Anconeus motor unit firing rates during isometric and muscle-shortening contractions comparing young and very old adults.

With effects of aging, voluntary neural drive to the muscle, measured as motor unit (MU) firing rate, is lower in older adults during sustained isometric contractions compared with young adults, but differences remain unknown during limb movements. Therefore, our purpose was to compare MU firing rates during both isometric and shortening contractions between two adult age groups. We analyzed intramuscular electromyography of single-MU recordings in the anconeus muscle of young (n = 8, 19-33 yr) and very old (n = 13, 78-93 yr) male adults during maximal voluntary contractions (MVCs). In sustained isometric and muscle-shortening contractions during limb movement, MU trains were linked with elbow joint kinematic parameters throughout the contraction time course. The older group was 33% weaker and 10% slower during movements than the young group (P < 0.01). In isometric contractions, median firing rates were 42% lower (P < 0.01) in the older group (18 Hz) compared with the young group (31 Hz), but during shortening contractions firing rates were higher for both age groups and not statistically different between groups. As a function of contraction time, firing rates at MU recruitment threshold were 39% lower in the older group, but the firing rate decrease was attenuated threefold throughout shortening contraction compared with the young group. At the single-MU level, age-related differences during isometric contractions (i.e., pre-movement initiation) do not remain constant throughout movement that comprises greater effects of muscle shortening. Results indicate that neural drive is task dependent and during movement in older adults it is decreased minimally.NEW & NOTEWORTHY Changes of neural drive to the muscle with adult aging, measured as motor unit firing rates during limb movements, are unknown. Throughout maximal voluntary efforts we found that, in comparison with young adults, firing rates were lower during isometric contraction in older adults but not different during elbow extension movements. Despite the older group being ∼33% weaker across contractions, their muscles can receive neural drive during movements that are similar to that of younger adults.

Analysis of Muscle Fatigue Progression using Cyclostationary Property of Surface Electromyography Signals.

Analysis of neuromuscular fatigue finds various applications ranging from clinical studies to biomechanics. Surface electromyography (sEMG) signals are widely used for these studies due to its non-invasiveness. During cyclic dynamic contractions, these signals are nonstationary and cyclostationary. In recent years, several nonstationary methods have been employed for the muscle fatigue analysis. However, cyclostationary based approach is not well established for the assessment of muscle fatigue. In this work, cyclostationarity associated with the biceps brachii muscle fatigue progression is analyzed using sEMG signals and Spectral Correlation Density (SCD) functions. Signals are recorded from fifty healthy adult volunteers during dynamic contractions under a prescribed protocol. These signals are preprocessed and are divided into three segments, namely, non-fatigue, first muscle discomfort and fatigue zones. Then SCD is estimated using fast Fourier transform accumulation method. Further, Cyclic Frequency Spectral Density (CFSD) is calculated from the SCD spectrum. Two features, namely, cyclic frequency spectral area (CFSA) and cyclic frequency spectral entropy (CFSE) are proposed to study the progression of muscle fatigue. Additionally, degree of cyclostationarity (DCS) is computed to quantify the amount of cyclostationarity present in the signals. Results show that there is a progressive increase in cyclostationary during the progression of muscle fatigue. CFSA shows an increasing trend in muscle fatiguing contraction. However, CFSE shows a decreasing trend. It is observed that when the muscle progresses from non-fatigue to fatigue condition, the mean DCS of fifty subjects increases from 0.016 to 0.99. All the extracted features found to be distinct and statistically significant in the three zones of muscle contraction (p < 0.05). It appears that these SCD features could be useful in the automated analysis of sEMG signals for different neuromuscular conditions.

Differential effects of stimulation frequency on isometric and concentric isotonic contractile function in human quadriceps.

