Two-dimensional spatial error distribution of key tensiomyographic parameters

Tensiomyography detects the contraction time (Tc) and amplitude (Dm) of muscle belly thickening during maximal isometric twitch contraction. The assessment of both parameters is highly reliable; however, it seems that their calculation depends on the measurement point. The aim of the study was to determine spatial relative error distribution of Tc and/or Dm within a two-dimensional array of 27 (3 × 9) measurement points in comparison to the reference point (RP) in 12 male participants (22.5 ± 3.1 years). The RPs were determined as follows: in the biceps brachii (BB) at 50% of the humerus length; in the erector spinae (ES) at the height of the iliac crest; in the vastus lateralis (VL), vastus medalis (VM), and rectus femoris (RF) at 30%, 20%, and 50% of femur length above the patella, respectively. The surface area under the 3% relative error in Dm (BB: 4.0; VL: 3.8; VM; 8.2; RF: 6.2; ES: 2.4 cm²) was lower than in Tc (BB: 6.9; VL: 3.8; VM; 4.6; RF: 9.5; ES: 3.7 cm²), yielding merged values (BB: 3.9; VL: 3.7; VM; 4.8; RF: 5.1; ES: 2.4 cm²). Dm show twice as steep relative error rate when moving away from the RP in comparison to Tc, which seems to be less sensitive to spatial sensor positioning.     

Correlation between timed up and go test and skeletal muscle tensiomyography in female nursing home residents

Objectives:Tensiomyography (TMG) derived contraction time (Tc) and amplitude (Dm) are related to muscle fibre composition and to muscle atrophy/tone, respectively. However, the link between mobility and TMG-derived skeletal muscle contractile properties in older persons is unknown. The aim of the study was to correlate lower limb skeletal muscle contractile properties with balance and mobility measures in senior female residents of retirement homes in Austria.Methods:Twenty-eight female participants (aged from 67-99 years) were included in measurements of contractile properties (TMG) of four skeletal muscles: vastus lateralis, vastus medialis, biceps femoris and gastrocnemius medialis. Their balance and mobility performance was measured using a timed up and go test (TUG).Results:Time needed to complete TUG is negatively correlated to biceps femoris (r= -0.490; p= 0.008), vastus lateralis (r= -0.414; p=0.028) and vastus medialis (r= -0.353; p=0.066) Dm and positively correlated to vastus lateralis Tc (r=0.456; p=0.015). Overall, vastus lateralis Tc and vastus medialis Dm explained 37% of TUG time variance.Conclusions:Our study demonstrates that TMG-derived quadriceps muscle contractile parameters are correlated with the balance and mobility function in female nursing home residents.     

The mechanics of agonistic muscles

IntroductionIn this study, we tested two assumptions that have been made in experimental studies on muscle mechanics: (i) that the torque-angle properties are similar among agonistic muscles crossing a joint, and (ii) that the sum of the torque capacity of individual muscles adds up to the torque capacity of the agonist group.MethodsMaximum isometric torque measurements were made using a specifically designed animal knee extension dynamometer for the intact rabbit quadriceps muscles (n = 10) for knee angles between 60 and 120°. The nerve branches of the vastus lateralis (VL), vastus medialis (VM) and rectus femoris (RF) muscles were carefully dissected, and a custom made nerve cuff electrode was implanted on each branch. Knee extensor torques were measured for four maximal activation conditions at each knee angle: VL activation, VM activation, RF activation, and activation of all three muscles together.ResultsWith the exception of VL, the torque-angle relationships of the individual muscles did not have the shape of the torque-angle relationship obtained when all muscles were activated simultaneously. Furthermore, the maximum torque capacity obtained by adding the individual torque capacities of VL, VM and RF was approximately 20% higher than the torques produced when the three muscles were activated simultaneously.DiscussionThese results bring into question our understanding of in-vivo muscle contraction and challenge assumptions that are sometimes made in human and animal muscle force analyses.     

