Changes in contractile and elastic properties of the triceps surae muscle induced by neuromuscular electrical stimulation training

Purpose

Neuromuscular electrical stimulation (NMES) training is known to induce improvement in force production capacities and fibre-type transition. The aim of this study was to determine whether NMES training also leads to changes in the mechanical properties of the human triceps surae (TS) muscle.

Methods

Fifteen young male subjects performed a training protocol (4 weeks, 18 sessions, 4–5 sessions per week) based on a high-frequency isometric NMES programme of TS muscle. Quick-release test was used to evaluate Musculo-Tendinous (MT) stiffness index (SI_MT) as the slope of the linear MT stiffness–torque relationships under submaximal contraction. Sinusoidal perturbations allowed the assessment of musculo-articular stiffness index (SI_MA) as well as the calculation of the maximal angular velocity ( \(\varTheta_{\hbox{max} }^{{\prime }}\) ) of TS muscle using an adaptation of Hill’s equation.

Results

After NMES training, Maximal Voluntary Contraction under isometric conditions and \(\varTheta_{\hbox{max} }^{{\prime }}\) increased significantly by 17.5 and 20.6 %, respectively, while SI_MT and SI_MA decreased significantly (?12.7 and ?9.3 %, respectively).

Conclusions

These changes in contractile and elastic properties may lead to functional changes of particular interest in sport-related activities as well as in the elderly.     

Changes in muscle and joint elasticity following long-term strength training in old age

The current investigation was designed (1) to examine the effect of a 48-week strength training on musculotendinous (MT) and musculoarticular (MA) stiffness characteristics in older men and women; and (2) to evaluate the influence of gender on stiffness behaviour in response to such training. The training was performed twice per week and mainly consisted of three series of 10 repetitions of calf-rise at 75% of the 3-repetition maximum. Two methods were used to perform stiffness measurements during plantar flexion: (1) the use of quick-release movements, allowing the calculation of MT stiffness; (2) the application of sinusoidal perturbations to the joint, allowing the calculation of MA stiffness. In each case, stiffness was linearly related to torque, leading to the calculation of a normalized stiffness index (SI) as the slope of this stiffness-torque relationship: SI_MT and SI_MA, respectively. Results showed a similar decrease in SI_MT among older men (−27%, P < 0.05) and women (−29%, P < 0.05) following training. A decrease in SI_MA was only observed among women (−11%, P < 0.05). The results suggest that (1) MT stiffness decreases following training in older individuals, counterbalancing the effect of ageing; and (2) older men and women respond differently to the same resistance-training stimulus in terms of MA stiffness. Gender-related differences in MA stiffness response may originate from passive MA elastic structures. This work was part of the Better Ageing Program and was supported by grants from the European Commission (Framework Program V, QLRT-2001-00323).     

Surface EMG and mechanomyogram disclose isokinetic training effects on quadriceps muscle in elderly people

Maximum voluntary contraction (MVC) and cross-sectional area (CSA) of fast and slow twitch fibers are reduced in the lower limb muscles of elderly subjects. Isokinetic training at medium and high velocities has been widely used to improve muscle performance and force in young as well as elderly subjects. EMG and mechanomyogram (MMG) are compound signals in which the electrical and mechanical activities of recruited motor units (MUs) are summated. The aim of the present study was to verify the hypothesis that isokinetic training in the elderly induces changes in EMG and MMG parameters, compatible with a functional retrieval of fast twitch fiber MUs. In ten sedentary males (62–78 years), the surface EMG and MMG were recorded from the vastus lateralis muscle during isometric contractions at 20, 40, 60, 80 and 100% of the MVC, before and after 12 weeks of isokinetic training (six series of ten repetitions, each at an angular velocity of 2.09 rad s^–1 and 4.19 rad s^–1, two times a week). With training: (a) MVC and CSA increased by about 35±5% and 8±1%, respectively (P<0.05); (b) the ratio MVC/CSA increased significantly in all subjects by 25±5%; (c) the EMG root mean square and MMG spectral mean frequency increased significantly at the highest workloads. In conclusion, our data indicate that isokinetic training in the elderly improved muscle size and performance significantly. The EMG and MMG changes suggest that these results may be due to a retrieval of the fast twitch fiber MUs, contributing to muscle action.     

