Sex- and osteoarthritis-related differences in muscle co-activation during weight-bearing tasks

BackgroundIndividuals with knee osteoarthritis (OA) demonstrate impairments in muscle function (i.e. muscle weakness, high muscle co-activation believed to have detrimental effects on joint integrity). Women with knee OA exhibit poorer health outcomes than men. Sex and muscle function are known risk factors for knee OA. It is unclear how these risk factors are associated with muscle function in knee OA and the implications for disease aetiology.Research questionHow does sex and knee osteoarthritis disease status relate to muscle function, specifically strength and muscle co-activation, during walking, stair negotiation and sit-to-walk activities.MethodsA cross-sectional study assessed muscle co-activation in 77 individuals with knee OA (mean[SD], 62.5[8.1] years; 48/29 women/men) and 18 age-matched controls (62.5[10.4] years; 9/9 women/men), during a series of walking, stair ascent and descent and sit-to-walk activities. Muscle strength of the knee extensors and flexors was assessed using maximal voluntary isometric contractions (MVIC). Electromyography was recorded from the vastus lateralis/medalis, rectus femoris, biceps femoris, semitendinosus, medial/lateral gastrocnemius normalised to MVIC. Multiple regression assessed the relationship between sex, disease status, and muscle strength on muscle co-activation.ResultsIndividuals with knee OA were weaker than controls, had higher hamstrings-quadriceps and medial-lateral co-activation for specific phases of gait. Women were weaker than men with higher muscle co-activation across all activities. Sex and muscle weakness, but not age or disease status predicted high muscle co-activation.SignificanceHigh muscle co-activation was associated with female sex and muscle weakness regardless of disease status and age. High muscle co-activation is believed to be a compensatory mechanism for muscle weakness to maintain a certain level of function. High muscle co-activation is also thought to have detrimental effects on cartilage and joint integrity this may explain high muscle co-activation in women with muscle weakness and contribute to increased risk of incidence and progression of knee OA in women.     

Surface electromyography and muscle force: limits in sEMG-force relationship and new approaches for applications

The estimation of the force generated by an activated muscle is of high relevance not only in biomechanical studies but also more and more in clinical applications in which the information about the muscle forces supports the physician's decisions on diagnosis and treatment. The surface electromyographic signal (sEMG) reflects the degree of activation of skeletal muscles and certain that the sEMG is highly correlated to the muscle force. However, the largest disadvantage in predicting the muscle force from sEMG is the fact that the force generated by a muscle cannot be directly measured non-invasively. Indirect measurement of muscle force goes along with other unpredictable factors which influence the detected force but not necessarily the sEMG data. In addition, the sEMG is often difficult to interpret correctly. The sEMG-force relationship has been investigated for a long time and numerous papers are available. This review shows the limitations in predicting the muscle force from sEMG signals and gives some perspectives on how these limitations could be overcome, especially in clinical applications, by using novel ways of interpretation.     

Electromyographic activity in the shoulder-neck region according to arm position and glenohumeral torque

Summary The electromyographical (EMG) response to isometric ramp contractions of the right arm, the left arm, and both arms was studied using four pairs of surface electrodes above the right upper trapezius muscle (UT) of six men and six women. Contractions were made against gravity with the active arm(s) in eight positions, ranging from flexion to abduction. To describe arm positions, a new, simple terminology was developed. Root mean square (rms)-converted EMG-signals were normalized (EMG_norm) with respect to a reference contraction. The EMG_norm corresponding to a 15 N · m torque in the right glenohumeral (GH) joint was strongly related to the position of the right arm (P<0.001). The shape of this relationship depended on the electrode position (P<0.001). The ratio between EMG_norm at 30 N · m and 15 N · m GH torques was related to arm position (P < 0.001) and differed between electrodes (P< 0.001). A left-side GH torque resulted in right-side (contralateral) EMG activity, typically corresponding to 20%–30% of that obtained during similar right-side GH torque. Bilateral GH torque implied 0%50% increase in EMG activity as compared to that obtained with the right arm alone. The results have shown that signals from one pair of surface electrodes above UT cannot be taken as representative of the EMG activity from electrodes located elsewhere above UT. The EMG recordings reflected a complex pattern of muscular activation, significantly related to both outwardly visible factors (arm position, GH torque), and within-body servosystems (motor control reflexes).     

