Voluntary Contraction Intensity Research Articles

Effect of knee joint angle on vastus medialis and vastus lateralis rigidity during isometric submaximal voluntary knee extensions

The use of shear wave elastography during voluntary contraction has enabled the non-invasive assessment of load sharing strategies between agonist muscles. However, the change in joint angle and voluntary contraction intensity can modify contribution between muscles. The aim of this study was to investigate the effect of knee joint angle on the local mechanical properties of the vastus medialis (VM) and the vastus lateralis (VL) during isometric submaximal voluntary contractions from shear wave elastography mapping. The VM and VL Young’s modulus at rest and during constant isometric submaximal voluntary contractions (i.e., 25%, 50% and 75% of maximal voluntary contraction [MVC]) were assessed for two knee angles (50° and 100° | knee fully extended = 0°) in twelve participants. No significant difference was found in the VM Young’s modulus among all torque levels and knee angles (p > 0.05). VL Young’s modulus was significantly higher at 25% MVC for a knee angle of 100° than at 75% MVC for the same knee angle and was greater at 25% MVC for a knee angle of 100° than for 50° (p < 0.05). In contrast to the VM, the contribution of the VL to the knee joint torque production during isometric voluntary contraction appears to depend on the muscle length and the relative knee extension torque level.

Firing rate trajectories of human occipitofrontalis motor units in response to triangular voluntary contraction intensity.

During voluntary contractions, limb muscle motor unit (MU) firing rates accelerate over a small force range and saturate in response to increasing contraction intensity. In comparison, facial muscles are cranially innervated, and some function without crossing joints. Therefore, the MU firing rate behaviour and characteristics of saturation were explored in a facial muscle that moves skin and facia during voluntary contractions. We evaluated the firing rate trajectory in response to triangular voluntary contraction ramps in the occipitofrontalis muscle of 11 adult participants. Intramuscular electromyography of the frontalis aspect was used to record single MU trains followed up to maximal voluntary contraction intensities. Firing rates were measured from each MU sample, with the firing rate trajectory fit as both exponential (i.e., saturation) and linear models that were compared statistically. The rate coding behaviour of frontalis MUs was broad, as the peak firing rate (mean 76Hz) was ninefold greater than the firing rate at recruitment threshold (mean 8Hz). Across 20 MU samples, only 40% (8 MU samples) were determined to have a firing rate trajectory that saturated and had slow acceleration in response to increasing voluntary drive until maximum. The exponential curve of the firing rate trajectory had ~ tenfold lower acceleration as compared to prior reports in limb muscles. These results across all MU samples indicated that voluntary control of the frontalis muscle requires relatively slower accelerating or linear MU firing rate trajectories, suggesting that movements of facial muscles may be directly representative of extrinsic synaptic inputs.

Motor unit firing rates during constant isometric contraction: establishing and comparing an age-related pattern among muscles.

Motor unit (MU) firing rates (FRs) are lower in aged adults, compared with young, at relative voluntary contraction intensities. However, from a variety of independent studies of disparate muscles, the age-related degree of difference in FR among muscles is unclear. Using a standardized statistical approach with data derived from primary studies, we quantified differences in FRs across several muscles between younger and older adults. The data set included 12 different muscles in young (18-35 yr) and older adults (62-93 yr) from 18 published and one unpublished study. Experiments recorded single MU activity from intramuscular electromyography during constant isometric contraction at different (step-like) voluntary intensities. For each muscle, FR ranges and FR variance explained by voluntary contraction intensity were determined using bootstrapping. Dissimilarity of FR variance among muscles was calculated by Euclidean distances. There were threefold differences in the absolute frequency of FR ranges across muscles in the young (soleus 8-16 and superior trapezius 20-49 Hz), but in the old, FR ranges were more similar and lower for nine out of 12 muscles. In contrast, the explained FR variance from voluntary contraction intensity in the older group had 1.6-fold greater dissimilarity among muscles than the young (P < 0.001), with FR variance differences being muscle dependent. Therefore, differences between muscle FR ranges were not explained by how FRs scale to changes in voluntary contraction intensity within each muscle. Instead, FRs were muscle dependent but were more dissimilar among muscles in the older group in their responsiveness to voluntary contraction intensity.NEW & NOTEWORTHY The mean frequency of motor unit firing rates were compared systematically among several muscles and between young and older adults from new and published data sets. Firing rates among muscles were lower and more similar during voluntary isometric contraction in older than younger adults. Firing rate responses from voluntary contraction intensity were muscle dependent and more dissimilar among muscles in the older than young adults.

