Faster intrinsic rate of torque development in elbow flexors than knee extensors: Effect of muscle architecture?

We studied the effect of pennate vs. fusiform muscle architecture on the rate of torque development (RTD) by examining the predominately fusiform elbow flexors (EF) and highly-pennate knee extensors (KE). Seventeen male volunteers (28.4 ± 6.2 years) performed explosive isometric EF and KE contractions (MVCs). Biceps brachii and vastus lateralis fascicle angles were measured to confirm their architecture, and both the rate of voluntary muscle activation (root-mean-square EMG in the 50 ms before contraction onset; EMG_-50) and electromechanical delay (EMD; depicting muscle-tendon series elasticity) were assessed as control variables to account for their influence on RTD. MVC torque, early (RTD₅₀) and late (RTD₂₀₀) RTDs were calculated and expressed as absolute and normalized values. Absolute MVC torque (+412%), RTD₅₀ (+215%), and RTD₂₀₀ (+427%) were significantly (p < 0.001) higher in KE than EF. However, EF RTD₅₀ was faster (+178%) than KE after normalization (p = 0.02). EMG_-50 and EMD did not differ between muscle groups. The results suggest that the faster absolute RTD in KE is largely associated with its higher maximal torque capacity, however in the absence of differences in rates of muscle activation, fiber type, and EMD the fusiform architecture of EF may be considered a factor allowing its faster early RTD relative to strength capacity.     

Contraction type influences the human ability to use the available torque capacity of skeletal muscle during explosive efforts

The influence of contraction type on the human ability to use the torque capacity of skeletal muscle during explosive efforts has not been documented. Fourteen male participants completed explosive voluntary contractions of the knee extensors in four separate conditions: concentric (CON) and eccentric (ECC); and isometric at two knee angles (101°, ISO101 and 155°, ISO155). In each condition, torque was measured at 25 ms intervals up to 150 ms from torque onset, and then normalized to the maximum voluntary torque (MVT) specific to that joint angle and angular velocity. Explosive voluntary torque after 50 ms in each condition was also expressed as a percentage of torque generated after 50 ms during a supramaximal 300 Hz electrically evoked octet in the same condition. Explosive voluntary torque normalized to MVT was more than 60 per cent larger in CON than any other condition after the initial 25 ms. The percentage of evoked torque expressed after 50 ms of the explosive voluntary contractions was also greatest in CON (ANOVA; p < 0.001), suggesting higher concentric volitional activation. This was confirmed by greater agonist electromyography normalized to M(max) (recorded during the explosive voluntary contractions) in CON. These results provide novel evidence that the ability to use the muscle's torque capacity explosively is influenced by contraction type, with concentric contractions being more conducive to explosive performance due to a more effective neural strategy.     

Early vs. late rate of torque development: Relation with maximal strength and influencing factors

We re-examined the relationship between rate of torque development (RTD) and maximal voluntary contractions (MVC) torque, and investigated some possible neuromuscular determinants of early (≤100 ms) and late (≥200 ms) RTD. Seventeen healthy men performed maximal explosive isometric knee extensions at five joint angles, from which MVC torque, RTD at different time intervals (50–250 ms), and early quadriceps EMG activity (EMG₅₀) were evaluated. Quadriceps muscle thickness (MT) was quantified by longitudinal ultrasonography. The relationship between MVC torque, EMG₅₀ and MT against RTD was assessed with Pearson’s and repeated measures correlation coefficients. Moderate-to-strong correlation coefficients were observed between MVC torque and RTD (r = 0.50–0.88, p < 0.001), with stronger relationships for late RTD than for early RTD. Weak-to-strong correlation coefficients were observed amongst RTD and EMG₅₀ (r = 0.37–0.83, p < 0.001), with stronger relationships for early RTD than for late RTD. Only late RTD was significantly correlated with MT, though only moderately (r = 0.50–0.52, p < 0.05). These findings suggest that early and late knee extension RTD are potentially governed by different neuromuscular factors. Neuromuscular activation seems to have a greater influence on early RTD than on late RTD, and vice versa for muscle mass.     

