Peak triceps surae muscle activity is not specific to knee flexion angles during MVIC

There is limited research on peak activity of the separate triceps surae muscles in select knee flexion (KF) positions during a maximum voluntary isometric contraction (MVIC) used to normalize EMG signals. The aim of this study was to determine how frequent peak activity occurred during an MVIC for soleus (SOL), gastrocnemius medialis (GM), and gastrocnemius lateralis (GL) in select KF positions, and if these peaks were recorded in similar KF positions. Forty-eight healthy individuals performed unilateral plantar-flexion MVIC in standing with 0°KF and 45°KF, and in sitting with 90°KF. Surface EMG of SOL, GM, and GL were collected and processed in 250 ms epochs to determine peak root-mean-square amplitude. Peak activity was most frequently captured in standing and rarely in sitting, with no position selective to SOL, GM or GL activity. Peak GM and GL activity was more frequent in 0°KF than 45°KF, and more often in similar KF positions than not. Peak SOL activity was just as likely in 45°KF as 0°KF, and more in positions similar to GM, but not GL. The EMG amplitudes were at least 20% greater in positions that captured peak activity over those that did not. The overall findings support performing an MVIC in more than one KF position to normalize triceps surae EMG. It is emphasized that no KF position is selective to SOL, GM, or GL alone.     

Transient and steady-state cardiopulmonary responses to combined rhythmic and isometric exercise

The transient and steady-state cardiopulmonary responses to combined rhythmic (R) and isometric (I) exercise were examined in nine subjects. Isometric exercise at 30% maximal voluntary contraction (MVC) was started 1.5 min prior to either a 50% or 75% maximal oxygen uptake (VO2max) cycle ride and continued for 1.5 min into the 10-min R. Systolic (Pas) and diastolic (P(ad)) blood pressure, heart rate (fc), inspired ventilation volumes (VI), and oxygen uptake (VO2) were recorded every 30 s throughout each experiment. Responses to I effort alone were recorded for comparison with experiments in which the combined exercises were performed during the first 1.5 min when R had not yet begun. Pas responses in the first 1.5 min of I (no R) showed the typical rapid linear increase. Addition of the R effort further increased Pas to levels which remained nearly constant (steady state) throughout R. R alone produced a slower Pas increase to approximately the same steady-state levels as those of the combined R and I exercise. For P(ad), the linear increase which occurred during the first 1.5 min of I was attenuated with the superimposition of R. Following cessation of I, P(ad) fell rapidly during continued R to levels not different from experiments with R alone. The fc during I alone increased slightly. As I continued, the onset of the R induced a further rapid increase in fc to levels not different from R alone. The VI showed a similar response to fc. VO2 during I alone did not change significantly.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of isokinetic training of the knee extensors on isometric strength and peak power output during cycling

Isokinetic training of right and left quadriceps femoris was undertaken three times per week for 16 weeks. One group of subjects (n = 13) trained at an angular velocity of 4.19 rad.s-1 and a second group (n = 10) at 1.05 rad.s-1. A control group (n = 10) performed no training. Maximal voluntary contraction (MVC) of the quadriceps, and peak pedal velocity nu p,peak) and peak power output (Wpeak) during all-out cycling (against loads equivalent to 9, 10, 11, 12, 13 and 14% MVC) were assessed before and after training. The two training groups did not differ significantly from each other in their training response to any of the performance variables (P > 0.05). No significant difference in MVC was observed for any group after the 16-week period (P = 0.167). The post-training increases in average Wpeak (7%) and nu p,peak (6%) during the cycle tests were each significantly different from the control group response (P = 0.018 and P = 0.008, respectively). It is concluded that 16 weeks of isokinetic strength training of the knee extensors is able to significantly improve nu p, peak and Wpeak during spring cycling, an activity which demands considerable involvement of the trained muscle group but with its own distinct pattern of coordination.     

Electromyography of knuckle-walking: results of four experiments on the forearm of Pan gorilla

Preliminary results of electromyographic (EMG) studies on the forearm of a gorilla provisionally support the hypothesis that special closepacked positioning mechanisms may characterize the wrist and metacarpophalangeal joints II–V in extant knuckle-walkers (chimpanzees and gorillas). We recommend that once the bony features, related to these close-packed positions are clearly identified, they may be employed strategically to discern evidence of a knuckle-walking heritage in the hands of extant hominoids, including man, and to trace the history of knuckle-walking in available fossils. This report contains results of the first successful employment of indwelling fine-wire electrode techniques to elucidate problems on the functional and evolutionary biology of great apes.     