Paired changes in electromechanical delay and musculo-tendinous stiffness after endurance or plyometric training

When measured in vivo electromechanical delay (EMD) depends mainly on the elastic properties of the muscle–tendon unit. Recent studies have shown changes in stiffness of the triceps surae (TS) following a period of training. To confirm the influence of musculo-tendinous stiffness on EMD, this study investigates paired changes in these two parameters after a training period. Two types of training known to induce opposite changes in stiffness were analysed. EMD and musculo-tendinous stiffness were measured on adult subjects before and after 10 weeks of endurance (n = 21) or plyometric (n = 9) trainings. EMD was defined as the time lag between the TS M-wave latency and the onset of muscle twitch evoked at rest by supramaximal electrical stimulations of the posterior tibial nerve. Quick release tests were used to evaluate the musculo-tendinous stiffness of the ankle plantar flexors. The stiffness index was defined as the slope of the relationship between angular stiffness and external torque values. Endurance training, known to preferentially activate the slow, stiffer muscle fibers, leads to a decrease in EMD and to an increase in stiffness index. Following plyometric training, which specifically recruits fast, more compliant fibers, EMD and the stiffness index exhibited adaptations directionally opposite to those seen with endurance training. When pooling the data for the two subject groups, a correlation was found between changes in EMD and changes in musculo-tendinous stiffness indexes. Thus, changes in EMD values are proposed to indirectly link to changes in musculo-tendinous stiffness for subjects involved in muscle training.     

Stunting delays maturation of triceps surae mechanical properties and motor performance in prepubertal children

Malnutrition can lead to possible irreversible consequences in the development of muscle function and some of them are yet poorly characterized. The present study evaluated the mechanical properties of the triceps surae and motor performance in stunted (S) and eutrophic (E) prepubertal children (9?years?±?6?months). Height-for-age ratio was used as indicator of stunting due to early malnutrition, according to the World Health Organization. Torque was determined by maximal voluntary contractions (MVC) and musculotendinous (MT) stiffness was achieved through quick-release tests to obtain MT stiffness index (SI_MT) and passive stiffness (K _p) from linear MT stiffness–torque relationships. Percutaneous supramaximal electrically elicited contractions determined twitch torque (Pt) and electromechanical delay (EMD). Motor performance was evaluated by balance test. S group presented significantly lower MVC and a trend of lower Pt values indicating lower capacities to develop force under voluntary or induced conditions. Significantly higher SI_MT and EMD values were observed, while K _p and motor performance in balance were significantly lower. Higher SI_MT values have been reported previously in youngest prepubertal children, indicating that immature activation capacities can mask MT stiffness assessment during voluntary contractions, taking into consideration the higher EMD values as a measure of muscle stiffness contribution. Lower K _p may indicate a delay in the maturation of tendinous tissue in S group, influencing motor performance in balance. The present study shows that malnutrition leads to adaptation of intrinsic MT elastic properties, but depends on the level of the observed structure.     

Repeatability of a dual gradient‐recalled echo MRI method for monitoring post‐isometric contraction blood volume and oxygenation changes