Electrically evoked contractions are used to assess the relationship between frequency input and contractile output to characterize inherent muscle function, and these have been done mostly with isometric contractions (i.e., no joint rotation). The purpose was to compare the electrically stimulated frequency and contractile function relationship during isometric (i.e., torque) with isotonic (i.e., concentric torque, angular velocity, and mechanical power) contractions. The knee extensors of 16 (5 female) young recreationally active participants were stimulated (∼1-2.5 s) at 14 frequencies from 1 to 100 Hz. This was done during four conditions, which were isometric and isotonic at loads of 0 (unloaded), 7.5%, and 15% isometric maximal voluntary contraction (MVC), and repeated on separate days. Comparisons across contractile parameters were made as a % of 100 Hz. Independent of the load, the mechanical power-frequency relationship was rightward shifted compared with isometric torque-frequency, concentric torque-frequency, and velocity-frequency relationships (all P ≤ 0.04). With increasing load (0%-15% MVC), the isotonic concentric torque-frequency relationship was shifted leftward systematically from 15 to 30 Hz (all P ≤ 0.04). Conversely, the same changes in load caused a rightward shift in the velocity-frequency relationship from 1 to 40 Hz (all P ≤ 0.03). Velocity was leftward shifted of concentric torque in the unloaded isotonic condition from 10 to 25 Hz (all P ≤ 0.03), but concentric torque was leftward shifted of velocity at 15% MVC isotonic condition from 10 to 50 Hz (all P ≤ 0.03). Therefore, isometric torque is not a surrogate to evaluate dynamic contractile function. Interpretations of evoked contractile function differ depending on contraction type, load, and frequency, which should be considered relative to the specific task.NEW & NOTEWORTHY In whole human muscle, we showed that the electrically stimulated power-frequency relationship was rightward shifted of the stimulated isometric torque-frequency relationship independent of isotonic load, indicating that higher stimulation frequencies are needed to achieve tetanus. Therefore, interpretations of evoked contractile function differ depending on contraction type (isometric vs. dynamic), load, and frequency. And thus, isometric measures may not be appropriate as a surrogate assessment when evaluating dynamic isotonic contractile function.

Influence of stimulation frequency on early and late phase rate of torque and velocity development.

The early (≤50 ms) rate of torque development (RTD) is dependent upon the speed of neuromuscular activation; however, few studies have evaluated the determinants of rate of velocity development (RVD), which may be load-dependent. The purpose here was to explore the relationship between stimulation frequency with the early and late (≥100 ms) phase isometric RTD and isotonic RVD. The knee extensors of 16 (five female) young recreationally active participants were stimulated using 14 frequencies from 1 to 100 Hz during isometric and isotonic ("unloaded" and 7.5% of the isometric maximal voluntary contraction [MVC]) contractions. Isometric RTD and isotonic RVD were evaluated for the early (0-50 ms) and late (0-100 ms) phases from torque and velocity onset, respectively. Sigmoid functions were fit and bilinear regressions were used to examine the slopes of the steep portion of the curve and the plateau frequency. RTD- and RVD-frequency relationships were well described by a sigmoid function (all r2 > 0.96). Compared with the late phase, early isometric RTD, and unloaded RVD displayed lower slopes (all P ≤ 0.001) and higher plateau frequencies (all P < 0.001). In contrast, early and late RVD of a moderately loaded isotonic contraction did not display different slopes (P = 0.055) or plateau frequencies (P = 0.690). Early isometric RTD and unloaded isotonic RVD are more dependent on changes in stimulation frequency compared with late phases. However, RVD for a moderately loaded isotonic contraction displayed similar responses for the early and late phases. Therefore, a high frequency of activation is critical for early torque and velocity generation but dependent upon the load for isotonic contractions.NEW & NOTEWORTHY We show that during an "unloaded" isotonic contraction, the early phase rate of velocity development is more dependent upon a high electrical activation frequency compared with the late phase, similar to isometric torque. However, early and late phase rates of velocity development of moderately loaded isotonic contractions display similar responses. These results indicate that the determinants of isotonic shortening function are dependent on the externally applied load, highlighting the importance of task-specificity of contraction.

Test-retest repeatability of electrically evoked isotonic power and isometric torque in the plantar flexors.

Electrically evoked isometric torque has good to excellent repeatability, but the degree of repeatability for electrically evoked isotonic power is unknown. We evaluated the test-retest repeatability of plantar flexion isometric torque and isotonic power evoked using brief tetanic trains at 10 and 50 Hz. Both torque and power had excellent (intraclass correlation coefficient >0.9) repeatability at low and high frequencies. Similar to isometric torque, electrically evoked isotonic power is a stable measure for studying dynamic muscle function.