Inter-rater reliability of muscle contractile property measurements using non-invasive tensiomyography

Tensiomyography (TMG) is a relatively novel technique to assess muscle mechanical response based on radial muscle belly displacement consecutive to a single electrical stimulus. Although intra-session reliability has been found to be good, inter-rater reliability and the influence of sensor repositioning and electrodes placement on TMG measurements is unknown. The purpose of this study was to analyze the inter-rater reliability of vastus medialis muscle contractile property measurements obtained with TMG as well as the effect of inter-electrode distance (IED). Five contractile parameters were analyzed from vastus medialis muscle belly displacement–time curves: maximal displacement (Dm), contraction time (Tc), sustain time (Ts), delay time (Td), and half-relaxation time (Tr). The inter-rater reliability and IED effect on these measurements were evaluated in 18 subjects. Intra-class correlation coefficients, standard errors of measurement, Bland and Altman systematic bias and random error as well as coefficient of variations were used as measures of reliability. Overall, a good to excellent inter-rater reliability was found for all contractile parameters, except Tr, which showed insufficient reliability. Alterations in IED significantly affected Dm with a trend for all the other parameters. The present results legitimate the use of TMG for the assessment of vastus medialis muscle contractile properties, particularly for Dm and Tc. It is recommended to avoid Tr quantification and IED modifications during multiple TMG measurements.     

Effect of the contraction of medial rotators of the tibia on the electromyographic activity of vastus medialis and vastus lateralis

PurposeThis study attempted to assess if the resisted contraction of medial rotators of the tibia increases the ratio between the activity of vastus medialis (VM) and vastus lateralis (VL) during maximal isometric contractions (MIC) of the quadriceps femoral (QF) muscle at 90° of knee flexion.MethodsAbout 24 female subjects participated in this study, performing four series MIC of the QF. In the first series subjects performed only MIC of the QF muscle, whereas in the other three there was MIC of the QF with resisted contraction of medial rotators of the tibia, with the tibia positioned in medial, neutral and lateral rotation. During each contraction, VM and VL electromyographic signal (EMGs) and QF force were collected, being the EMGs root mean square (RMS) used to access the activity level of these muscles.ResultsThe use of the General Linear Model (GLM) test showed that for α = 0.05 there was a significant increase in the VM:VL ratio when the resisted contraction of medial rotators of the tibia was performed with the tibia in medial (p = <0.0001), neutral (p = <0.0001) and lateral rotation (p = 0.001). The same test showed that during MIC of the QF associated to resisted contraction of medial rotators of the tibia there were no significant differences in the VM:VL ratio between the three tibial rotation positions adopted (p = 0.866 [medial–neutral]; p = 0.106 [medial–lateral]; p = 0.068 [neutral–lateral]).ConclusionsThe resisted contraction of medial rotators of the tibia increases the VM:VL ratio during MIC of the QF and the tibial rotation position does not influence the VM:VL ratio during MIC associated to resisted contraction of medial rotators of the tibia.     

Skeletal muscle morphology in power-lifters with and without anabolic steroids

The morphological appearance of the vastus lateralis (VL) muscle from high-level power-lifters on long-term anabolic steroid supplementation (PAS) and power-lifters never taking anabolic steroids (P) was compared. The effects of long- and short-term supplementation were compared. Enzyme-immunohistochemical investigations were performed to assess muscle fiber type composition, fiber area, number of myonuclei per fiber, internal myonuclei, myonuclear domains and proportion of satellite cells. The PAS group had larger type I, IIA, IIAB and IIC fiber areas (p<0.05). The number of myonuclei/fiber and the proportion of central nuclei were significantly higher in the PAS group (p<0.05). Similar results were seen in the trapezius muscle (T) but additionally, in T the proportion of fibers expressing developmental myosin isoforms was higher in the PAS group compared to the P group. Further, in VL, the PAS group had significantly larger nuclear domains in fibers containing ≥5 myonuclei. The results of AS on VL morphology in this study were similar to previously reported short-term effects of AS on VL. The initial effects from AS appear to be maintained for several years.     

