flexors with flexion angle. The location of the peak flexion moment is determined by the wrist flexion moment arms (which tend to increase with wrist flexion) in combination with the force-length characteristics of these muscles.

Estimation of muscle forces about the wrist joint during isometric tasks using an EMG coefficient method.

A technique for estimating isometric muscle forces based on EMGs and anatomical parameters is presented. In the present study, we record EMGs from five muscles acting at the wrist, during a series of isometric contractions in flexion, extension, ulnar deviation and radial deviation. The method then uses these EMG signals and the necessary anatomical data to estimate individual muscle forces. For one subject, complete anatomical parameters were estimated by MRI reconstruction of muscle moment arms and lines of muscle action. In all subjects, the errors associated with variability in the EMG signals were reduced through the use of signal processing techniques and intensive subject training. These EMG-based force estimates were then validated by evaluations at torque directions in which no mechanical redundancy existed. The stability of the solution space was examined using Monte Carlo simulations. The results of our study show that individual muscle forces at the wrist can be estimated with considerable accuracy, without assuming any control strategy (as is done with optimization theories). However, due to the limited mechanical redundancy of the wrist, it is uncertain whether the method can be used to estimate muscle forces in more highly redundant systems.

Evidence that maximum muscle stress is not a constant: differences in specific tension in elbow flexors and extensors.

The specific tension of muscle (or maximum muscle stress) is the maximum force developed per unit cross-sectional area and is a frequently used parameter by investigators estimating muscle force. Generally, it is assumed to be a constant value for all muscles and, when multiplied by a muscle's cross-sectional area, is used to provide a measure of a muscle's maximum force production. In this study, the specific tension for elbow flexors and for extensors were compared to evaluate the validity of this assumption. Maximum muscle stress was determined using maximum joint moments measured as a function of joint angle and using anatomical parameters reported in the literature. It was observed that the specific tension for elbow flexors was considerably larger than for extensors when measured a variety of ways. The exact reasons for the differences are unknown, but variations in specific tension of individual fibers may play a role. It was concluded that the use of a constant value for specific tension in muscle models is questionable in studies that demand accurate results.

Dynamic knee stability: current theory and implications for clinicians and scientists.

We will discuss the mechanisms by which dynamic knee stability may be achieved and relate this to issues that interest clinicians and scientists concerned with dynamic knee stability. Emphasis is placed on the neurophysiologic evidence and theory related to neuromuscular control. Specific topics discussed include the ensemble firing of peripheral mechanoreceptors, the potential for muscle stiffness modulation via force and length feedback, postural control synergies, motor programs, and the neural control of gait. Factors related to answering the difficult question of whether or not knee ligament injuries can be prevented during athletic activities are discussed. Prevention programs that train athletes to perform their sport skills in a safe fashion are put forth as the most promising prospect for injury prevention. Methods of assessing neuromuscular function are reviewed critically and the need for future research in this area is emphasized. We conclude with a brief review of the literature regarding neuromuscular training programs.

Ankle inversion injury and hypermobility: effect on hip and ankle muscle electromyography onset latency.

OBJECTIVE: Changes in reflexes associated with chronically sprained ankles were examined by measuring the reflex response latency of hip and ankle muscles during instantaneous ankle/foot inversion. DESIGN: Randomized trials. SETTING: All studies were performed in the Research Department laboratories at a major rehabilitation center in a large metropolitan area. PATIENTS AND OTHER PARTICIPANTS: Twenty subjects were assigned to 2 groups (normal and hypermobile) based on goniometry testing. Subjects were recruited from hospital and University staff and had a mean age of 31 +/- 5 years. OUTCOME MEASURES: Subjects stood on a platform constructed such that either foot/ankle could be instantaneously inverted. Latency was measured by EMG surface electrodes placed over the right and left gluteus medius and peroneal muscles. Two-factor analysis of variance was calculated to determine significant muscle onset latency differences (p < .01) between groups. RESULTS: Significant EMG latency differences were found in comparing right gluteus medius of the hypermobile group (127.35 +/- 6.02msec) with the normal group (150.49 +/- 6.49msec) during right ankle perturbation, and the left gluteus medius of the hypermobile group (120.71 +/- 6.16msec) with the normal group (136.24 +/- 5.88msec) during left ankle perturbation. CONCLUSIONS: These data suggest that there is decreased latency of hip muscle activation after ankle inversion in the hypermobile population. In treating ankle instability, clinicians must decide to address the altered hip muscle recruitment pattern or accept this recruitment pattern as an injury-adaptive strategy and thus accept unknown long-term consequences of premature muscle activation (ie, possible articular predisposition to degenerative changes, altered joint reaction forces, and muscle imbalances).

