GH responses to two consecutive bouts of neuromuscular electrical stimulation in healthy adults

BACKGROUND

It is well established that repeated GHRH administration or repeated voluntary exercise bouts are associated with a complete blunting of GH responsiveness when the administration of the second stimulus follows the first one after a 2-h interval.

AIM

To evaluate GH responses to neuromuscular electrical stimulation (NMES) in healthy adults.

METHODS

Six volunteers (mean age+/-s.d. 31.7+/-5.5 years) were studied before and after two consecutive bouts of NMES exercise (a series of 20 contractions at the maximum of individual tolerance, frequency: 75 Hz, pulse duration: 400 mus, on-off ratio: 6.25-20 s) administered at a 2-h interval.

RESULTS

Baseline GH levels (mean: 0.3+/-0.2 ng/ml) significantly increased after the first NMES (peak: 4.2+/-3.7 ng/ml), with a complete normalization after 120 min (0.3+/-0.3 ng/ml). The administration of the second bout of NMES of comparable characteristics also resulted in a significant GH increase (peak: 5.2+/-3.2 ng/ml), which was comparable with that observed after the previous one. GH net incremental area under the curve after the first and second bouts of NMES were not significantly different (155.1+/-148.5 and 176.9+/-123.3 ng/ml per h, P=0.785).

CONCLUSIONS

Unlike repeated pharmacological stimuli and voluntary exercise bouts, subsequent sessions of NMES administered at a 2-h interval appear to circumvent feedback mechanisms and to re-induce the GH responses, thus indicating a possible different underlying mechanism elicited by different GH-releasing stimuli.

Comparison between voluntary and stimulated contractions of the quadriceps femoris for growth hormone response and muscle damage

This study aimed to compare voluntary and stimulated exercise for changes in muscle strength, growth hormone (GH), blood lactate, and markers of muscle damage. Nine healthy men had two leg press exercise bouts separated by 2 wk. In the first bout, the quadriceps muscles were stimulated by biphasic rectangular pulses (75 Hz, duration 400 μs, on-off ratio 6.25–20 s) with current amplitude being consistently increased throughout 40 contractions at maximal tolerable level. In the second bout, 40 voluntary isometric contractions were performed at the same leg press force output as the first bout. Maximal voluntary isometric strength was measured before and after the bouts, and serum GH and blood lactate concentrations were measured before, during, and after exercise. Serum creatine kinase (CK) activity and muscle soreness were assessed before, immediately after, and 24, 48, and 72 h after exercise. Maximal voluntary strength decreased significantly ( P < 0.05) after both bouts, but the magnitude of the decrease was significantly ( P < 0.05) greater for the stimulated contractions (−22%) compared with the voluntary contractions (−9%). Increases in serum GH and lactate concentrations were significantly ( P < 0.05) larger after the stimulation compared with the voluntary exercise. Increases in serum CK activity and muscle soreness were also significantly ( P < 0.05) greater for the stimulation than voluntary exercise. It was concluded that a single bout of electrical stimulation exercise resulted in greater GH response and muscle damage than voluntary exercise.

Effect of electrostimulation training-detraining on neuromuscular fatigue mechanisms

The aim of this study was to evaluate the effects of neuromuscular electrical stimulation (NMES) training and subsequent detraining on neuromuscular fatigue mechanisms. Ten young healthy men completed one NMES fatigue protocol before and after a NMES training program of 4 weeks and again after 4 weeks of detraining. Muscle fatigue (maximal voluntary torque loss), central fatigue (activation failure), and peripheral fatigue (transmission failure and contractile failure) of the plantar flexor muscles were assessed by using a series of electrically evoked and voluntary contractions with concomitant electromyographic and torque recordings. At baseline, maximal voluntary torque decreased significantly with fatigue (P<0.001), due to both activation and transmission failure. After detraining, maximal voluntary torque loss was significantly reduced (P<0.05). In the same way, the relative decrease in muscle activation after training and detraining was significantly lower compared to baseline values (P<0.05). Short-term NMES training-detraining of the plantar flexor muscles significantly reduced the muscle fatigue associated to one single NMES exercise session. This was mainly attributable to a reduction in activation failure, i.e., lower central fatigue, probably as a result of subject's accommodation to pain and discomfort during NMES.