The purpose of this study was to assess the repeatability of a dual gradient‐recalled echo (GRE) muscle functional MRI technique. On 2 days, subjects (n = 8) performed 10 s isometric dorsiflexion contractions under conditions of: (1) maximal voluntary contraction (MVC), (2) 50% MVC (50% MVC), or (3) 50% MVC with concurrent proximal arterial cuff occlusion (50% MVC_cuff). Functional MRI data were acquired using single‐slice dual GRE (TR/TE = 1000/6, 46 ms)‐echo planar imaging for 20 s before, during, and for 180 s after each contraction. The mean signal intensity (SI) time courses at each TE (SI₆ and SI₄₆, reflecting variations in blood volume and %HbO₂, respectively) from the tibialis anterior (TA) and extensor digitorum longus (EDL) muscles were characterized with the post‐contraction change in SI and the time‐to‐peak SI (ΔSI and TTP, respectively). ΔSI₆ following an MVC was 36% higher than that obtained after a 50% MVC (p = 0.048). For ΔSI₆, the highest intraclass correlation coefficients (ICCs) were observed for the TA muscle in the 50% MVC and MVC conditions, with values of 0.83 (p = 0.01) and 0.88 (p = 0.005), respectively. Bland–Altman plots revealed repeatability coefficients (RCs) for the 50% MVC and MVC conditions in the TA muscle of 1.9 and 1.4, respectively. The most repeatable measures for ΔSI₄₆ were obtained for the 50% MVC and MVC conditions in the EDL muscle (p = 0.01 and p = 0.04, respectively). Bland–Altman plots revealed RC's for 50% MVC and MVC conditions in the EDL muscle of 3.9 and 5.7, respectively. ΔSI₆ and ΔSI₄₆ increased as a function of the contraction intensity. The repeatability of the method depends on the muscle and contraction condition being evaluated, and in general, is higher following an MVC. Copyright © 2009 John Wiley & Sons, Ltd.     

Influence of training background on the relationships between plantarflexor intrinsic stiffness and overall musculoskeletal stiffness during hopping

The aim of this study was to characterize the influence of intrinsic musculotendinous and musculoarticular stiffness of plantarflexor muscles on (1) the overall musculoskeletal stiffness and (2) the performance during stretch-shortening cycles-type exercise. The influence of plyometric training background on these relationships was also analyzed. Musculotendinous (SI_MT), passive (K _P ) and active (SI_MA) musculoarticular stiffnesses were quantified, using quick-release and sinusoidal perturbation tests, on nine French elite long or triple jumpers (athlete group, AG) and nine control subjects (CG). These ergometric parameters were related with the lower-limb stiffness (K _leg) and the maximal performance (H _max) measured from a force platform during vertical hopping. AG showed a significantly higher SI_MT (2.76 rad⁻¹), K _P (55.6 N m rad⁻¹), K _leg (30.3 kN m⁻¹) and H _max (0.48 m) compared to CG (1.83 rad⁻¹, 37.8 N m rad⁻¹, 19.6 kN m⁻¹ and 0.38 m, respectively). K _leg was not significantly correlated with any of the intrinsic stiffness parameters (SI_MT, SI_MA or K _P ). For AG, a strong and negative correlation was observed between H _max and K _leg. These data indicate that, while elite jumpers presented higher stiffness of both musculotendinous and passive musculoarticular structures, a high compliance of musculoskeletal system was beneficial to optimize the performance in vertical hopping for these athletes. We suggested that neuromuscular strategies were designed to counterbalance this higher intrinsic stiffness to solve the problem of the conflicting requirement of the musculotendinous elements: increase in compliance to enhance the elastic recoil and increase in stiffness for a better force transmission to the periphery.     

Silent period to transcranial magnetic stimulation: construction and properties of stimulus–response curves in healthy volunteers