Selective block of the brachialis motor point. An anatomic investigation of musculocutaneous nerve branching.

BACKGROUND AND OBJECTIVES: Injections of neurolytic agents designed to block the musculocutaneous nerve often eliminate all elbow flexion movements, leaving the patient with a flail arm. In such patients, motor point blocks of the biceps brachii or brachialis muscle, or both, may be indicated. By virtue of its relative cross-section area, the brachialis is the largest contributor to elbow flexion. This factor, together with this muscle's lack of a role in supination, makes it the target of choice for controlling flexion spasticity. There are few descriptions of brachialis motor point blocks, and they fail to provide satisfactory instructions for the procedure. The goal of this study was to determine the brachialis motor point site and to quantitatively describe its location. METHODS: In this prospective, randomized study of 26 cadaver arms, the innervation site of the brachialis muscle from the musculocutaneous nerve was measured. Measurements were taken from the lateral epicondyle and were compared with the distance to the biceps motor point. These lengths were normalized across subjects by dividing by the arm length (from lateral epicondyle to the acromion). RESULTS: The brachialis was found to be innervated at approximately one third of the distance from the elbow to the acromion. This site is significantly different (P < .05) from that of the biceps brachii, which was found to be located at approximately half of the distance from the elbow to the acromion. CONCLUSIONS: An injection one third of the distance from the lateral epicondyle to the acromion along the medial aspect is recommended to provide best access to the brachialis motor point. By injecting from the medial aspect, one avoids the humerus (encountered in a lateral approach) and the need to pass through the biceps brachii (as in an anterior approach).

A model of load sharing between muscles and soft tissues at the human knee during static tasks.

In this study, we have subjects voluntarily generate various forces in a transverse plane just above their ankles. The contributions of their muscles and soft tissues to the support of the total external knee joint moment were determined by analyzing the experimental data using a biomechanical model of the knee. In this model, muscle forces were estimated using the recorded EMGs. To account for subject variability, various muscle parameters were adjusted using a nonlinear least-squares fit of the model's estimated flexion and extension joint moments to those recorded externally. Using the estimated muscle forces, the contributions from the muscles and other soft tissues to the total joint moment were obtained. The results showed that muscles were primarily used to support flexion and extension loads at the knee, but in so doing, were able to support some part of the varus or valgus loads. However, soft tissue loading was still required. Soft tissues supported up to an average maximum of 83 percent of the external load in pure varus and valgus. Soft tissue loading in pure varus and valgus was less than 100 percent of the external load as the muscles, on average, were able to support 17 percent of the external load. This muscle support was by virtue of muscle cocontraction and/or specific muscle activation.

An evaluation of optimization techniques for the prediction of muscle activation patterns during isometric tasks.

The purpose of this study was to critically evaluate the modeling potential of proposed optimization cost functions for predicting muscle forces during isometric loading. Models of the muscles about the elbow (eleven muscles) and wrist (five muscles) were constructed. The models accounted for muscle moment arms, physiological cross-sectional area, specific tension, and percent fiber type. Five nonlinear optimization cost functions, a representative sample of those proposed to date, were analyzed: minimizing the sums of muscle force2, stress2, stress3, (normalized force)2, and minimizing fatigue. Several different protocols were implemented, including elbow models which balanced combinations of flexion-extension, supination-pronation, and varus-valgus loads. Theoretical predictions were compared with EMG data of muscle activation changes as a function of load direction and muscle coactivation relationships. Results indicate a strong dependence of muscle coordination predictions on the number of degrees of freedom balanced. The choice of cost function had little influence on the results. The cost functions examined were not able to reliably estimate muscle activation as a function of load direction. Furthermore, specific synergic relationships between muscle pairs could not be accurately represented. An error analysis indicated that the discrepancies between predicted values and actual values could not be explained by errors in physiological measurements, as the differences between these two were relatively insensitive to changes in the anatomical parameters. In short, no particular cost function was found to adequately represent actual muscle activity at the elbow, although predictions at the wrist were more favorable due to differences in the degrees of freedom at the joints.