Delayed onset of electromyographic activity of the vastus medialis relative to the vastus lateralis may be related to physical activity levels in females with patellofemoral pain

The aims of this study were to examine group differences in muscle activation onset of the vastus medialis (VM) in relation to the vastus lateralis (VL) and pain level during stair ascent in females with patellofemoral pain (PFP) who maintain high and moderate levels of physical activity; to determine the association between physical activity level and muscle activation onset. Forty-three females with PFP and thirty-eight pain-free females were recruited and divided into four groups based on their level of physical activity: females with PFP (n = 26) and pain-free females (n = 26) who practiced a moderate level of physical activity and females with PFP (n = 17) and pain-free females (n = 12) who practiced an intense amount of physical activity. Participants were asked to ascend a seven-step staircase and the VM and VL activation onset was determined. Females with PFP who practiced high level of physical activity demonstrated delayed onset of VM (4.06 ms) compared to healthy females (−14.4 ms). Conversely, females with PFP who practiced moderate level of physical activity did not present VM delay (−2.48 ms) in comparison to healthy females (−9.89 ms). Furthermore, physical activity significantly correlated to the muscle activation onset difference (p = 0.005; R = 0.60). These findings may explain why controversial results regarding VM and VL muscle activation onset have been found.     

The contraction-induced increase in Achilles tendon moment arm: A three-dimensional study

The present study aimed to re-examine the influence of the isometric plantarflexors contraction on the Achilles tendon moment arm (ATMA) and the factors influencing the ATMA in three-dimensions. A series of coronal magnetic resonance images of the right ankle were recorded at foot positions of 10° of dorsiflexion, neutral position, and 10° of plantarflexion for the rest condition and the plantarflexors contraction condition at 30% maximal voluntary effort. The shortest distance between the talocrural joint axis and the line of action of the Achilles tendon force projected to the orthogonal plane of the talocrural joint axis was determined as the ATMA. The ATMA determined in the contraction condition was significantly greater by 8 mm than that determined in the rest condition. The talocrural joint axis was displaced anteriorly by 3 mm and distally by 2 mm due to the muscle contraction. As the same time, the line of action of the Achilles tendon force was displaced posteriorly by 5 mm and medially by 2 mm. These linear displacements of the talocrural joint axis and the line of action of the Achilles tendon force accounted for the difference in the ATMAs between the two conditions by 35.9 and 62.4%, respectively. These angular displacements accounted for the total of 0.4% increase in the ATMA. These results confirm the previous findings reported in two-dimensional studies and found that the linear displacement of the line of action of the Achilles tendon force is the primary source of the contraction-induced increase in the ATMA.     

The minimum number of contractions required to examine the EMG amplitude versus isometric force relationship for the vastus lateralis and vastus medialis

Studies have demonstrated that the electromyographic (EMG) amplitude versus submaximal isometric force relationship is relatively linear. The purpose of this investigation was to determine the minimum number of contractions required to study this relationship. Eighteen men (mean age = 23 years) performed isometric contractions of the leg extensors at 10–90% of the maximum voluntary contraction (MVC) in 10% increments while surface EMG signals were detected from the vastus lateralis and vastus medialis. Linear regression was used to determine the coefficient of determination, slope coefficient, and y-intercept for each muscle and force combination with successively higher levels included in the model (i.e., 10–30%, … 10–90% MVC). For the slope coefficients, there was a main effect for force combination (P < .001). The pairwise comparisons showed there was no difference from 10–60% through 10–90% MVC. For the y-intercepts, there were main effects for both muscle (vastus lateralis [4.3 μV RMS] > vastus medialis [−3.7 μV RMS]; P = .034) and force combination (P < .001), with similar values shown from 10–50% through 10–90% MVC. The linearity of the absolute EMG amplitude versus isometric force relationship for the vastus lateralis and vastus medialis suggests that investigators may exclude high force contractions from their testing protocol.     

Development of software for human muscle force estimation

Muscle force estimation (MFE) has become more and more important in exploring principles of pathological movement, studying functions of artificial muscles, making surgery plan for artificial joint replacement, improving the biomechanical effects of treatments and so on. At present, existing software are complex for professionals, so we have developed a new software named as concise MFE (CMFE). CMFE which provides us a platform to analyse muscle force in various actions includes two MFE methods (static optimisation method and electromyographic-based method). Common features between these two methods have been found and used to improve CMFE. A case studying the major muscles of lower limb of a healthy subject walking at normal speed has been presented. The results are well explained from the effect of the motion produced by muscles during movement. The development of this software can improve the accuracy of the motion simulations and can provide a more extensive and deeper insight in to muscle study.