Silent period (SP) is widely used in transcranial magnetic stimulation studies. Methodologically, SP is usually elicited at stimulus intensities corresponding to a certain percentage of corticomotor threshold. Because this approach might lead to factitious SP changes, the present study was designed to develop, in a stepwise manner, a method for investigating SP independently of corticomotor threshold. First, stimulus–response (S–R) curves of SP against stimulus intensity (SI) were constructed and quantitatively described in healthy volunteers. Second, various methodological issues such as the optimum model for describing the relationship between SP duration and SI and the importance of the type of stimulating coil were addressed. Finally, the proposed method and a commonly used method (eliciting SPs at 130% MT SI) were directly compared for a group of epileptic patients for whom administration of oxcarbazepine resulted in significant corticomotor threshold elevation. Twenty-one subjects (eleven females, median age, 38 years) were studied. SPs were obtained with a figure-of-eight coil using a standardized procedure (recording, FDI). Pilot experiments indicated that at least four trials were required, at each intensity level, to estimate the mean SP duration within 10% of the true mean. Therefore, SPs were determined from the average of four trials with 5% increments from 5 to 100% maximum SI. In a second set of experiments, SPs were obtained for fifteen subjects using a circular coil. In a third set of experiments, eight epileptic patients were studied before and after administration of oxcarbazepine (mean dose 1553 mg, range 900–1800 mg). The S–R curves were fitted to a Boltzman function and to first-order to fourth-order polynomial and sigmoid functions. The Boltzman function described the data accurately (R²=0.947–0.990). In addition, direct comparison of the six models with an F-test proved the superiority of the first. The best-fit parameters of the reference curve, i.e. the maximum and minimum values, the slope, and V₅₀ (the SI at which SP duration is halfway between Min and Max) were 230.8±3.31 ms (x±SEM), –11.51±3.31 ms, 11.56±0.65%, and 49.82±0.65%, respectively. When the curves obtained with the circular coil were compared with those obtained with the figure-of-eight coil, there were differences between V₅₀ (51.69±0.72 vs 47.95±0.82, P<0.001) and SP threshold (31.15 vs 24.77, P<0.01) whereas the other best-fit values did not differ significantly. Oxcarbazepine increased corticomotor threshold from 45.3±5.8% at baseline to 59.4±10.4% (P<0.001). According to the commonly used method, the drug significantly prolonged SP (from 117.6±42.4 ms to 143.5±46.5 ms, P<0.001) and, consequently, enhanced brain inhibition. In contrast, study of the SP curves led to the conclusion that oxcarbazepine does not affect the Max value and slope but significantly increases V₅₀ and SP threshold (from 54.5±4.9% to 59.9±7.2% and from 29.1±6.4% to 34.6±6.8%, respectively, P<0.01). These findings imply that oxcarbazepine does not enhance brain inhibitory mechanisms. Thus, in situations characterized by significant changes in corticomotor threshold the proposed method provides results clearly different from a commonly used approach. It is concluded that S–R curves obtained with a figure-of-eight coil in 5% increments and fitted to a Boltzman function provide an accurate, comprehensive, and clinically applicable method for exploring SP.Presented in part at the meeting of the EFNS, Helsinki, September 2003     

Static balance improvement in elderly after dorsiflexors electrostimulation training

The purpose of the present study was to determine the effect of dorsiflexors ElectroStimulation (ES) training, on postural tasks of increasing difficulty in the elderly. Twenty-one elderly adults were randomly assigned into one of two groups: a Training (TG) and a Control Group (CG). The TG (n=10) performed (4 weeks, 4 s/week, 40 min/session) superimposed (electrically evoked and voluntary activation) isometric dorsiflexions (ankle 100°) while seated. Biphasic, rectangular symmetrical pulses (300 ms, 70 Hz, 20–60 mA) were used to provoke maximal muscle activation. Participants performed three static balance tasks (Normal Quiet Stance, Sharpened Romberg, and One-Legged Stance) during which postural sway was quantified using maximum range and standard deviation of Centre of Pressure displacement (Kistler 9281C, 1,000 Hz). Bipolar surface electrodes were used to record the Electromyographic activity (EMG) of Tibialis Anterior, Medial Gastrocnemius, Rectus Femoris and Semi-Tendineous. Two-dimensional kinematic data were collected (60 Hz) and analyzed using the APAS Motion Analysis software. The body was modeled as a five-segment rigid link system. Isometric dorsiflexion moment/angular position relationship was also established using a Cybex dynamometer. ES training resulted in decreased postural sway (P<0.05), greater ankle muscles EMG activity (P<0.001), greater stability of the ankle joint (P<0.05) and significant changes in mean position of all three joints of the lower limb. In addition, dorsiflexion moment significantly (P<0.001) increased as a result of ES training. It is concluded that dorsiflexors ES training, could reduce postural sway and the use of ankle muscles, more characteristic of young adults, might appear in the elderly as well.     