膝骨性关节炎患者登梯时下肢肌肉活动和膝关节负荷的分析

目的:分析膝骨性关节炎患者(KOA)登梯时下肢肌群肌电活动与关节角冲量与正常人的差异,为康复方案设计提供生物力学参考。方法:采用Qualisys三维运动分析系统以及Delsys无线表面肌电系统对招募10名符合纳排标准的膝骨性关节炎患者和10名正常人进行登梯活动的步态检测,采用下肢肌群均方根值、股内外侧肌协同收缩比值、股二头肌和股外侧肌共同活动比值和髋、膝关节在冠状面和矢状面上角冲量对比分析与两组登梯时下肢肌群收缩模式对关节负荷的影响。结果:与正常对照相比,上梯时膝骨性关节炎患者股直肌均方根值RMS(Root Mean Square)增大(P0.05),膝骨性关节炎患者股内外侧肌收缩均方根值比值(RMS(Vastus Medialis)VM/(Vastus Lateralis)VL)减小(P0.05),膝骨性关节炎患者腘绳肌与股外侧肌收缩比值(RMS(Biceps Femoris)BF/VL增大(P0.05)。下梯时,膝骨性关节炎患者股直肌均方根值(RMS)增大(P0.05),臀大肌均方根值(RMS)减小(P0.05),股内外侧肌收缩均方根比值(RMS VM/VL)减小(P0.05)。上梯时,膝骨性关节炎患者髋、膝关节冠状面上的关节角冲量大于正常人(P0.05),膝关节在矢状面上关节角冲量大于正常组(P0.05),下梯髋、膝关节冠状面、矢状面上的角冲量无统计学差异(P0.05)。KOA组VM/VL、BF/VL与膝关节在冠状面和矢状面上的角冲量的改变没有直接的相关性(P0.05)。结论:膝骨性关节炎患者在登梯活动时股直肌的收缩活动增加,股内外侧肌的协同收缩下降,主动肌与拮抗肌的共同收缩增加,膝骨性关节炎患者在面对登梯活动时下肢肌群选择性激活和高激活状态协调一致,促进关节稳定。虽然下肢神经肌肉的收缩模式和膝关节负荷之间没有直接的相关性,可能是对膝关节负荷产生影响的生物力学因素较多,神经肌肉的收缩模式只是部分影响因素,后续将增加其他生物力学因素进一步研究。     

Knee and Hip Joint Kinematics Predict Quadriceps and Hamstrings Neuromuscular Activation Patterns in Drop Jump Landings

PurposeThe purpose was to assess if variation in sagittal plane landing kinematics is associated with variation in neuromuscular activation patterns of the quadriceps-hamstrings muscle groups during drop vertical jumps (DVJ).MethodsFifty female athletes performed three DVJ. The relationship between peak knee and hip flexion angles and the amplitude of four EMG vectors was investigated with trajectory-level canonical correlation analyses over the entire time period of the landing phase. EMG vectors consisted of the {vastus medialis(VM),vastus lateralis(VL)}, {vastus medialis(VM),hamstring medialis(HM)}, {hamstring medialis(HM),hamstring lateralis(HL)} and the {vastus lateralis(VL),hamstring lateralis(HL)}. To estimate the contribution of each individual muscle, linear regressions were also conducted using one-dimensional statistical parametric mapping.ResultsThe peak knee flexion angle was significantly positively associated with the amplitudes of the {VM,HM} and {HM,HL} during the preparatory and initial contact phase and with the {VL,HL} vector during the peak loading phase (p<0.05). Small peak knee flexion angles were significantly associated with higher HM amplitudes during the preparatory and initial contact phase (p<0.001). The amplitudes of the {VM,VL} and {VL,HL} were significantly positively associated with the peak hip flexion angle during the peak loading phase (p<0.05). Small peak hip flexion angles were significantly associated with higher VL amplitudes during the peak loading phase (p = 0.001). Higher external knee abduction and flexion moments were found in participants landing with less flexed knee and hip joints (p<0.001).ConclusionThis study demonstrated clear associations between neuromuscular activation patterns and landing kinematics in the sagittal plane during specific parts of the landing. These findings have indicated that an erect landing pattern, characterized by less hip and knee flexion, was significantly associated with an increased medial and posterior neuromuscular activation (dominant hamstrings medialis activity) during the preparatory and initial contact phase and an increased lateral neuromuscular activation (dominant vastus lateralis activity) during the peak loading phase.     

Task-Dependent Intermuscular Motor Unit Synchronization between Medial and Lateral Vastii Muscles during Dynamic and Isometric Squats

Purpose

Motor unit activity is coordinated between many synergistic muscle pairs but the functional role of this coordination for the motor output is unclear. The purpose of this study was to investigate the short-term modality of coordinated motor unit activity–the synchronized discharge of individual motor units across muscles within time intervals of 5ms–for the Vastus Medialis (VM) and Lateralis (VL). Furthermore, we studied the task-dependency of intermuscular motor unit synchronization between VM and VL during static and dynamic squatting tasks to provide insight into its functional role.