Physical activity and age-related biomechanical risk factors for knee osteoarthritis

BackgroundKnee osteoarthritis (OA) is a highly prevalent disease leading to mobility disability in the aged that could, in part, be initiated by age-related alterations in knee mechanics. However, if and how knee mechanics change with age remains unclear.Research questionWhat are the impacts of age and physical activity (PA) on biomechanical characteristics that can affect the loading environment in the knee during gait?MethodsThree groups (n = 20 each, 10 male and 10 female) of healthy adults were recruited: young (Y, 21–35 years), mid-life highly active (MHi, 55–70 years, runners), and mid-life less active (MLo, 55–70 years, low PA). Outcome measures included knee kinematics and kinetics and co-activation during gait, and knee extensor muscle torque and power collected at baseline and after a 30-minute treadmill trial to determine the impact of prolonged walking on knee function.ResultsAt baseline, high-velocity concentric knee extensor power was lower for MLo and MHi compared with Y, and MLo displayed greater early (6.0 ± 5.8 mm) and peak during stance (11.3 ± 7.8 mm) femoral anterior displacement relative to the tibia compared with Y (0.2 ± 5.6 and 4.4 ± 6.8 mm). Also at baseline, MLo showed equal quadriceps:hamstrings activation, while Y showed greater relative hamstrings activation during midstance. The walking bout induced substantial knee extensor fatigue (decrease in maximal torque and power) in Y and MLo, while MHi were fatigue-resistant.SignificanceThese results indicate that maintenance of PA in mid-life may impart small but measurable effects on knee function and biomechanics that may translate to a more stable loading environment in the knee through mid-life and thus could reduce knee OA risk long-term.     

Co-activation and tension-regulating phenomena during isokinetic knee extension in sedentary and highly skilled humans

The aim of this study was to examine isokinetic torque produced by highly skilled (HS) and sedentary (S) human subjects, during knee extension, during maximal voluntary and superimposed electrical activation. To verify the level of activation of agonist (vastus lateralis, VL, and vastus medialis, VM) and antagonist muscles (semi-tendineous, ST), during maximal voluntary activation, their myo-electrical activities were detected and quantified as root mean square (rms) amplitude. Ten HS and ten S subjects performed voluntary and superimposed isometric actions and isokinetic knee extensions at 14 angular velocities (from –120 to 300°·s^–1). The rms amplitude of each muscle was normalized with respect to its rms amplitude when acting as agonist at 15°·s^–1. Whatever the angular velocity considered, peals torque and constant angular torque at 65° HS were significantly higher (P < 0.05) than those of S. Eccentric superimposed torque of S, but not HS, was significantly higher (P < 0.05) than voluntary torque at –120, –90, –60 and –30°·s^–1 angular velocities. For a given velocity, the rms amplitude of VL and VM were significantly lower (P < 0.05), during eccentric than during concentric actions, in S, but not in HS. However, whatever the angular velocity, ST co-activation in HS was significantly lower (P < 0.05) than in S. We concluded that co-activation phenomenon could partly explain differences in isokinetic performances. Differences between voluntary and superimposed eccentric torques as well as lower agonist rms amplitude during eccentric action in S, support the possibility of the presence of a tension-regulating mechanism in sedentary subjects.     

Activation of agonist and antagonist muscles at different joint angles during maximal isometric efforts

The purpose of this study was to investigate the influence of different angles of the knee joint on the activation level of an agonist (quadriceps femoris muscle) and antagonist (biceps femoris muscle) from electromyographic activities and activation levels (twitch interpolation). Isometric torque measurements were performed on 23 healthy subjects at 10° intervals between 40° and 110° of knee joint flexion. Superimposed twitches at maximal voluntary contraction were applied and the voluntary activation estimated. To quantify the antagonist muscle activity, we normalized its integrated EMG (iEMG) value at each joint angle with respect to its iEMG value at the same angle when acting as an agonist at maximal effort. The activation levels at the knee-flexed position (80–110°) were higher than that at the knee-extended position (40–70°). The co-activation levels at 90, 100, and 110° were significantly higher than that the other knee angle. These results suggest that the activation level of an agonist (quadriceps femoris) muscle and the co-activation level of an antagonist (biceps femoris) muscle were higher in longer muscles than in shorter muscles. It was also concluded that the risk of knee injuries could be reduced by applying these mutual relationships between activation levels of agonist and antagonist muscles.     