Stroke volume response to progressive exercise in athletes engaged in different types of training

Summary Using the impedance cardiography method, heart rate ( _c) matched changes on indexed stroke volume (SI) and cardiac output (CI) were compared in subjects engaged in different types of training. The subjects consisted of untrained controls (C), volleyball players (VB) who spent about half of their training time (360 min · week^–1) doing anaerobic conditioning exercises and who had a maximal oxygen uptake ( ) 41% higher than the controls, and distance runners (D) who spent all their training time (366 min·week^–1) doing aerobic conditioning exercises and who had a 26% higher than VB. The subjects performed progressive submaximal cycle ergometer exercise (10 W·min^–1) up to _c of 150 beats·min^–1. In group C, SI had increased significantly (P<0.05) at _c of 90 beats·min^–1 ( + 32%) and maintained this difference up to 110 beats·min^–1, only to return to resting values on reaching 130 beats·min^–1 with no further changes. In group VB, SI peaked (+ 54%) at _c of 110 beats·min^–1, reaching a value significantly higher than that of group C, but decreased progressively to 22010 of the resting value on reaching 150 beats·min^–1. In group D, SI peaked at _c of 130 beats·min^–1 (+ 54%), reaching a value significantly higher than that of group VB, and showed no significant reduction with respect to this peak value on reaching 150 beats·min^–1. As a consequence, the mean CI increase per _c unit was progressively higher in VB than in C (+46%) and in D than in VB (+ 105%). It was concluded that thef _c value at which SI ceased to increase during incremental exercise was closely related to the endurance component in the training programme.     

Changes induced by eccentric training on force-velocity relationships of the elbow flexor muscles

The aim of this study was to examine the effects of a short term eccentric training period on force-velocity relationships of the elbow flexor muscles. From a muscle model, the maximal shortening velocity V_O(x) and the af parameter which varies according to the curvature of the force-velocity relationship of the muscle were determined. Sixteen volunteer subjects divided into 2 groups participated in this study (Group Eccentric GE, n=8 . Group Control GC, n=8). The subjects performed, on an isokinetic ergometer, 2 maximal concentric elbow flexions at different angular velocities (60, 120, 180; 240, 300, 360 °s^–1) and held maximal and submaximal isometric actions at an elbow flexion angle of 90°. Under isometric conditions, myoelectrical activity (EMG) of the biceps was recorded and quantified as a RMS value. All tests were performed before and after training sessions. Training was conducted 3 times a week for 4 weeks by the GE, and included 6×5 eccentric actions with a load of 100% of 1 RM. After training and for the GE, the af parameter and V_o(x) increased significantly (p<0.05). These changes were accompanied by a significant increase (p<0.05) of the RMS value of the maximal isometric action. This evolution towards faster characteristics for the elbow flexor muscles after training could be partly due to nervous adaptation.     

Effects of different strength training regimes on moment and power generation during dynamic knee extensions

This study examined the effect of different training regimes on moment and power generation during maximal knee extensions at low to very high extension velocities (0–1000°·s^–1 individual range). A group of 24 soccer players performed 12 weeks of progressively adjusted strength training of the knee extensors at either high resistance (HR,n=7), low resistance (LR,n=6), loaded kicking movements (FU,n=6), while one group served as controls (n=5). Moment and power generation of the knee extensors were determined before and after the training period with a nonisokinetic measuring method recently described. Following HR training, knee extension moment increased 9%–10% at knee angular velocities 0 (isometric) and 30° · s^–1 (P<0.05), peak moment increased 20% at 240–300°·s^–1 (P<0.05), while power generation increased 5%–29% at 240–480° · s^–1 (P<0.01). In addition, in the HR group maximal recorded power increased 45% (P<0.01). After FU training a 7%–13% increase in moment and power was observed at 30–180° · s^–1 (P<0.05). Following LR training, peak moment increased 9% at 120° · s^–1 (P<0.05). Improvements in knee extension moment and power were generally related to the angular velocities employed during training. However, as evaluated using the present measuring method, moment and power increased not only at very low but also at high knee angular velocities following the high-resistance strength training.     