Random motor unit activation by electrostimulation

Whether the involvement of motor units is different between surface neuromuscular electrostimulation and voluntary activation remains an unresolved issue. The aim of this pilot study was to verify if motor unit activation during electrostimulation is nonselective/random (i.e., without obvious sequencing related to fibre type), as recently suggested by Gregory and Bickel [6]. Sixteen healthy men randomly performed submaximal isometric contractions (10-s duration) of the quadriceps femoris muscle at 20, 40 and 60 % of maximal voluntary torque under both stimulated and voluntary conditions. During the contractions, paired stimuli were delivered to the femoral nerve (twitch interpolation technique) and the characteristics of the superimposed doublet were compared between the two conditions. For each torque level, time-to-peak torque was significantly longer (p range = 0.05 - 0.0002) during electrostimulation compared to voluntary contractions. Moreover, time-to-peak torque during voluntary trials decreased significantly when increasing the torque level from 20 to 60 % of maximal voluntary torque (p range = 0.03 - 0.0001), whereas it was unchanged during electrostimulation. In conclusion, over-the-muscle electrostimulation would neither result in motor unit recruitment according to Henneman's size principle nor would it result in a reversal in voluntary recruitment order. During electrostimulation, muscle fibres are activated without obvious sequencing related to fibre type.

Differences in quadriceps muscle strength and fatigue between lean and obese subjects

The present study aimed to compare quadriceps femoris muscle strength and fatigue between obese (grade II and III) and nonobese adults. Ten obese (mean age: 25 years; mean BMI: 41 kg/m(2)) and ten lean (mean age: 27 years; mean BMI: 23 kg/m(2)) men were tested. Quadriceps muscle fatigue was quantified as the (percent) torque loss during a voluntary isokinetic (50 maximal contractions at 180 degrees /s) and an electrostimulated (40 Hz) isometric protocol (5 min, 10% of the maximal torque). Maximal voluntary isometric and isokinetic torque and power were also measured. Voluntary torque loss was significantly higher (P < 0.05) in obese (-63.5%) than in lean subjects (-50.6%). Stimulated torque decreased significantly (P < 0.05) but equally in the two subject groups. Obese subjects displayed higher absolute (+20%; P < 0.01) but lower relative (i.e., normalized to body mass) (-32%; P < 0.001) muscle torque and power than their lean counterparts. Obese individuals demonstrated lower fatigue resistance during voluntary but not during stimulated knee extensions compared to their nonobese counterparts. Peripheral mechanisms of muscle fatigue -- at least those associated to the present stimulated test -- were not influenced by obesity. The observed quadriceps muscle function impairments (voluntary fatigue and relative strength) probably contribute to the reduced functional capacity of obese subjects during daily living activities.

Central and peripheral contributions to fatigue after electrostimulation training

PURPOSE

We examined the effect of 4 (WK4) and 8 wk (WK8) of neuromuscular electrical stimulation (NMES) training on both endurance time and mechanisms contributing to task failure.

METHODS

Ten males performed a fatiguing isometric contraction with the knee extensor muscles at 20% of maximal voluntary contraction (MVC) until exhaustion before (B), at WK4, and at WK8 of NMES training. The electromyographic (EMG) activity and muscle activation obtained under MVC were recorded before and after the fatiguing task to assess central fatigue. Torque and EMG responses obtained under electrically evoked contractions were examined before and after the fatiguing task to analyze peripheral fatigue.

RESULTS

Knee extensor MVC torque increased significantly between B and WK4 (+16%), between WK4 and WK8 (+10%), and between B and WK8 (+26%), which meant that the average target torque sustained during the fatiguing contraction increased between the testing sessions. Endurance time decreased significantly over the three sessions (493+/-101 s at B, 408+/-159 s at WK4, and 338+/-126 s at WK8) despite a similar reduction in knee extensor MVC (approximately 25%). Negative correlations were found between endurance time absolute changes and target torque absolute gains. Average EMG activity of the knee extensor muscles was lower after training, but the mean rate of increase was similar over the three sessions. Single-twitch contractile properties were not affected by the task.