Age-related fatigability in knee extensors and knee flexors during dynamic fatiguing contractions.

This study investigated the effects of dynamic knee extension and flexion fatiguing task on torque and neuromuscular responses in young and older individuals. Eighteen young (8 males; 25.1 ± 3.2 years) and 17 older (8 males; 69.7 ± 3.7 years) volunteered. Following a maximal voluntary isometric contraction test, participants performed a fatiguing task involving 22 maximal isokinetic (concentric) knee extension and flexion contractions at 60°/s, while surface EMG was recorded simultaneously from the knee extensors (KE) and flexors (KF). Fatigue-induced relative torque reductions were similar between age groups for KE (peak torque decrease: 25.15% vs 26.81%); however, KF torque was less affected in older individuals (young vs older peak torque decrease: 27.6% vs 11.5%; p < 0.001) and this was associated with greater increase in hamstring EMG amplitude (p < 0.001) and hamstrings/quadriceps peak torque ratio (p < 0.01). Furthermore, KE was more fatigable than KF only among older individuals (peak torque decrease: 26.8% vs 11.5%; p < 0.001). These findings showed that the age-related fatigue induced by a dynamic task was greater for the KE, with greater age-related decline in KE compared to KF.

Sensor Anchoring Improves the Correlation Between Intramuscular Pressure and Muscle Tension in a Rabbit Model.

Intramuscular pressure (IMP) shows promise for estimating individual muscle tension in vivo. However, previous pressure measurements show high variability during isometric contraction and poor correlation with tension during dynamic contraction. We hypothesized that enhanced sensor anchoring/orientation would improve tension estimation and thus developed a novel pressure sensor with a barbed housing. Sensors were inserted into the tibialis anterior (TA) of New Zealand White rabbits (N = 8) both parallel and perpendicular to the fiber orientation. We measured muscle stress and IMP during both isometric and dynamic contractions. Passive stress showed good agreement for both insertion directions across muscle lengths (ICC > 0.8). Active stress and IMP agreement were good (ICC = 0.87 ± 0.04) for perpendicular insertions but poor (ICC = 0.21 ± 0.22) for parallel insertions across both dynamic contractions and isometric contractions within the muscle's range of motion. These findings support use of IMP measurements to estimate muscle tension across a range of contraction conditions.

Analysis of Muscle Load-Sharing in Patients With Lateral Epicondylitis During Endurance Isokinetic Contractions Using Non-linear Prediction.

The aim of this paper is to analyze muscle load-sharing in patients with Lateral Epicondylitis during dynamic endurance contractions by means of non-linear prediction of surface EMG signals. The proposed non-linear cross-prediction scheme was used to predict the envelope of an EMG signal and is based on locally linear models built in a lag-embedded Euclidean space. The results were compared with a co-activation index, a common measure based on the activation of a muscle pair. Non-linear prediction revealed changes in muscle coupling, that is load-sharing, over time both in a control group and Lateral Epicondylitis (p < 0.05), even when subjects did not report pain at the end of the exercise. These changes were more pronounced in patients, especially in the first part of the exercise and up to 50% of the total endurance time (p < 0.05). By contrast, the co-activation index showed no differences between groups. Results reflect the changing nature of muscular activation strategy, presumably because of the mechanisms triggered by fatigue. Strategies differ between controls and patients, pointing to an altered coordination in Lateral Epicondylitis.

Effects of short-term fatigue on biomechanical and physiological aspects of double poling in high-level cross-country skiers.

The study aim was to evaluate biomechanical and physiological alterations in double poling technique (DP) after a short-term fatiguing exercise. Eight high-level skiers performed a sub-maximal DP trial (20kmh(-1), 1°) before (PRE) and after (POST) a DP test to exhaustion while roller skiing on a treadmill. An integrated analysis of DP technique during PRE and POST included measurement of pole, joint, and centre of mass (COM) kinematics, poling forces, cycle timing, and metabolic parameters. Muscle fatigue in three upper-body muscles was assessed by calculating the Dimitrov' fatigue index (FInms5) of specific electromyographic segments. FInms5 tended to increase in the latissimus dorsi and teres major muscles (P=0.023 and P=0.030, respectively) across consecutive DP cycles, as did blood lactate concentration (P=0.001) and rating of perceived exertion (P=0.005). The changes indicated a state of fatigue during POST and coincided with the reduction in poling force exertion capacity (P=0.020). Pole, joint and COM kinematics did not differ between PRE and POST (P>0.050), whereas recovery phase and cycle times were shorter at POST (P<0.001 and P=0.001, respectively). Short-term fatigue led to a reduction in poling force exertion capacity and cycle time in high-level skiers, without altering body and pole kinematics.