Physical attractiveness of face and body as indicators of physical fitness in men

Human physical attractiveness appears to be an important signal of mate value that is utilized in mate choice We argue that performance-related physical fitness (PF) was an important facet of ancestral male mate value and, therefore, that a positive relationship exists between PF and physical attractiveness as well as mating success. We investigated these relationships in a sample of 80 young men. In line with our predictions, we found that (i) a composite measure of PF correlated substantially with body attractiveness (r=.43, after controlling for confounds) but not with facial attractiveness; (ii) PF was positively related to various measures of self-reported mating success (r_S ≈ .22); (iii) the relationship between PF and self-reported mating success was partly mediated by body attractiveness. We conclude it is a key function of men's body attractiveness to signal their PF and that men's faces and bodies signal different facets of mate value.     

Validity of trunk extensor and flexor torque measurements using isokinetic dynamometry

This study aimed to evaluate the validity and test–retest reliability of trunk muscle strength testing performed with a latest-generation isokinetic dynamometer. Eccentric, isometric, and concentric peak torque of the trunk flexor and extensor muscles was measured in 15 healthy subjects. Muscle cross sectional area (CSA) and surface electromyographic (EMG) activity were respectively correlated to peak torque and submaximal isometric torque for erector spinae and rectus abdominis muscles. Reliability of peak torque measurements was determined during test and retest sessions. Significant correlations were consistently observed between muscle CSA and peak torque for all contraction types (r = 0.74−0.85; P < 0.001) and between EMG activity and submaximal isometric torque (r ⩾ 0.99; P < 0.05), for both extensor and flexor muscles. Intraclass correlation coefficients were comprised between 0.87 and 0.95, and standard errors of measurement were lower than 9% for all contraction modes. The mean difference in peak torque between test and retest ranged from −3.7% to 3.7% with no significant mean directional bias. Overall, our findings establish the validity of torque measurements using the tested trunk module. Also considering the excellent test–retest reliability of peak torque measurements, we conclude that this latest-generation isokinetic dynamometer could be used with confidence to evaluate trunk muscle function for clinical or athletic purposes.     

Muscle activation and the isokinetic torque-velocity relationship of the human triceps surae

The influence of muscle activation and the time allowed for torque generation on the angle-specific torque-velocity relationship of the triceps surae was studied during plantar flexion using supramaximal electrical stimulation and a release technique on six male subjects [mean (SD) age 25 (4) years]. Torque-velocity data were obtained under different levels of constant muscle activation by varying the stimulus frequency and the time allowed for isometric torque generation prior to release and isokinetic shortening. To eliminate the effects of the frequency response on absolute torque the isokinetic data were normalized to the maximum isometric torque values at 0.44 rad. There were no significant differences in the normalized torques generated at any angular velocity using stimulus frequencies of 20, 50 or 80 Hz. When the muscle was stimulated at 50 Hz the torques obtained after a 400 ms and 1 s pre-release isometric contraction did not differ significantly. However, with no pre-release contraction significantly less torque was generated at all angular velocities beyond 1.05 rad · s^–1 when compared with either the 200, 400 ms or 1 s condition. With a 200 ms pre-release contraction significantly less torque was generated at angular velocities beyond 1.05 rad · s^–1 when compared with the 400 ms or 1 s conditions. It would seem that the major factor governing the shape of the torque-velocity curve at a constant level of muscle activation is the time allowed for torque generation.     

The N400 in a semantic categorization task across 6 decades

Objectives: To characterize the effects of normal aging on the amplitude, latency and scalp distribution of the N400 congruity effect.Methods: Event-related brain potentials (ERPs) were recorded from 72 adults (half of them men) between the ages of 20 and 80 years (12/decade) as they performed a semantic categorization task. Participants listened to spoken phrases (e.g. `a type of fruit' or `the opposite of black') followed about 1 s later by a visually-presented word that either did or did not fit with the sense of the preceding phrase; they reported the word read and whether or not it was appropriate. ERP measurements (mean amplitudes, peak amplitudes, peak latencies) were subjected to analysis of variance and linear regression analyses.Results: All participants, regardless of age, produced larger N400s to words that did not fit than to those that did. The N400 congruity effect (no-fit ERPs−fit ERPs) showed a reliable linear decrease in the amplitude (0.05–0.09 μV per year, r=0.40) and a reliable linear increase peak latency (1.5–2.1 ms/year, r=0.60) with age.Conclusions: In sum, the N400 semantic congruity effect at the scalp gets smaller, slower and more variable with age, consistent with a quantitative rather than qualitative change in semantic processing (integration) with normal aging.     