Methods

Sixteen healthy male and female participants completed four tasks: Bipedal squats, single-leg squats, an isometric squat, and single-leg balance. Monopolar surface electromyography (EMG) was used to record motor unit activity of VM and VL. For each task, intermuscular motor unit synchronization was determined using a coherence analysis between the raw EMG signals of VM and VL and compared to a reference coherence calculated from two desynchronized EMG signals. The time shift between VM and VL EMG signals was estimated according to the slope of the coherence phase angle spectrum.

Results

For all tasks, except for singe-leg balance, coherence between 15–80Hz significantly exceeded the reference. The corresponding time shift between VM and VL was estimated as 4ms. Coherence between 30–60Hz was highest for the bipedal squat, followed by the single-leg squat and the isometric squat.

Conclusion

There is substantial short-term motor unit synchronization between VM and VL. Intermuscular motor unit synchronization is enhanced for contractions during dynamic activities, possibly to facilitate a more accurate control of the joint torque, and reduced during single-leg tasks that require balance control and thus, a more independent muscle function. It is proposed that the central nervous system scales the degree of intermuscular motor unit synchronization according to the requirements of the movement task at hand.     

Spatial fiber type distribution in normal human muscle Histochemical and tensiomyographical evaluation

The variability of fiber type distribution in nine limb muscles was examined with histochemical and tensiomyographical (TMG) methods in two groups of 15 men aged between 17 and 40 years. The aim of this study was to determine the extent to which the relative occurrence of different fiber types and subtypes varies within human limb muscles in function to depth and to predict fiber type proportions with a non-invasive TMG method.The distribution of different fiber types varied within the muscles, as a function of depth, with a predominance of type 2b fibers at the surface and type 1 fibers in deeper regions of the muscle. For all the analyzed muscles the contraction times measured at stimulus intensity 10% of supramaximal stimulus (10% MS) were significantly (p<0.05) shorter than the contraction times measured at 50% of supramaximal stimulus intensity (50% MS). The Pearson's correlation coefficient between percentage of type 1 muscle fibers measured at the surface of the muscle and contraction time at 10% MS, obtained by TMG was statistically significant (r=0.76,P<0.01). Also the Pearson's correlation coefficient between percentage of type 1 muscle fibers measured in the deep region of the muscle and contraction time at 50% MS obtained by TMG was also statistically significant (r=0.90,P<0.001).These findings suggest that the contraction time obtained by TMG may be useful for non-invasive examining of muscle fiber types spatial distribution in humans.     

The muscle contractile properties in female soccer players: inter-limb comparison using tensiomyography

Objective:The present study aimed to: i) determine the contractile properties of the major lower limb muscles in female soccer players using tensiomyography; ii) investigate inter-limb differences; and iii) compare inter-limb differences between different selections and playing positions.Methods:A total of 52 female soccer players (A team; U19 and U17) were recruited. The vastus lateralis (VL), vastus medialis (VM), rectus femoris (RF), biceps femoris (BF), gastrocnemius medialis (GM), lateralis (GL) and tibialis anterior (TA) of both lower limbs were evaluated.Results:When the entire sample was assessed regardless of selection or playing position, there were significant inter-limb differences in all measured muscles except BF. Compared to the non-dominant limb, the dominant limb had higher delay time in VL (p=0.008), while showing lower values in VM (p=0.023), GL (p=0.043) and GM (p=0.006). Contraction time was lower in the RF of the dominant limb (p=0.005) and VM (p=0.047), while showing higher values in VL (p=0.036) and TA (p<0.001) as compared to the non-dominant limb.Conclusion:Given the differences found between the limbs in the whole sample studied, it is necessary to examine both limbs to gather a more in-depth understanding of underlying mechanisms related to neuromuscular functions in female soccer players.Level of evidence:Prognostic study, Level II.     