Motor unit discharge rate in dynamic movements of the aging soleus.

Aging is related to a variety of changes at the muscular level. It seems that the age-related changes in motor unit activation are muscle- and intensity dependent. The purpose of this study was to examine the motor unit discharge rate (MUDR) in both isometric and dynamic contractions of the aging soleus muscle. Eight elderly males participated in the study. The subjects performed isometric and dynamic plantar flexions while seated in an ankle dynamometer. The force levels studied were 10, 20, 40, 60, 80 and 100% of the isometric (ISO) maximal voluntary contractions (MVC) in ISO and 10, 20 and 40% in concentric (CON) and eccentric (ECC) contractions. Soleus intramuscular EMG was recorded with bipolar fine-wire electrodes and decomposed to individual trains of motor unit discharges. In ISO the MUDR increased with each force level from 40 to 100% MVC. In dynamic contractions the descriptive analysis showed a higher MUDR in CON compared to ISO or ECC. The difficulties of recording single motor units in dynamic contractions, especially in the elderly is discussed.

Study of muscle fatigue in isokinetic exercise with estimated conduction velocity and traditional electromyographic indicators

INTRODUCTION: In the study of human biomechanics, it is often desirable to evaluate fatigue in the muscles that are involved in performing a particular task. Identifying the phenomena responsible for this condition is a problem that in most cases is complex and requires appropriate research mechanisms. Isokinetic dynamometry (ID) and surface electromyography (SEMG) are two techniques widely used in studies on strength and muscle fatigue. Their effectiveness is conditioned upon a good understanding of their limitations and the adoption of procedures to fully exploit the potential of each one. The main goal of the present study is to verify whether the electromyographic parameters, especially the conduction velocity (CV), are sensitive to the fatigue instauration process within sets of maximal isokinetic contractions. CV is a basic physiological parameter directly related to muscle activity and still little explored in experiments combining ID and SEMG. METHODS: Instrumentation architecture that combines ID and SEMG was used to estimate electromyographic and biomechanical parameters in protocols of maximum intensity isokinetic knee extension exercises. This architecture allows for limiting the parameter estimates to a specific region of isokinetic exercise, called the isokinetic load range (ILR), where one can consider that the angular velocity is constant and the SEMG signals are cyclo-stationary. Electromyographic signals were acquired using an array of electrodes. CONCLUSION: The results suggest that CV and the other SEMG parameters, including amplitude and frequency descriptors, are sensitive to detect a fatigue process only in protocols that restrict the analysis to ILR and that also bring the subject to a state of fatigue quickly.

CARDIOVASCULAR RESPONSE TO STATIC AND DYNAMIC MUSCLE CONTRACTIONS AT EQUIVALENT TENSION-TIME INDICES / 体力科学

In the present study, we examined cardiovascular response to static and dynamic hand-grip exercise at equivalent work load (peak tension) and tension-time index (TTI, integrated tension for time) in healthy young (n=8) and elderly (n=8) males. Static and dynamic exercises were conducted for 75 s and 150 s at 30% of maximal voluntary contraction (MVC) and for 45 s and 90 s at 50%MVC, respectively. Arterial pressure was continuously measured on a beat basis. Blood pressure at the end of exercise and the magnitude of pressor response induced by exercise did not differ significantly between static and dynamic exercises at the two work loads. The magnitude of pressor response tended to depend on work load. These findings were the same in both age groups. Consequently, it was indicated that blood pressure responses to static and dynamic hand-grip exercise at equivalent work load and TTI did not differ both in young and elderly people. Furthermore, it was suggested that central command and muscle metabolite induced stimulation of the exercise pressor reflex during static and dynamic exercise were similar based on the results of relative perceived exertion and blood pressure response during post-exercise arterial occlusion.