Effect of low‐load resistance training on the tendon properties in middle‐aged and elderly women

Aim: The purposes of this study were to determine the age‐related changes in the tendon‐aponeurosis structures and to investigate the effects of low‐load resistance training on the tendon‐aponeurosis structures in middle‐aged and elderly women. Methods: Fifty‐one women (55.8 ± 13.7 years, range: 21–77 years) volunteered to take part in the present study. Furthermore, 11 middle‐aged and elderly women (49.7 ± 9.2 years) performed the low‐load resistance training, i.e. squat using body weight, for 6 months. The elongation of the tendon and aponeurosis of the vastus lateralis muscle was directly measured by ultrasonography, while the subjects performed ramp isometric knee extension up to the voluntary maximum, followed by a ramp relaxation. The relationship between the estimated muscle force (F_m) and tendon elongation (L) during the ascending phase was fitted to a linear regression, the slope of which was defined as stiffness. The percentage of the area within the F_m‐L loop to the area beneath the curve during the ascending phase was calculated as hysteresis. Results: Maximal strain (L/initial tendon length) and stiffness of the tendon‐aponeurosis structures decreased significantly with ageing. In contrast, the hysteresis increased significantly with ageing. In addition, low‐load resistance training produced no significant change in stiffness and hysteresis, but significantly increased the maximal elongation of tendon‐aponeurosis structures from 23.3 ± 2.1 mm to 24.8 ± 2.2 mm (P = 0.045). Conclusion: These results suggest that increasing age results in a decrease in the elasticity of tendon‐aponeurosis structures and an increase in their viscosity. Furthermore, the low‐load resistance training made the elasticity of tendon‐aponeurosis structures increase.     

Muscle strength, volume and activation following 12-month resistance training in 70-year-old males

In elderly males muscle plantar flexor maximal voluntary contraction (MVC) torque normalised to muscle volume (MVC/VOL) is reduced compared to young males as a result of incomplete muscle activation in the elderly. The aim of the present study was to determine the influence of a 12-month resistance training programme on muscle volume, strength, MVC/VOL, agonist activation and antagonist coactivation of the plantarfexors in elderly males. Thirteen elderly males aged 70 years and over (range 70–82 years), completed a 12-month whole body resistance-training programme (TRN), training three times a week. Another eight males (range 18–30 years), who maintained their habitual physical activity for the same 12-month period as the TRN group acted as controls (CTRL). Isometric plantarflexor maximal voluntary contraction (MVC) torque increased in the TRN group by 20% (P<0.01), from 113.1±22.0 Nm to 141.5±19.2 Nm. Triceps surae volume (TS VOL) assessed using MRI, increased by 12%, from 796.3±78.9 cm³ to 916.8±144.4 cm³ . PF activation, measured using supramaximal double twitch interpolation, increased from 83.6±11.0% pre training, to 92.1±7.6% post training (P<0.05). Dorsiflexion MVC and antagonist coactivation (assessed using surface electromyography) did not change with training. Plantarflexor MVC torque normalized for triceps surae muscle volume (MVC/VOL) was 142.6±32.4 kN m^–2 before training and 157.0± 27.9 kN m^–2 after training (a non-significant increase of 8%). No significant change in any measurement was observed in the CTRL group. This study has shown that the gain in muscle strength in response to long-term (12-month) training in older men is mostly accounted for by an increased muscle volume and activation.     

Reaction time and electromyographic activity during a sprint start

Summary Eight male sprinters were filmed running three maximal starts over 3 m on a long force platform. The subjects were divided into two groups (n=4) according to the leg on which the electromyograph (EMG) electrodes were fixed. When in the set position one group had electrodes on the front leg (FLG) and the other group on the rear leg (RLG). The EMG activities of the gastrocnemius caput laterale muscle (GA), vastus lateralis muscle (VL), biceps femoris caput longum muscle (BF), rectus femoris muscle (RF) and gluteus maximus muscle (GM) were recorded telemetrically using surface electrodes. Total reaction time (TRT) was defined as the time from the gun signal until a horizontal force was produced with a value 10% above the base line. Pre-motor time was defined as the time from the gun signal until the onset of EMG activity and motor time (MT) as the time between the onset of EMG activity and that of force production. Reproducibility of the reaction time variables was satisfactory (r=0.792–0.89; coefficient of variation= 8.8%–11.6%). The TRT was 0.121 s, SD 0.014 in FLG and 0.119 s, SD 0.011 in RLG. The MT ranged from 0.008 s, SD 0.009 (GM) to 0.057 s, SD 0.050 (GA) in FLG and from 0.018 s, SD 0.029 (GA) to 0.045 s, SD 0.009 (GM) in RLG. In some individual cases there were no MT values before horizontal force production. Significant positive correlations were observed between MT and maximal horizontal force and the velocity of the centre of gravity during the last observable contact on the blocks (P<0.05–0.01). The EMG activities of the muscles analysed demonstrated large individual variations until the end of the first contact after the blocks. This resulted in non-observable MT in some individual cases. In general, however, despite the complex multijoint character of TRT, its fractions could be analysed during the early phases on the blocks. To optimize starting action it is desirable that all the important muscles should be activated before any force can be detected against the blocks.     