Dynamics of wrist and forearm rotations

Human movement generally involves multiple degrees of freedom (DOF) coordinated in a graceful and seemingly effortless manner even though the underlying dynamics are generally complex. Understanding these dynamics is important because it exposes the challenges that the neuromuscular system faces in controlling movement. Despite the importance of wrist and forearm rotations in everyday life, the dynamics of movements involving wrist and forearm rotations are currently unknown.     

Zero crossing rate of electromyograms during occupational work and endurance tests as predictors for work related myalgia in the shoulder/neck region

The relationship between electromyographic signs of fatigue (ESF) during work and occupation-related myalgia in the shoulder/neck region was investigated in a longitudinal study. Forty-three healthy female assembly workers were studied over 2 years. Measurements were performed at the start of the study with follow-up measurements after 1 and 2 years. The ESF were estimated as the zero crossing rate of electromyograms (EMG) detected during short test contractions performed during short breaks in normal work. As a complement, an endurance test using EMG records was performed and analysed with the zero crossing technique. The occurrence of shoulder/neck disorders was assessed by a clinical investigation and a questionnaire. No significant relationship between ESF during work in year 0 and deterioration of the disorder was seen. On the other hand, the absolute zero crossing rate and the time constant of the zero crossing decline from the endurance test showed a significant relationship with deterioration of the disorder. The ESF during work year 2, showed a significant relationship with disorder year 2, while the endurance test parameters year 2 did not. It was concluded that ESF during work was not a predictor of muscle injury, whereas it could be useful as a diagnostic tool.     

Normalization of electromyogram in the neck-shoulder region

Linear and curvilinear electromyogram (EMG) normalization methods were compared among ten healthy men during a simulated work cycle demanding attention and static holding of the arm (Solitaire test). Maximal voluntary contractions (MVC) and gradually increasing contractions up to 70% of MVC were used for normalization in different arm postures. The test contractions studied included inward and outward rotations, abduction, shoulder elevation, and flexion in different arm positions. The shoulder load moment was calculated for the flexion tests using a simple two-dimensional model. The effect of arm posture on the EMG versus shoulder load moment relationship was studied on the following muscles: supraspinatus, infraspinatus, trapezius (three parts), deltoid (two parts) and pectoralis major. All muscles participated in the MVC tests performed, and its was not possible to suggest a single recommended test for each muscle. Differences in normalized EMG median values ranging up to 30% of MVC were found between linear and curvilinear normalization methods. Short-term repeatability of normalization based on a contraction with gradually increasing force was good. Arm posture affected the relationships between shoulder load moment and EMG activity of all muscles studied. Arm posture did not, however, have a significant effect on the estimated amplitude probability distribution functions during the simulated work task. Therefore, at least for the tasks studied, the principle of normalizing in the middle position of the range of movement was deemed acceptable.     

Velocity-dependent muscle strategy during plantarflexion in humans

This work examines the relative contribution of the triceps surae heads and the tibialis anterior (TA) to tension development with reference to voluntary plantarflexion at various velocities and at two articular positions of the knee joint (extended and flexed at 90 °). Subjects were instructed to perform plantarflexion at various submaximal and maximal velocities with no intention of stopping the movement. Voluntary electromyographic (EMG) activity was recorded and the amplitude, duration and integral were analysed. Integrated EMG (IEMG) was normalized with respect to duration. The maximal M wave and the Hoffmann (H) reflex elicited by electrical stimulation of the tibial nerve were recorded in the triceps surae to estimate the effects in gastrocnemii (G) length and motoneuron excitability differences, respectively, in the two knee positions. The results indicate that: (a) although the largest EMG activity was recorded in the extended limb, the greatest maximal velocities were performed in the flexed knee position; (b) with increasing velocity of movement, all triceps surae muscles showed enhanced IEMG activities; (c) at a low velocity of movement the soleus (So1)/G IEMG ratio was larger in the flexed compared to the extended knee; and (d) with increasing velocity, co-activation of agonist and antagonist muscles appeared. It is concluded that the larger maximal velocity of movement observed in the flexed compared to the extended knee was not primarily related to the neural command of the different triceps surae components, but rather to their mechanical properties. Furthermore, co-activation of agonist and antagonist muscles may contribute to the performance of the contractile strategy during rapid movements.     