Contractile function and motor unit firing rates of the human hamstrings.

We studied motor unit firing rates (MUFRs) at various voluntary contraction intensities in the hamstrings, one of the only major lower limb muscles to have MUFRs affected by muscle length changes. Within the hamstrings muscle-specific differences have greater impact on MUFRs than length changes, with the biceps femoris having reduced neural drive compared with the semimembranosus-semimembranosus. Comparing our results to other lower limb muscles, flexors have inherently higher firing rate compared with extensors.

Golgi tendon organ reflex inhibition following manually applied acute static stretching

Golgi tendon organ disinhibition may contribute to exercise-associated muscle cramp (henceforth referred to as “cramps”) genesis. Static stretching pre-exercise is prescribed to prevent cramps based on the assumption golgi tendon organ inhibition remains elevated post-stretching. We determined whether stretching increased gastrocnemius golgi tendon organ inhibition and, if so, the time course of this inhibition post-stretching. Twelve participants’ dominant limb medial gastrocnemius inhibition was measured before, and at 1, 5, 10, 15 and 30 min after investigators applied three, 1-min duration stretches. Participants maintained voluntary contraction intensities of 5% of their maximum while the Achilles tendon was stimulated transcutaneously 50 times. Five-hundred millisecond epochs of raw electromyographic activity were band-pass filtered, full-wave rectified and averaged. An algorithm identified inhibitory points and calculated the area, maximum and duration of inhibition. Area of inhibition (F1,14 = 1.5, P = 0.25), maximum inhibition (F1,14 = 0.2, P = 0.72) and duration of inhibition (F1,14 = 1.5, P = 0.24) were unaffected by static stretching over the 30-min post-stretching period. If pre-stretching does prevent fatigue-induced cramping, the mechanism is unlikely to involve the autoinhibition produced by the golgi tendon organ reflex. Further empirical research is needed to validate the proposed link between static stretching and cramping and then to investigate alternative mechanisms.

The Effects Of Voluntary Contraction Intensity On The Electromechanical Delay

The Effects Of Voluntary Contraction Intensity On The Electromechanical Delay

Test–retest reliability of the soleus H-reflex is affected by joint positions and muscle force levels

The purpose of this study was to determine the test–retest reliability of the soleus (SOL) H-reflex during rest and isometric contractions at 10%, 30%, and 50% of the maximal voluntary force (MVC) at the ankle joint angles of neutral (0°), plantarflexion (20°), and dorsiflexion (−20°) respectively, in a sitting position. Ten healthy participants, with mean age of 24.9 ± 5.0 (SD) years, height 168.3 ± 8.8 cm, weight 62.7 ± 12.3 kg, were tested for the SOL H-reflex (H max) on two separate occasions within 7 days. The intraclass correlation coefficient (ICC) for the test–retest of the SOL H-reflex during rest was found to be high at ankle joint angle of neutral (ICC = 0.92) and plantarflexion (0.96), and moderate at dorsiflexion (0.75). Inconsistent ICC values (range from 0.62 to 0.97) were found during the submaximal voluntary contractions at the three ankle joint positions. High ICCs were also found in H max/M max ratio at neutral (0.86), plantarflexion (0.96), and dorsiflexion (0.84) positions. It was concluded that the test–retest reliability of the SOL H-reflex was affected by the intensity of voluntary contraction and ankle joint position. The H-reflex demonstrated a higher reliability at the neutral and plantarflexion positions than that at the dorsiflexion position during rest, and a higher reliability at 10% MVC than that at 30% and 50% MVC.