Redox-potentiometric titrations of the electrochromic absorption change in chloroplasts

Two phases of the electrochromic 515 nm absorption change in chloroplasts elicited by microsecond flashes can be resolved kinetically. Redox-potentiometric titrations indicate that the initial amplitude appearing within 0.5 ms, and designated as phase a, has three components in the low-potential region with E_m7.5 values of +60 mV, −195 mV and less than −400 mV. From the insensitivity to DCMU, we propose that the species with E_m7.5 values of −195 mV and less than −400 mV are both related to Photosystem I. This conclusion was supported by the loss of both components when the Photosystem I reaction centre (P-700) was chemically oxidised (E_m7.5 = +370 mV). The species having an E_m7.5 less than −400 mV is presumed to be the Photosystem I primary acceptor, while the E_m7.5 = −195 mV wave could be due to a secondary electron acceptor, such as cytochrome b-563_LP, whose photoreduction is possible owing to the long duration of the excitation flash. The DCMU-sensitive component with an E_m7.5 of +60 mV is assumed to be the primary quinone acceptor (Q_A) of Photosystem II. Unlike the Photosystem I redox components, the midpoint potential of this species is sensitive to the background ionic level: the E_m7.5 is shifted to −100 mV when the cation concentration is lowered to facilitate membrane unstacking. The slow phase of the electrochromic signal (phase b) has been estimated by measuring the 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone-sensitive amplitude of the absorption change at 20 ms. The signal appears with an estimated E_m7.5 = +50 mV, becomes maximal at −50 mV and attenuates with an E_m7.5 of about −180 mV. These results suggest that phase b occurs when the plastoquinone pool is reduced and cytochrome b-563_LP is oxidised.     

Antagonist mechanical contribution to resultant maximal torque at the ankle joint in young and older men

A recorded muscular torque at one joint is a resultant torque corresponding to the participation of both agonist and antagonist muscles. This study aimed to examine the effect of aging on the mechanical contributions of both plantar- and dorsi-flexors to the resultant maximal voluntary contraction (MVC) torques exerted at the ankle joint, in dorsi-flexion (DF) and plantar-flexion (PF). The estimation of isometric agonist and antagonist torques by means of an EMG biofeedback technique was made with nine young (mean age 24 years) and nine older (mean age 80 years) men. While there was a non-significant age-related decline in the measured resultant DF MVC torque (?15%; p = 0.06), there was a clear decrease in the estimated agonist MVC torque exerted by the dorsi-flexors (?39%; p = 0.001). The DF-to-PF resultant MVC torque ratio was significantly lower in young than in older men (0.25 vs. 0.31; p = 0.006), whereas the DF-to-PF agonist MVC torque ratio was no longer different between the two populations (0.38 vs. 0.35; p > 0.05). Thus, agonist MVC torques in PF and DF would be similarly affected by aging, which could not be deduced when only resultant torques were examined.     

Isometric torque–angle relationships of the elbow flexors and extensors in the transverse plane

Maximal voluntary isometric torque–angle relationships of elbow extensors and flexors in the transverse plane (humerus elevation angle of 90°) were measured at two different horizontal adduction angles of the humerus compared to thorax: 20° and 45°. For both elbow flexors and extensors, the torque–angle relationship was insensitive to this 25° horizontal adduction of the humerus. The peak in torque–angle relationship of elbow extensors was found at 55° (0° is full extension). This is closer to full elbow extension than reported by researchers who investigated this relationship in the sagittal plane. Using actual elbow angles during contraction, as we did in this study, instead of angles set by the dynamometer, as others have done, can partly explain this difference.We also measured electromyographic activity of the biceps and triceps muscles with pairs of surface electrodes and found that electromyographic activity level of the agonistic muscles was correlated to measured net torque (elbow flexion torque: Pearson’s r = 0.21 and extension torque: Pearson’s r = 0.53). We conclude that the isometric torque–angle relationship of the elbow extensors found in this study provides a good representation of the force–length relationship and the moment arm–angle relationship of the elbow extensors, but angle dependency of neural input gives an overestimation of the steepness.     