Neuromuscular interactions around the knee in children,adults and elderly

Although injury and neuromuscular activation patterns may be common for all individuals, there are certain factors which differentiate neuromuscular activity responses between children, adults and elderly. The purpose of this study is to review recent evidence on age differences in neural activation and muscle balances around the knee when performing single joint movements. Particularly, current evidence indicates that there are some interesting similarities in the neuromuscular mechanisms by which children or the elderly differ compared with adults. Both children and elderly display a lower absolute muscle strength capacity than adults which cannot fully be explained by differences in muscle mass. Quadriceps activation failure is a common symptom ofall knee injuries, irrespective of age but it is likely that its effect is more evident in children or adults. While one might expect that antagonist co-activation would differ between age categories, it appears that this is not the case. Although hamstring: quadriceps ratio levels are altered after knee injury, it is not clear whether this is an age specific response. Finally, evidence suggests that both children and the elderly display less stiffness of the quadriceps muscle-tendon unit than adults which affects their knee joint function.     

Hamstrings co-activation in ACL-deficient subjects during isometric whole-leg extensions

It has been reported that anterior cruciate ligament (ACL)-deficient subjects increase the level of hamstrings activation and this has been interpreted as a means to cope with increased anterior tibial laxity in the knee. This study aimed to establish to what extent co-activation strategies in ACL-deficient subjects are load level and knee angle dependent. Eleven chronic ACL-deficient and 15 control subjects were positioned in a range of postures and asked to exert a feedback controlled vertical ground reaction force (GRF; 30, 60% and maximum), while horizontal forces were not constrained. Surface electromyography of the leg muscles and GRF were measured. In postures with the knee over and in front of the ankle, ACL-deficient subjects generated, respectively, 2.4 and 5.1% MVC more hamstrings activation than control subjects. Enhanced hamstrings co-activation in ACL-deficient subjects was more apparent in extended than in flexed knee angles. For both ACL-deficient and control subjects, hamstrings co-activation was larger in males than in females. It is concluded that ACL-deficient subjects show a task dependent increase in hamstrings co-activation, but its clinical significance remains to be shown.     

Exploring relationships for visceral and somatic pain with autonomic control and personality

The autonomic nervous system (ANS) integrates afferent and motor activity for homeostatic processes including pain. The aim of the study was to compare hitherto poorly characterised relations between brainstem autonomic control and personality in response to visceral and somatic pain. Eighteen healthy subjects (16 females, mean age 34) had recordings during rest and pain of heart rate (HR), cardiac vagal tone (CVT), cardiac sensitivity to baroreflex (CSB), skin conductance level (SC), cardiac sympathetic index (CSI) and mean blood pressure (MBP). Visceral pain was induced by balloon distension in proximal (PB) and distal (DB) oesophagus and somatic pain by nail-bed pressure (NBP). Eight painful stimuli were delivered at each site and unpleasantness and intensity measured. Personality was profiled with the Big Five inventory. (1) Oesophageal intubation evoked “fight-flight” responses: HR and sympathetic (CSI, SC, MBP) elevation with parasympathetic (CVT) withdrawal (p < 0.05). (2) Pain at all sites evoked novel parasympathetic/sympathetic co-activation with elevated HR but vasodepression (all p < 0.05). (3) Personality traits correlated with slope of distal oesophageal pain-related CVT changes wherein more neurotic-introvert subjects had greater positive pain-related CVT slope change (neuroticism r 0.8, p < 0.05; extroversion r −0.5, p < 0.05). Pain-evoked heart rate increases were mediated by parasympathetic and sympathetic co-activation – a novel finding in humans but recently described in mammals too. Visceral pain-related parasympathetic change correlated with personality. ANS defence responses are nuanced and may relate to personality type for visceral pain. Clinical relevance of these findings warrants further exploration.     

膝关节角度对屈伸肌共同收缩及伸膝力矩的影响

目的 应用表面电极肌电图（EMG）测量膝关节在不同角度做开放链最大等长伸膝时屈膝肌的共同兴奋能力，比较不同角度膝屈伸肌的共同兴奋水平。方法 12名健康在校大学生参与测试。每名受试者在Cybex测试仪上髋关节屈曲90°，单侧膝关节分别于屈膝10，60和90°时各做1组开放链的最大等长伸膝动作。使用表面电极EMG测量每次膝关节伸展时股四头肌与胭绳肌的电信号。在每次伸膝的同时用cy—bex测量股四头肌的最大伸膝力矩。结果所有角度下股四头肌的最大等长收缩均伴胭绳肌的共同兴奋。平均最大伸膝力矩出现在屈膝60°（P〈0．05）。结论膝关节在屈膝60°时可产生最大等长伸膝力矩，开放链的伸膝肌等长收缩伴有屈膝肌的共同兴奋。