Voluntary activation in the triceps brachii at short and long muscle lengths

The aim was of this study was to determine whether voluntary activation calculated using the interpolated twitch technique (ITT) would be underestimated by muscle length-induced changes in the twitch amplitude evoked at rest after maximum voluntary contraction (MVC) in the elbow extensors. In 12 healthy men, calculated voluntary activations were compared at short (20 degrees of elbow flexion) and long muscle lengths (120 degrees ) using the actual post-MVC doublet, and the predicted post-MVC doublet estimated from linear or nonlinear extrapolations. Actual post-MVC doublet amplitudes were smaller at 20 degrees versus 120 degrees . At 20 degrees , the predicted post-MVC doublet obtained from nonlinear extrapolation was larger, and voluntary activation values improved by 5-33% at the submaximal voluntary contraction intensities (< or =80% of MVC). When voluntary drive is compromised, this method of extrapolation is useful to account for mechanical limitations that blunt the actual post-MVC doublet, which otherwise leads to underestimation of voluntary activation.

The effect of arm abduction angle and contraction intensity on perceived exertion

The objective of the present investigation was to examine the effects of voluntary contraction intensity and arm elevation angle on ratings of perceived exertion in healthy young adults. Twelve healthy young adults participated in six separate experimental sessions, during which maximal and sub-maximal arm abduction contractions were performed at one of the following randomly assigned arm angles: 15 degrees , 30 degrees , 45 degrees , 60 degrees , 75 degrees , and 90 degrees arm angles (degrees). The sub-maximal arm abduction contractions (10 s) were performed to target intensities ranging from 10 to 90% of the maximal voluntary contraction (MVC), in randomly ordered 10% increments. Perceived exertion was sampled with a modified Borg category-ratio scale immediately, following each sub-maximal contraction. Voluntary arm abduction torque was significantly (p < 0.05) lower at 45 degrees , 60 degrees , 75 degrees and 90 degrees , as compared to 15 degrees . The ratings of perceived exertion increased significantly (p < 0.05) across the range of contraction intensities, with no significant differences between arm angles. The perceived exertion response, as a function of contraction intensity, was best described as linear across the 30 degrees -90 degrees , while a power function described the response at 15 degrees . The major findings demonstrated that arm position plays a minimal role in mediating the growth in the perceived exertion response, as a function of contraction intensity.

Effect of voluntary contraction intensity on the H-reflex and V-wave responses

This study examined the evolution of H-reflex and V-wave responses of soleus muscle during maximal voluntary plantar-flexor contraction. We also investigated the relationship between the V response and force level and between V-wave during maximal voluntary contraction (MVC) and the maximal H reflex at rest. The H-reflex and the V-wave responses are measures of motoneuron excitability and also reflect the magnitude of presynaptic inhibition on Ia afferents and the magnitude of descending motor drive. Both may be influenced by postsynaptic inhibition. Twenty male subjects participated in the study and were assigned to one of two groups. The maximal M wave (M max) was evoked at rest in the 20 subjects, who then performed 10 maximal voluntary contraction. During MCV performance, a stimulus was delivered at supra-maximal intensity, which allowed us to record the superimposed M wave (M sup) and V wave of the soleus muscle. These parameters were also recorded during sub-maximal contractions (20, 40, 60, 80% of one MVC) in 10 subjects. The maximal H reflex (H max), was evoked at rest in the other 10 subjects. These subjects then performed 10 MVC and the H sup (superimposed H, evoked by means of stimulus at H max intensity) was recorded. The results show that the amplitude of maximal M wave increased during MVC (gain 44.52 ± 10.71%). No significant difference between H max/M max at rest and the H sup/M sup ratios during MVC was observed, while an effect of force level on the V/M sup ratio was found. V/M sup and H max/M max were linearly correlated ( r 2 = 0.81), but V/M sup was significantly lower ( P < 0.01) than H max/M max. In conclusion, the present study shows that maximal voluntary contractions potentiate some reflex responses. The V wave, which reflects motoneuron excitability presynaptic inhibition of Ia afferents and the magnitude of descending central motor drive to spinal motoneurons, may be a relatively simple method to analyse the modulation adaptive neural alterations at spinal and supraspinal level during voluntary contractions.