Effects of aerobic exercise conditioning at intensities corresponding to lactate threshold in the elderly

Summary In this study we attempted to determine the effects of exercise training at the intensity corresponding to lactate threshold (Th_la ^–) on various health-related variables in sedentary but apparently healthy elderly subjects. Six men and five women volunteers [mean age 68.9 (SD 3.4) years] performed supervised endurance-type training on stationary cycle ergometers for 30 min and recreational activities for 30 min, 3 days a week for 12 weeks. Four men and four women served as the control group [68.8 (SD 4.4) years]. As a result of the training programme, statistically significant increases in maximal oxygen consumption (10%), oxygen consumption at Th_la ^–(18%), distance covered in 12-min walk, side step, and leg extensor power were found in the training group, while no changes occurred in the control group. The changes in serum cholesterol and triglyceride concentrations from the pre- to post-training period were statistically significant. High-density lipoprotein cholesterol remained unchanged, and low-density lipoprotein cholesterol tended to decrease following the training programme. These data would indicate that exercise training at the intensity corresponding to Th_la ^–may have favourable effects on overall physical fitness and some serum lipid variables in older individuals.     

Adaptation of surround inhibition in the human motor system

Adaptation of a rapid ballistic movement requires that commands for the next movement are updated on the basis of sensory error signals from the current movement. Previous experiments, mostly using visual feedback, have demonstrated that adaptation is highly sensitive to the timing of feedback and can be substantially impaired by delays of 100 ms or so. Here, we use the phenomenon of surround inhibition (SI) to explore the consequences of somatosensory feedback delay in a task requiring participants to flex the index finger without generating any electromyographical (EMG) activity in other fingers. Participants were requested to perform brief isolated flexion movements of the index finger. After a short period of practice, SI in the distant abductor digiti minimi (ADM) muscle was quantified by measuring the amplitude of EMG responses evoked by a standard pulse of transcranial magnetic stimulation to the contralateral motor cortex at the onset of flexion. SI indicates that the response during flexion was smaller than the response at rest. After this, two training blocks were performed in which the ADM muscle was vibrated (80 Hz, 100 ms) either at the onset (VIB_onset) of finger flexion or with a delay of 100 ms (VIB₁₀₀). SI was reassessed after training. SI measured after VIB_onset training was transiently more effective than at baseline. In contrast, SI was unchanged compared to baseline after VIB₁₀₀. The present study demonstrates that SI can be modified by experience. The timing of the sensory stimulation was found to be critical for the modification of SI, suggesting that only sensory signals closely related to the movement onset can induce adaptive changes, presumably through a feed-forward process.     

Differences in strength and surface electromyogram characteristics between pre-pubertal gymnasts and untrained boys during brief and maintained maximal isometric voluntary contractions