C1q/tumor necrosis factor-related protein-3 enhances the contractility of cardiomyocyte by increasing calcium sensitivity

C1q/tumor necrosis factor-related protein-3 (CTRP3) is an adipokine that protects against myocardial infarction-induced cardiac dysfunction through its pro-angiogenic, anti-apoptotic, and anti-fibrotic effects. However, whether CTRP3 can directly affect the systolic and diastolic function of cardiomyocytes remains unknown. Adult rat cardiomyocytes were isolated and loaded with Fura-2AM. The contraction and Ca²⁺ transient data was collected and analyzed by IonOptix system. 1 and 2 μg/ml CTRP3 significantly increased the contraction of cardiomyocytes. However, CTRP3 did not alter the diastolic Ca²⁺ content, systolic Ca²⁺ content, Ca²⁺ transient amplitude, and L-type Ca²⁺ channel current. To reveal whether CTRP3 affects the Ca²⁺ sensitivity of cardiomyocytes, the typical phase-plane diagrams of sarcomere length vs. Fura-2 ratio was performed. We observed a left-ward shifting of the late relaxation trajectory after CTRP3 perfusion, as quantified by decreased Ca²⁺ content at 50% sarcomere relaxation, and increased mean gradient (μm/Fura-2 ratio) during 500–600 ms (-0.163 vs. −0.279), 500–700 ms (-0.159 vs. −0.248), and 500–800 ms (-0.148 vs. −0.243). Consistently, the phosphorylation level of cardiac troponin I at Ser23/24 was reduced by CTRP3, which could be eliminated by preincubation of okadaic acid, a type 2A protein phosphatase inhibitor. In summary, CTRP3 increases the contraction of cardiomyocytes by increasing the myofilament Ca²⁺ sensitivity. CTRP3 might be a potential endogenous Ca²⁺ sensitizer that modulates the contractility of cardiomyocytes.     

Does adiposity affect muscle function during walking in children?

The biomechanical mechanisms responsible for the altered gait in obese children are not well understood, particularly as they relate to increases in adipose tissue. The purpose of this study was to test the hypotheses that as body-fat percentage (BF%) increased: (1) knee flexion during stance would decrease while pelvic obliquity would increase; (2) peak muscle forces normalized to lean-weight would increase for gluteus medius, gastrocnemius, and soleus, but decrease for the vasti; and (3) the individual muscle contributions to center of mass (COM) acceleration in the direction of their primary function(s) would not change for gluteus medius, gastrocnemius, and soleus, but decrease for the vasti. We scaled a musculoskeletal model to the anthropometrics of each participant (n=14, 8–12 years old, BF%: 16–41%) and estimated individual muscle forces and their contributions to COM acceleration. BF% was correlated with average knee flexion angle during stance (r=−0.54, p=0.024) and pelvic obliquity range of motion (r=0.78, p<0.001), as well as with relative vasti (r=−0.60, p=0.023), gluteus medius (r=0.65, p=0.012) and soleus (r=0.59, p=0.026) force production. Contributions to COM acceleration from the vasti were negatively correlated to BF% (vertical— r=−0.75, p=0.002, posterior— r=−0.68, p=0.008), but there were no correlation between BF% and COM accelerations produced by the gastrocnemius, soleus and gluteus medius. Therefore, we accept our first, partially accept our second, and accept our third hypotheses. The functional demands and relative force requirements of the hip abductors during walking in pediatric obesity may contribute to altered gait kinematics.     

CNTF genotype is associated with muscular strength and quality in humans across the adult age span.