CONCLUSION

We conclude that the endurance time was shorter after 4 and 8 wk of NMES training, and this was associated with higher absolute contraction intensity. Despite endurance time reduction, NMES training did not affect the amount of fatigue at exhaustion nor the central and peripheral contributions to fatigue.

Late neural adaptations to electrostimulation resistance training of the plantar flexor muscles

The present study aimed to examine early and late neural adaptations to short-term electrostimulation training of the plantar flexor muscles. Changes in triceps surae muscle activation (twitch interpolation), maximal electromyographic (EMG) activity, H-reflex amplitudes and antagonist coactivation were investigated after electrostimulation training (4 weeks) and after 4 weeks of detraining in a group of ten young healthy men. Maximal voluntary contraction torque was significantly higher (P < 0.01) after training (+19.4%) and detraining (+17.2%) with respect to baseline. Activation level, soleus and lateral gastrocnemius EMG normalized to the maximal M-wave significantly increased as a result of training (P < 0.05), and these gains were preserved after detraining, excepted for soleus EMG. Maximal H reflex to maximal M wave ratio increased significantly between baseline and detraining for both soleus and lateral gastrocnemius muscles (P < 0.05). Tibialis anterior coactivation was unchanged after training but significantly decreased after the detraining period (P < 0.01). Short-term electrostimulation resistance training was accompanied by early (increased muscle activation and EMG activity) and late neural adaptations (increased spinal reflex amplitude and decreased coactivation), likely explaining the increase and then the preservation of the maximal voluntary strength. These effects may help in conceiving and programming effective electrostimulation therapy programs for both healthy and immobilized plantar flexor muscles.

A follow-up of GH-dependent biomarkers during a 6-month period of the sporting season of male and female athletes

In order to verify the effects of the sporting season (entailing periods of training, competition, recovery, resting) on GH-dependent parameters in male and female athletes from different sporting disciplines, 47 male and female athletes (3 rowers, 5 swimmers, 7 alpine skiers, 3 soccer players, 7 middle distance runners, 14 sprinters, 4 triathletes, 1 road walker, 3 cyclists) were followed-up for a period of 6 months. Blood samples were taken every two months for the evaluation of IGF-I, N-terminal propeptide of type III procollagen (PIIINP) and C-terminal cross-linked telopeptide of type I collagen (ICTP). Abnormal IGF-I, PIIINP and ICTP levels were observed during the follow-up period in 7/100 (7%), 9/100 (9.0%) and 8/100 (8%) samples of the male group, respectively, and in 9/88 (10.2%), 1/88 (1.1%) and 0/88 (0%) samples of the female group, respectively. Abnormal levels appeared to be randomly distributed over the different periods of the sporting season and within male and female subjects, with the large majority of abnormal values being found in the younger athletes. Taking into account all the tests done during the 6-month period (no. 564), individual markers falling outside the normal range (for age) were observed in a small number of instances (34/564 tests done, 24/300 for males and 10/264 for females). When our method for the detection of exogenous recombinant GH (rhGH) administration, based on the concomitant determination of these three peripheral GH-dependent markers and on the attribution of specific scores, was applied in the same athlete at a given time point of the 6-month period, the prevalence of a positive score was extremely low (ie, 3/188 samples or 1.6%). Total positive scores were actually recorded in only three male athletes (2 swimmers and 1 skier, aged <21 yr) at one occasion during the 6-month period considered. In contrast, no total positive scores were found in female athletes (ie, 0/88 samples). In conclusion, the concentrations of IGF-I, PIIINP and ICTP were stable and not significantly modified during 6 months of a sporting season (entailing periods of training, competition, recovery, resting) in athletes from different sporting disciplines. Therefore our method, based on the concomitant determination of three peripheral GH-dependent biomarkers appears safe, acceptable, relatively inexpensive and repeatable (in case of positive or suspected values) immediately or at different intervals of the sporting season. Further additional studies are requested to precise the cut-off values for narrower age-class subdivisions in both genders in order to improve the proposed method.