The present study was aimed at investigating differences of maximal strength (F _max) of the elbow flexors and characteristics of the surface electromyogram (EMG) between six gymnasts (G) and six untrained (UT) 10-year-old boys during brief and maintained maximal voluntary isometric contraction (MVC). The F _max was estimated during 5 s MVC (maximal test, MT) and normalized to the cross sectional area (CSA) of the arm. The EMG signal of the biceps brachii was recorded during MT and during a 25 s maintained MVC (fatigue test). Values were calculated for root-mean-square (rms_MT) and mean power frequency (MPF_MT) of the EMG signal for the duration of the MT. For the fatigue test, MPF were normalized to the initial value (MPF_n) and kinetics were expressed by the slope coefficient of linear regression. Although F _max and F _max/CSA tended to be higher for G than UT, the differences did not reach significance. The MPF_MT was significantly higher for G [mean (SD)][136 (8) Hz] than for UT [125 (9) Hz]. The MPF_n slope coefficients were significantly greater for G than for UT [–1.0 (0.2) and –0.5 (0.3), respectively]. When all the children were considered, F _max was significantly correlated to MPF_MT (r=0.61). These results showed that gymnasts tend to have higher F _max and F _max/CSA accompanied by a significantly higher MPF_MT and a steeper MPF downshift. Moreover, children with greater strength tended to have higher MPF_MT. It is suggested that spatial and/or temporal recruitment of more fatigable fast motor units could have been enhanced in G and more generally, that it could be a mechanism that would explains, in part, the level of force production in children. Electronic Publication     

Occurrence of electromyographic and ventilatory thresholds in professional road cyclists

The temporal relationship between the electromyographic (EMG) and ventilatory thresholds was investigated during incremental exercise performed by eight professional road cyclists. The exercise, performed on a cycloergometer, started at 100 W with successive increments of 26 W·min^–1 until exhaustion. Gas exchange and the root mean square value of EMG (RMS) from eight lower limb muscles were examined throughout the exercise period. Professional cyclists achieved a maximal oxygen consumption, i.e. O_2max, of 5.4 (0.5) l·min^–1 [74.6 (2.5) ml·min^–1·kg^–1, range: 67.8–82.4 ml·min^–1·kg^–1] and a maximum power (W_max) of 475 (30) W (range: 438–516 W). Our results showed at least the occurrence of a first EMG threshold (EMG_Th1) in 50% (gastrocnemius lateralis) of the subjects and a second EMG threshold (EMG_Th2) in 63% (gastrocnemius medialis). EMG_Th1 occurred significantly before the first ventilatory threshold (VT₁), i.e. at 52 (2)% and 62 (9)% of W_max, respectively. Inversely, no significant difference was observed between the occurrence of EMG_Th2 and the second ventilatory threshold (VT₂), i.e. at 86 (1)% and 89 (7)% of W_max, respectively. These results suggest that the use of EMG may be a useful non-invasive method for detecting the second ventilatory threshold in most of the muscles involved in cycling exercise.     

The relationship between anaerobic threshold and electromyographic fatigue threshold in college women

Summary The purpose of this study was to investigate the relationship between anaerobic threshold (Th_an) and muscle fatigue threshold (EMG_FT) as estimated from electromyographic (EMG) data taken from the quadriceps muscles (vastus lateralis) during exercise on a cycle ergometer. The subjects in this study were 20 female college students, including highly trained endurance athletes and untrained sedentary individuals, whose fitness levels derived from their maximal oxygen consumption ranged from 24.9 to 62.2 ml · kg^–1·min^–1. The rate of increase in integrated EMG (iEMG) activity as a function of time (iEMG slope) was calculated at each of four constant power outputs (350, 300, 250, 200 W), sufficiently high to bring about muscle fatigue. The iEMG slopes so obtained were plotted against the exercise intensities imposed, resulting in linear plots which were extrapolated to zero slope to give an intercept on the power axis which was in turn interpreted as the highest exercise intensity sustainable without electromyographic evidence of neuromuscular fatigue (EMGF_FT). The Th_an was estimated from gas exchange parameters during an incremental exercise test on the same cycle ergometer. The mean results indicated that oxygen uptake (VO₂) at Th_an was 1.391·min^–1, SD 0.44 andVO₂ at EMG_FT was 1.33 1·min^–1, SD 0.57. There was no significant difference between these mean values (P>0.05) and there was a highly significant correlation betweenVO₂ at Th_an andVO₂ at EMG_FT (r=0.823,P<0.01). These data supported the concept of Th_an on the basis that Than was associated with the highest exercise intensity that could be sustained without evidence of neuromuscular fatigue and thus suggested that EMG_FT may provide an attractive alternative to the measurement of Th_an.