The relationship between muscle deoxygenation and activation in different muscles of the quadriceps during cycle ramp exercise

The relationship between muscle deoxygenation and activation was examined in three different muscles of the quadriceps during cycling ramp exercise. Seven young male adults (24 ± 3 yr; mean ± SD) pedaled at 60 rpm to exhaustion, with a work rate (WR) increase of 20 W/min. Pulmonary oxygen uptake was measured breath-by-breath, while muscle deoxygenation (HHb) and activity were measured by time-resolved near-infrared spectroscopy (NIRS) and surface electromyography (EMG), respectively, at the vastus lateralis (VL), rectus femoris (RF), and vastus medialis (VM). Muscle deoxygenation was corrected for adipose tissue thickness and normalized to the amplitude of the HHb response, while EMG signals were integrated (iEMG) and normalized to the maximum iEMG determined from maximal voluntary contractions. Muscle deoxygenation and activation were then plotted as a percentage of maximal work rate (%WR(max)). The HHb response for all three muscle groups was fitted by a sigmoid function, which was determined as the best fitting model. The c/d parameter for the sigmoid fit (representing the %WR(max) at 50% of the total amplitude of the HHb response) was similar between VL (47 ± 12% WR(max)) and VM (43 ± 11% WR(max)), yet greater (P < 0.05) for RF (65 ± 13% WR(max)), demonstrating a "right shift" of the HHb response compared with VL and VM. The iEMG also showed that muscle activation of the RF muscle was lower (P < 0.05) compared with VL and VM throughout the majority of the ramp exercise, which may explain the different HHb response in RF. Therefore, these data suggest that the sigmoid function can be used to model the HHb response in different muscles of the quadriceps; however, simultaneous measures of muscle activation are also needed for the HHb response to be properly interpreted during cycle ramp exercise.     

Nature of the coupling between neural drive and force-generating capacity in the human quadriceps muscle

The force produced by a muscle depends on both the neural drive it receives and several biomechanical factors. When multiple muscles act on a single joint, the nature of the relationship between the neural drive and force-generating capacity of the synergistic muscles is largely unknown. This study aimed to determine the relationship between the ratio of neural drive and the ratio of muscle force-generating capacity between two synergist muscles (vastus lateralis (VL) and vastus medialis (VM)) in humans. Twenty-one participants performed isometric knee extensions at 20 and 50% of maximal voluntary contractions (MVC). Myoelectric activity (surface electromyography (EMG)) provided an index of neural drive. Physiological cross-sectional area (PCSA) was estimated from measurements of muscle volume (magnetic resonance imaging) and muscle fascicle length (three-dimensional ultrasound imaging) to represent the muscles'' force-generating capacities. Neither PCSA nor neural drive was balanced between VL and VM. There was a large (r = 0.68) and moderate (r = 0.43) correlation between the ratio of VL/VM EMG amplitude and the ratio of VL/VM PCSA at 20 and 50% of MVC, respectively. This study provides evidence that neural drive is biased by muscle force-generating capacity, the greater the force-generating capacity of VL compared with VM, the stronger bias of drive to the VL.     

Alternate muscle activity observed between knee extensor synergists during low-level sustained contractions.

To determine quantitatively the features of alternate muscle activity between knee extensor synergists during low-level prolonged contraction, a surface electromyogram (EMG) was recorded from the rectus femoris (RF), vastus lateralis (VL), and vastus medialis (VM) in 11 subjects during isometric knee extension exercise at 2.5% of maximal voluntary contraction (MVC) for 60 min (experiment 1). Furthermore, to examine the relation between alternate muscle activity and contraction levels, six of the subjects also performed sustained knee extension at 5.0, 7.5, and 10.0% of MVC (experiment 2). Alternate muscle activity among the three muscles was assessed by quantitative analysis on the basis of the rate of integrated EMG sequences. In experiment 1, the number of alternations was significantly higher between RF and either VL or VM than between VL and VM. Moreover, the frequency of alternate muscle activity increased with time. In experiment 2, alternating muscle activity was found during contractions at 2.5 and 5.0% of MVC, although not at 7.5 and 10.0% of MVC, and the number of alternations was higher at 2.5 than at 5.0% of MVC. Thus the findings of the present study demonstrated that alternate muscle activity in the quadriceps muscle 1) appears only between biarticular RF muscle and monoarticular vasti muscles (VL and VM), and its frequency of alternations progressively increases with time, and 2) emerges under sustained contraction with force production levels < or =5.0% of MVC.     