Neuromuscular Adaptations to Electrostimulation Resistance Training

A combination of in vivo and in vitro analyses was performed to investigate muscular and neural adaptations of the weaker (nondominant) quadriceps femoris muscle of one healthy individual to short-term electrostimulation resistance training. The increase in maximal voluntary strength (+12%) was accompanied by neural (cross-education effect and increased muscle activation) and muscle adaptations (impairment of whole-muscle contractile properties). Significant changes in myosin heavy chain (MHC) isoforms relative content (+22% for MHC-2A and -28% for MHC-2X), single-fiber cross-sectional area (+27% for type 1 and +6% for type 2A muscle fibers), and specific tension of type 1 (+67%) but not type 2A fibers were also observed after training. Plastic changes in neural control confirm the possible involvement of both spinal and supraspinal structures to electrically evoked contractions. Changes at the single muscle fiber level induced by electrostimulation resistance training were significant and preferentially affected slow, type 1 fibers.

Central and Peripheral Fatigue of the Knee Extensor Muscles Induced by Electromyostimulation

The main purpose of this study was to characterise neuromuscular fatigue induced by 30 contractions of the knee extensor muscles evoked by electromyostimulation (EMS). Twelve healthy subjects were tested before and after a typical EMS session (frequency: 75 Hz, on-off ratio: 6.25 s on-20 s off) used for quadriceps femoris muscle strengthening. Surface electromyographic (EMG) activity and torque obtained during maximal voluntary and electrically evoked contractions were analysed to distinguish peripheral from central fatigue. Maximal voluntary torque of the knee extensor muscles decreased approximately 20 % (p < 0.001) following EMS. In the same way, peak torque associated to single (p < 0.05) and paired (p < 0.001) stimuli as well as M-wave amplitude (p < 0.05) significantly decreased as a result of EMS. The raw EMG activity of both vastus lateralis and rectus femoris muscle recorded during maximal voluntary isometric contraction significantly decreased after the session (-17.3 and -14.5 %, respectively) whereas no changes were observed when EMG signals were normalised to respective M-wave amplitudes. Similarly, voluntary activation estimated by using the twitch interpolation technique was unchanged following EMS. In conclusion, a typical session of EMS of the knee extensor muscles mainly induced neuromuscular propagation failure while excitation-contraction coupling and neural mechanisms were not significantly affected. It is recommended to interpret surface EMG data together with the corresponding M wave, at least for the knee extensor muscles, in order to distinguish peripheral from central causes of fatigue.

Central and peripheral fatigue after electrostimulation-induced resistance exercise

PURPOSE

To investigate central and peripheral fatigue induced by a typical session of electromyostimulation (EMS) of the triceps surae muscle.

METHODS

A series of neuromuscular tests including voluntary and electrically evoked contractions were performed before and immediately after 13 min of EMS (75 Hz) in 10 healthy individuals.

RESULTS

Maximal voluntary contraction torque of the plantar flexor muscles significantly decreased (-9.4%; P < 0.001) after EMS, and this was accompanied by an impairment of central activation, as attested by twitch interpolation results (P < 0.05), whereas soleus maximal Hoffmann reflex and tibialis anterior coactivation did not change significantly. Contractile properties associated with paired stimuli and maximal M-wave amplitude for both soleus and medial gastrocnemius muscles (-9.4 and -38.7%, respectively) were significantly affected by EMS (P < 0.05), whereas postactivation potentiation did not change.

CONCLUSION

A single bout of EMS resulted in fatigue attributable to both central and peripheral factors. The most obvious alteration in the function of the central nervous system is a decrease in the quantity of the neural drive to muscle from the supraspinal centers. On the other hand, neuromuscular propagation failure was more evident for the muscle with the higher percentage of Type II fibers.