Using voluntary motor commands to inhibit involuntary arm movements

A hallmark of voluntary motor control is the ability to stop an ongoing movement. Is voluntary motor inhibition a general neural mechanism that can be focused on any movement, including involuntary movements, or is it mere termination of a positive voluntary motor command? The involuntary arm lift, or ‘floating arm trick’, is a distinctive long-lasting reflex of the deltoid muscle. We investigated how a voluntary motor network inhibits this form of involuntary motor control. Transcranial magnetic stimulation of the motor cortex during the floating arm trick produced a silent period in the reflexively contracting deltoid muscle, followed by a rebound of muscle activity. This pattern suggests a persistent generator of involuntary motor commands. Instructions to bring the arm down voluntarily reduced activity of deltoid muscle. When this voluntary effort was withdrawn, the involuntary arm lift resumed. Further, voluntary motor inhibition produced a strange illusion of physical resistance to bringing the arm down, as if ongoing involuntarily generated commands were located in a ‘sensory blind-spot’, inaccessible to conscious perception. Our results suggest that voluntary motor inhibition may be a specific neural function, distinct from absence of positive voluntary motor commands.     

Muscle spindle feedback differs between the soleus and gastrocnemius in humans

The Hoffmann (H) reflex and motor (M) response were studied in soleus and gastrocnemius during voluntary contraction in eight male volunteers. Aims: To determine if the strength of spindle input to the muscles is the same. To assess if the M response size changes during contraction. Results: The size of the maximum M response (M max) changed during contraction in each subject. Hence, all H reflex measurements were normalized to the M max at each level of contraction for each subject. The largest H/M max was bigger in soleus than gastrocnemius at every contraction level. The overall largest H/M max for soleus (97%) and gastrocnemius (55%) were achieved at 40 and 100% maximum voluntary contraction (MVC), respectively. Conclusion: Soleus receives greater spindle feedback than the gastrocnemius both at rest and during voluntary contraction.     

The effect of local skin cooling before a sustained,submaximal isometric contraction on fatigue and isometric quadriceps femoris performance: A randomized controlled trial

The central- and peripheral mechanisms by which heat strain limits physical performance are not fully elucidated. Nevertheless, pre-cooling is often used in an attempt to improve subsequent performance. This study compared the effects of pre-cooling vs. a pre-thermoneutral application on central- and peripheral fatigue during 60% of isometric maximum voluntary contraction (MVC) of the right quadriceps femoris muscle. Furthermore, the effects between a pre-cooling and a pre-thermoneutral application on isometric MVC of the right quadriceps femoris muscle and subjective ratings of perceived exertion (RPE) were investigated. In this randomized controlled trial, 18 healthy adults voluntarily participated. The participants received either a cold (experimental) application (+8 °C) or a thermoneutral (control) application (+32 °C) for 20 min on their right thigh (one cuff). After the application, central (fractal dimension – FD) and peripheral (muscle fiber conduction velocity – CV) fatigue was estimated using sEMG parameters during 60% of isometric MVC. Surface EMG signals were detected from the vastus medialis and lateralis using bidimensional arrays. Immediately after the submaximal contraction, isometric MVC and RPE were assessed. Participants receiving the cold application were able to maintain a 60% isometric MVC significantly longer when compared to the thermoneutral group (mean time: 78 vs. 46 s; p=0.04). The thermoneutral application had no significant impact on central fatigue (p>0.05) compared to the cold application (p=0.03). However, signs of peripheral fatigue were significantly higher in the cold group compared to the thermoneutral group (p=0.008). Pre-cooling had no effect on isometric MVC of the right quadriceps muscle and ratings of perceived exertion. Pre-cooling attenuated central fatigue and led to significantly longer submaximal contraction times compared to the pre-thermoneutral application. These findings support the use of pre-cooling procedures prior to submaximal exercises of the quadriceps muscle compared to pre-thermoneutral applications.     

Redundancy or heterogeneity in the electric activity of the biceps brachii muscle? Added value of PCA-processed multi-channel EMG muscle activation estimates in a parallel-fibered muscle

Conventional bipolar EMG provides imprecise muscle activation estimates due to possibly heterogeneous activity within muscles and due to improper alignment of the electrodes with the muscle fibers. Principal component analysis (PCA), applied on multi-channel monopolar EMG yielded substantial improvements in muscle activation estimates in pennate muscles. We investigated the degree of heterogeneity in muscle activity and the contribution of PCA to muscle activation estimates in biceps brachii (BB), which has a relatively simply parallel-fibered architecture. EMG-based muscle activation estimates were assessed by comparison to elbow flexion forces in isometric, two-state isotonic contractions in eleven healthy male subjects. Monopolar EMG was collected over the entire surface of the BB with about 63 electrodes. Estimation quality of different combinations of EMG channels showed that heterogeneous activation was found mainly in medio-lateral direction, whereas adding channels in the longitudinal direction added largely redundant information. Multi-channel bipolar EMG amplitude improved muscle activation estimates by 5–14% as compared to a single bipolar. PCA-processed monopolar EMG amplitude yielded a further improvement of (12–22%). Thus multi-channel EMG, processed with PCA, substantially improves the quality of muscle activation estimates compared conventional bipolar EMG in BB.