Effect of stimulation of cold receptors with menthol on EMG activity of quadriceps muscle during low load contraction

Purpose: Facilitatory and inhibitory responses of spinal motor neurons are influenced by somatosensory input from the skin. The purpose of this study, employing electromyography, was to examine the neuromuscular changes that occur with menthol applied to the skin over the quadriceps muscle.

Methods: Forty-two healthy volunteers performed isometric knee extensions at 35% maximum voluntary contraction (MVC) in three groups (Adult Placebo, Adult Menthol, Older Adult Menthol). Stimulation used was application of 5% menthol gel to the skin. Surface electromyography (sEMG) from the vastus lateralis (VL), vastus medialis (VM), and rectus femoris (RF) was recorded using miniature pair electrodes.

Results: Root mean square electromyography (rmsEMG) in VL and VM significantly increased with menthol stimulation both in Adult and Older Adult, but no significant difference was observed between Adult Menthol and Older Adult Menthol. There was a significant decrease in mean power frequency (MPF) in VM with menthol stimulation in Older Adult, but no significant changes were observed in Adult Menthol.

Conclusion: Neuromuscular modulation was observed with the application of menthol gel at low loads in the present study. These findings could lead to a new method of muscular training that targets the recruitment of fast type muscle safe for older adults.     

Relationship between neuromuscular fatigue and spasticity in chronic stroke patients: A pilot study

IntroductionThe aim of this study was to assess the effects of neuromuscular fatigue on stretch reflex-related torque and electromyographic activity of spastic knee extensor muscles in hemiplegic patients. The second aim was to characterize the time course of quadriceps muscle fatigue during repetitive concentric contractions.MethodsEighteen patients performed passive, isometric and concentric isokinetic evaluations before and after a fatigue protocol using an isokinetic dynamometer. Voluntary strength and spasticity were evaluated following the simultaneous recording of torque and electromyographic activity of rectus femoris (RF), vastus lateralis (VL) and biceps femoris (BF).ResultsIsometric knee extension torque and the root mean square (RMS) value of VL decreased in the fatigued state. During the fatigue protocol, the normalized peak torque decreased whereas the RMS of RF and BF increased between the first five and last five contractions. There was a linear decrease in the neuromuscular efficiency-repetitions relationships for RF and VL. The peak resistive torque and the normalized RMS of RF and VL during passive stretching movements were not modified by the fatigue protocol for any stretch velocity.DiscussionThis study showed that localized quadriceps muscle fatigue caused a decrease in voluntary strength which did not modify spasticity intensity. Changes in the distribution of muscle fiber type, with a greater number of slow fibers on the paretic side, may explain why the stretch reflex was not affected by fatigue.     

Males and females have different muscle activity patterns during gait after ACL injury and reconstruction

Kinematic and kinetic changes following anterior cruciate ligament (ACL) rupture and reconstruction (ACLR) have been fundamental to the understanding of mechanical disrupted load as it contributes to the development of posttraumatic osteoarthritis. These analyses overlook the potential contribution of muscle activity as it relates to the joint loading environment. Males and females classified as non-copers present with unique knee kinematics and kinetics after ACL injury. The purpose of this study was to perform sex-specific analyses in these individuals to explore muscle activity timing during gait after ACL rupture. Thirty-nine participants (12 females, 27 males) were enrolled. Muscle activity during gait was evaluated before and after pre-operative physical therapy, and six months after ACLR. Surface electromyography data were evaluated to determine timing (e.g., the time the muscle activity begins (‘On’) and ends (‘Off’)) for seven muscles: vastus lateralis and medialis (VL, VM), lateral and medial hamstrings (LH, MH), lateral and medial gastrocnemius (LG, MG), and soleus (SOL). General linear models with generalized estimating equations detected the effects of limb and time for muscle activity timing. Males presented with more limb asymmetries before and after pre-operative PT in the VL On (p < 0.001) and Off (p = 0.007), VM On and Off (p < 0.001), and MH off (p < 0.001), but all limb differences resolved by six months post ACLR. Changes in muscle activity in males were pervasive over time in both limbs. Females presented with no interlimb differences pre-operatively, and only involved limb VL off (p = 0.027) and VM off (p = 0.003) and the LH off in both limbs (p < 0.038) changed over time. Our data indicate that inter-limb differences in muscle activity across time points and changes in muscle activity timing over the course of physical therapy were sex specific. Males presented with more inter-limb differences in muscle activity across time points, and females presented with fewer asymmetries before and after pre-operative physical therapy. These data support that sex-specific adaptations should be taken into consideration when assessing biomechanical changes after ACLR.     