The relationship between ciliary neurotrophic factor (CNTF) genotype and muscle strength was examined in 494 healthy men and women across the entire adult age span (20-90 yr). Concentric (Con) and eccentric (Ecc) peak torque were assessed using a Kin-Com isokinetic dynamometer for the knee extensors (KE) and knee flexors (KF) at slow (0.52 rad/s) and faster (3.14 rad/s) velocities. The results were covaried for age, gender, and body mass or fat-free mass (FFM). Individuals heterozygous for the CNTF null (A allele) mutation (G/A) exhibited significantly higher Con peak torque of the KE and KF at 3.14 rad/s than G/G homozygotes when age, gender, and body mass were covaried (P < 0.05). When the dominant leg FFM (estimated muscle mass) was used in place of body mass as a covariate, Con peak torque of the KE at 3.14 rad/s was also significantly greater in the G/A individuals (P < 0.05). In addition, muscle quality of the KE (peak torque at 3.14 rad x s(-1) x leg muscle mass(-1)) was significantly greater in the G/A heterozygotes (P < 0.05). Similar results were seen in a subanalysis of subjects 60 yr and older, as well as in Caucasian subjects. In contrast, A/A homozygotes demonstrated significantly lower Ecc peak torque at 0.52 rad/s for both KE and KF compared with G/G and G/A groups (P < 0.05). No significant relationships were observed at 0.52 rad/s between genotype and Con peak torque. These data indicate that individuals exhibiting the G/A genotype possess significantly greater muscular strength and muscle quality at relatively fast contraction speeds than do G/G individuals. Because of high positive correlations between fast-velocity peak torque and muscular power, these findings suggest that further investigations should address the relationship between CNTF genotype and muscular power.     

Dynamic stability and compensatory stepping responses during anterior gait–slip perturbations in people with chronic hemiparetic stroke

To examine the control of dynamic stability and characteristics of the compensatory stepping responses to an unexpected anterior gait slip induced under the non-involved limb in people with hemi-paretic stroke (PwHS) and to examine any resulting adaptive changes in these on the second slip due to experience from prior slip exposure. Ten PwHS experienced overground slip (S1) during walking on the laboratory walkway after 5–8 regular walking (RW) trials followed by a second consecutive slip trial (S2). The slip outcome (backward loss of balance, BLOB and no loss of balance, NLOB) and COM state (i.e. its COM position and velocity) stability were examined between the RW and S1 and S1 and S2 at touchdown (TD) of non-involved limb and at liftoff (LO) of the contralateral limb. At TD there was no difference in stability between RW and S1, however at LO, subjects demonstrated a lower stability on S1 than RW resulting in a 100% backward loss of balance (BLOB) with compensatory stepping response (recovery step, RS, 4/10 or aborted step, AS, 6/10). On S2, although there was no change in stability at TD, there was a significant improvement in stability at LO with a 40% decrease in BLOB. There was also a change in step strategy with a decrease in AS response (60% to 35%, p<0.05) which was replaced by an increase in the ability to step (increased compensatory step length, p<0.05) either via a recovery step or a walkover step. PwHS have the ability to reactively control COM state stability to decrease fall-risk upon a novel slip; prior exposure to a slip did not significantly alter feedforward control but improved the ability to use such feedback control for improved slip outcomes.     

Moving beyond quiet stance: Applicability of the inverted pendulum model to stooping and crouching postures

Background

Currently, it is unknown whether the inverted pendulum model is applicable to stooping or crouching postures. Therefore, the aim of this study was to determine the degree of applicability of the inverted pendulum model to these postures, via examination of the relationship between the centre of mass (COM) acceleration and centre of pressure (COP)–COM difference.

Methods

Ten young adults held static standing, stooping and crouching postures, each for 20 s. For both the anterior–posterior (AP) and medio-lateral (ML) directions, the time-varying COM acceleration and the COP–COM were computed, and the relationship between these two variables was determined using Pearson?s correlation coefficients. Additionally, in both directions, the average absolute COM acceleration, average absolute COP–COM signal, and the inertial component (i.e., −I/Wh) were compared across postures.

Results

Pearson correlation coefficients revealed a significant negative relationship between the COM acceleration and COP–COM signal for all comparisons, regardless of the direction (p<0.001). While no effect of posture was observed in the AP direction (p=0.463), in the ML direction, the correlation coefficients for stooping were different (i.e., stronger) than standing (p=0.008). Regardless of direction, the average absolute COM acceleration for both the stooping and crouching postures was greater than standing (p<0.002).

Conclusion

The high correlations indicate that the inverted pendulum model is applicable to stooping and crouching postures. Due to their importance in completing activities of daily living, there is merit in determining what type of motor strategies are used to control such postures and whether these strategies change with age.     