The Effects of Contraction intensity on Quadriceps Femoris Muscle Recruitment

1224 Introduction: Previous work has suggested that differential recruitment of the separate portions of the superficial quadriceps femoris muscle varies as a function of voluntary contraction intensity, and tends to converge at near-maximal intensities (Alkner et al, Med Sci Sports Exerc, 2000; Pincivero and Coelho, Muscle Nerve, 2000). Purpose: To examine recruitment behavior of the superficial quadriceps femoris muscle across different contraction intensities. Procedures: Thirty-seven physically active young adults (12 men, 25 women) performed five isometric maximal voluntary contractions (MVC) at 90 deg knee flexion on a Biodex isokinetic dynamometer. The mean peak torque was calculated from the three highest values. Subjects then performed five s isometric knee extension contractions from 10 to 90% MVC, in 10% increments, in random order. Muscle recruitment was evaluated via electromyography (EMG) from the vastus lateralis (VL), vastus medialis (VM), and rectus femoris (RF) muscles. EMG signals were collected at 2000 Hz, band pass filtered, full-wave rectified, integrated over the middle three seconds of each isometric contraction, and normalized to the MVC. Descriptive statistics and repeated measures analysis of variance (ANOVA) of muscle and intensity was conducted. Statistical significance was determined at p<0.05. Results: The results demonstrated significant differences between the three muscles at 10% (F2,72 = 5.47, p = 0.006), 20% (F2,72 = 3.32, p = 0.041), and 30% (F2,72 = 6.89, p = 0.002) MVC intensities. Pair-wise comparisons demonstrated significant differences between the VM (5.10 ± 2.06%) and VL (6.88 ± 1.86%), and between the VM and RF (7.24 ± 1.75%) at 10% MVC. At 20% MVC, significant differences were found between VM (12.41 ± 1.80%) and VL (15.62 ± 1.64%). Significant differences were found between VM (20.83 ± 1.77%) and VL (24.90 ± 1.24%), and VL and RF (17.71 ± 1.40%) at the 30% intensity. CONCLUSIONS: The major findings demonstrate that the recruitment behavior of the different portions of the superficial quadriceps femoris muscle, are similar across moderate to near-maximal voluntary contraction intensities.

Immediate exercise hyperemia in humans is contraction intensity dependent: evidence for rapid vasodilation.

We tested the hypothesis that rapid vasodilation proportional to contraction intensity contributes to the immediate (first cardiac cycle after initial contraction) exercise hyperemia. Ten healthy subjects performed single 1-s isometric forearm contractions at 5, 10, 15, 20, 30, 50, and 70% maximal voluntary contraction intensity (MVC) in arm above heart (AH) and below heart (BH) positions. Forearm blood flow (FBF; brachial artery mean blood velocity, Doppler ultrasound), mean arterial pressure (arterial tonometry), and heart rate (electrocardiogram) were measured beat by beat. Venous emptying (measured with a forearm strain gauge) was already maximized at 5% MVC, indicating that increases in contraction intensity did not further empty the forearm veins. Immediate increases in FBF were linearly proportional to contraction intensity from 5 to 70% MVC in AH (slope = 4.4 +/- 0.5%DeltaFBF/%MVC). In BH, the immediate increase in FBF demonstrated a curvilinear relationship with increasing contraction intensity and was greater than AH at 15, 20, 30, and 50% MVC (P < 0.05). Peak changes in FBF were greater in BH vs. AH from 10 to 50% MVC, even when venous refilling was complete (P < 0.05). These data support the existence of a rapid-acting vasodilatory mechanism(s) at the onset of human forearm exercise.

随意収縮レベルによって調節されるヒト母指筋の粘弾性

随意収縮レベルによって調節されるヒト母指筋の粘弾性