Non-uniform mechanical activity of quadriceps muscle during fatigue by repeated maximal voluntary contraction in humans

To determine the non-uniform surface mechanical activity of human quadriceps muscle during fatiguing activity, surface mechanomyogram (MMG), or muscle sound, and surface electromyogram (EMG) were recorded from the rectus femoris (RF), vastus lateralis (VL), and vastus medialis (VM) muscles of seven subjects during unilateral isometric knee extension exercise. Time- and frequency-domain analyses of MMG and of EMG fatigued by 50 repeated maximal voluntary contractions (MVC) for 3 s, with 3-s relaxation in between, were compared among the muscles. The mean MVC force fell to 49.5 (SEM 2.0)% at the end of the repeated MVC. Integrated EMG decreased in a similar manner in each muscle head, but a marked non-uniformity was found for the decline in integrated MMG (iMMG). The fall in iMMG was most prominent for RF, followed by VM and VL. Moreover, the median frequency of MMG and the relative decrease in that of EMG in RF were significantly greater (P < 0.05) than those recorded for VL and VM. These results would suggest a divergence of mechanical activity within the quadriceps muscle during fatiguing activity by repeated MVC. Accepted: 19 January 1999     

Effects of exercise training modality on skeletal muscle fatigue in men with coronary heart disease

Cardiopulmonary and skeletal muscle effects of combined aerobic and resistance training vs. aerobic training were studied in men with coronary heart disease. Sixteen men with coronary heart disease underwent a cardiopulmonary exercise testing and a quadriceps skeletal muscle fatigue assessment. Patients were divided into two groups and trained in a combined aerobic and resistance or aerobic training group during 7 weeks. Maximal voluntary contraction and isometric endurance time were measured with electromyographic signals recorded from vastus lateralis (VL), rectus femoris (RF) and vastus medialis (VM) during isometric endurance time. Exercise tolerance increased only in the combined group (p < 0.05). Maximal voluntary contraction and isometric endurance time did not change after training in either group but was performed at 5.8% higher force output for the combined group. After training, median frequency values were higher for the VL and VM (p < 0.001) in the aerobic group and also higher for the VL, RF (p < 0.001) and VM (p < 0.05) in the combined group. Combined aerobic and resistance training was more effective to improve exercise tolerance, decrease skeletal muscle fatigue and correct neuromuscular alterations in men with coronary heart disease.     

Force fluctuations are modulated by alternate muscle activity of knee extensor synergists during low-level sustained contraction.

The study examined the hypothesis that altered synergistic activation of the knee extensors leads to cyclic modulation of the force fluctuations. To test this hypothesis, the force fluctuations were investigated during sustained knee extension at 2.5% of maximal voluntary contraction force for 60 min in 11 men. Surface electromyograms (EMG) were recorded from the rectus femoris (RF), vastus lateralis (VL), and vastus medialis (VM) muscles. The SD of force and average EMG (AEMG) of each muscle were calculated for 30-s periods during alternate muscle activity. Power spectrum of force was calculated for the low- (< or =3 Hz), middle- (4-6 Hz), and high-frequency (8-12 Hz) components. Alternate muscle activity was observed between RF and the set of VL and VM muscles. The SD of force was not constant but variable due to the alternate muscle activity. The SD was significantly greater during high RF activity compared with high VL and VM activity (P < 0.05), and the correlation coefficient between the SD and AEMG was significantly greater in RF [0.736 (SD 0.095), P < 0.05] compared with VL and VM. Large changes were found in the high-frequency component. During high RF activity, the correlation coefficient between the SD and high-frequency component [0.832 (SD 0.087)] was significantly (P < 0.05) greater compared with other frequency components. It is suggested that modulations in knee extension force fluctuations are caused by the unique muscle activity in RF during the alternate muscle activity, which augments the high-frequency component of the fluctuations.