Compliant characteristics of single joints: Preservation of equifinality with phasic reactions

Electromyograms (EMGs) of elbow flexors and extensors, elbow joint angle and torque were recorded in healthy subjects in the do not react voluntarily paradigm. Random loading and unloading torque perturbations of different rise times (T _r) were applied. Perturbations withT _r less than 300 ms evoke a phasic increase (in case of loading) or decrease (in case of unloading) in the flexor EMGs. The shortest EMG latency of about 30 ms was observed with very rapid perturbations (T _r less or equal to 20 ms). Smooth torque changes (T _r of 500–800 ms) were not accompanied by noticeable phasic reactions.Two types of the compliant characteristics were recorded: those with a zone of relatively high stiffness at the starting point (S-shaped curves) and those with a relatively constant stiffness. The presence or absence of theS-shape does not depend onT _r and therefore, does not seem to depend on phasic reflex or triggered reactions. In some of the subjects who were tested twice with an intervals of 2 months, the form of the compliant characteristics changed. This finding suggests that presence or absence of theS-shape can depend upon the subject's ability to follow the do not react voluntarily instruction. The invariance of the compliant characteristics in the presence or absence of reflex reactions can be regarded as an example of equifinality in a case of transient perturbations generated within the motor control system.     

Increased membrane surface positive charge and altered membrane fluidity leads to cationic antimicrobial peptide resistance in Enterococcus faecalis

To understand the role of cell membrane phospholipids during resistance development to cationic antimicrobial peptides (CAMPs) in Enterococcus faecalis, gradual dose-dependent single exposure pediocin-resistant (Ped^r) mutants of E. faecalis (Efv2.1, Efv3.1, Efv3.2, Efv4.1, Efv4.2, Efv5.1, Efv5.2 and Efv5.3), conferring simultaneous resistance to other CAMPs, selected in previous study were characterized for cell membrane phospholipid head-groups and fatty acid composition. The involvement of phospholipids in resistance acquisition was confirmed by in vitro colorimetric assay using PDA (polydiacetylene)-biomimetic membranes. Estimation of ratio of amino-containing phospholipids to amino-lacking phospholipids suggests that phospholipids in cell membrane of Ped^r mutants loose anionic character. At moderate level of resistance, the cell-membrane becomes neutralized while at further higher level of resistance, the cell-surface acquired positive charge. Increased expression of mprF gene (responsible for lysinylation of phospholipids) was also observed on acquiring resistance to pediocin in Ped^r E. faecalis. Decreased level of branched chain fatty acids in Ped^r mutants might have contributed in enhancing rigidification of cell membrane and contributing towards resistance. The interaction of pediocin with PDA-biomimetic membranes prepared from wild-type and Ped^r mutants was monitored by measuring percent colorimetric response (%CR). Increased %CR of pediocin against PDA-biomimetic membranes prepared from Ped^r mutants confirmed that cell membrane phospholipids are involved in the interactions of pore formation by CAMPs. There was a direct linear relationship between percent colorimetric response and IC₅₀ of CAMPs for wild-type and Ped^r mutants. This relationship further reveals that in vitro colorimetric assay can be used effectively for quantification of resistance to CAMPs.     

Single and multiple step balance recovery responses can be different at first step lift-off following lateral waist-pull perturbations in older adults

An inability to recover lateral balance with a single step is predictive of future falls in older adults. This study investigated if balance stability at first step lift-off (FSLO) would be different between multiple and single stepping responses to lateral perturbations. 54 healthy older adults received left and right waist-pulls at 5 different intensities (levels 1–5). Crossover stepping responses at and above intensity level 3 that induced both single and multiple steps were analyzed. Whole-body center of mass (COM) and center of pressure (COP) positions in the medio-lateral direction with respect to the base of support were calculated. An inverted pendulum model was used to define the lateral stability boundary, which was also adjusted using the COP position at FSLO (functional boundary). No significant differences were detected in the COP positions between the responses at FSLO (p ≥ 0.075), indicating no difference in the functional boundaries between the responses. Significantly smaller stability margins were observed at first step landing for multiple steps at all levels (p ≤ 0.024), while stability margins were also significantly smaller at FSLO for level 3 and 4 (p ≤ 0.048). These findings indicate that although reduced stability at first foot contact would be associated with taking additional steps, stepping responses could also be attributable to the COM motion state as early as first step lift-off, preceding foot contact. Perturbation-based training interventions aimed at improving the reactive control of stability would reduce initial balance instability at first step lift-off and possibly the consequent need for multiple steps in response to balance perturbations.