Biomedical subjects

V Strojnik

Publications and source records attributed to V Strojnik.

13 recordsLinked to original sources

PubMed2006

Maximal force during eccentric and isometric actions at different elbow angles.

The purpose of this study was to examine a course of force potentiation and/or inhibition during maximal voluntary eccentric action. Maximal voluntary force (MVC) of elbow flexion of ten healthy male volunteers was measured during isometric and isokinetic eccentric action starting from 80 degrees or 110 degrees and ending at 140 degrees elbow angle. Surface EMG was recorded from biceps brachii (BB) and brachioradialis (BR) muscles. Maximal voluntary eccentric force during the first 10 degrees of the movement was higher (P<0.001) than the maximal voluntary isometric preactivation force both in 80 degrees and in 110 degrees starting position at all three velocities (1, 2, and 4 rad s(-1)). The relative force potentiation was velocity dependent being smallest at the lowest stretching speed (P<0.01). Average EMG (aEMG) of BB and BR decreased as the joint angle increased both in eccentric and in isometric actions but the decrease in aEMG towards extension was somewhat higher in eccentric actions as compared to isometric. It was concluded that the force measured during the first 10 degrees of eccentric contraction always exceeded the maximal voluntary isometric preactivation force regardless of the joint angle or of the movement velocity. When maximal voluntary preactivation preceded the stretch, the relative force potentiation seemed to be greater at higher stretching velocities (velocity dependent) while at lower preactivation levels, the velocity dependence was not observed. Decreased muscle activation and lower maximal voluntary force towards the end of the movement suggested inhibition during maximal voluntary eccentric actions.

Adult↗

PubMed2006

Neuro-muscular fatigue and recovery dynamics following anaerobic interval workload.

The aim of this study was to determine the influence of anaerobic running on muscle contractile characteristics and voluntary muscle activation level during MVC as well as the dynamics of their recovery during a 2-hour period. Seven well-trained runners performed 5 x 300 m at submaximal velocity with a 1-minute active recovery interval between the runs. The average run velocity was 6.69 m.s(-1), which represented 77 % of their top velocity. Contractile characteristics of the vastus lateralis and activation level of quadriceps femoris muscles were measured before and immediately after the runs and within the 120-minute time interval that followed the workload. To do this we used: single twitch, low- and high-frequency electrical stimulation, maximal voluntary knee extension test, and muscle activation level test. After the exercise the maximal twitch torque (T(TW)) decreased for 28 +/- 3.7 % (p < 0.001) and torque at stimulation with 20 Hz and 100 Hz were 19.2 +/- 4.6 % (p < 0.01) and 7.5 +/- 2.3 % (p < 0.05) lower, respectively, while MVC torque and activation level remained unchanged. Subjects with higher blood lactate accumulation level showed significant decrease in the torque at low frequency stimulation (T(F20)) (r = - 0.80; p < 0.01) and T(TW) (r = - 0.92; p < 0.01). The restoration of twitch torque took a short time despite the fact that blood lactate concentration remained high. Ten minutes after the last interval run the twitch torque exceeded the pre-workload value by 11 % (p < 0.01). Potentiation lasted until the 40th min. It was concluded that fatigue after the anaerobic interval workload was peripheral in character and caused by contractile mechanisms disturbances.

Adult↗

PubMed2006

Neuromuscular fatigue and recovery dynamics following prolonged continuous run at anaerobic threshold.

OBJECTIVE: The aim of this study was to determine the influence of intensive aerobic running on some muscle contractile characteristics and the dynamics of their recovery during a 2 hour period afterwards. METHODS: Seven well trained runners performed a 6 km run at anaerobic threshold (V(OBLA)). Knee torque during single twitch, low and high frequency electrical stimulation (ES), maximum voluntary knee extension, and muscle activation level test of the quadriceps femoris muscles were measured before and immediately after the run, and at several time points during a 120 minute interval that followed the run. RESULTS: After exercise, the mean (SE) maximum twitch torque (T(TW)) and torque at ES with 20 Hz (low frequency ES; T(F20)) dropped by 14.1 (5.1)% (p<0.05) and 20.6 (7.9)% (p<0.05) respectively, while torque at stimulation with 100 Hz (high frequency ES; T(F100)), maximum isometric knee extension torque (maximum voluntary contraction torque; T(MVC)), and activation level did not change significantly. Twitch contraction time was shortened by 8 (2)% (p<0.05). Ten minutes after the run, T(TW) was 40% higher than immediately after the run and 10% (p<0.05) higher than before the run. T(F20), T(F100), and T(MVC) remained lower for 60 minutes (p<0.05) than before the run. CONCLUSIONS: A 6 km continuous run at V(OBLA) caused peripheral fatigue by impairing excitation-contraction coupling. Twitch torque recovered very quickly. However, the process of torque restoration at maximum isometric knee extension torque and at high and low frequency ES took much longer.

Adult↗

PubMed2002

EMG power spectrum and features of the superimposed M-wave during voluntary eccentric and concentric actions at different activation levels.

Muscle fiber conduction velocity (CV) may be affected by the level of voluntary activation and by the diameter of the fiber. Both the frequency component of the electromyography (EMG) power spectrum, such the as median frequency (MF) or mean power frequency, and the duration of muscle compound action potential to single supramaximal electrical impulse (maximal M-wave) may be related to CV. The aim of the present study was to examine how changes in the activation level in lengthening and in shortening conditions would affect the EMG power spectrum during voluntary effort, and compare them to changes in M-wave shape in similar conditions. Ten male subjects performed eccentric and concentric knee extensions at force levels of 40%, 60%, 80% and 100% of maximal eccentric and concentric knee extension force (maximum voluntary contraction, MVC) at an angular velocity of 2 rad.s(-1). In order to measure the M-wave at each force level and in a relaxed condition, a supramaximal electrical stimulus was given to the femoral nerve. The surface EMG was recorded from the vastus lateralis, vastus medialis, and rectus femoris muscles, and the average EMG (aEMG) and MF were calculated. The results show that although the absolute force was greater, the aEMG was generally lower in eccentric as compared to concentric actions at all of the force levels tested. Although the aEMG increased as force increased, no consistent differences were observed in the amplitude of the maximal M-wave in any of the conditions, or in the duration of the M-wave between eccentric and concentric actions. However, as the force level increased the duration of the M-wave decreased significantly ( P<0.01) for both eccentric and concentric actions. On average, no major differences were observed in MF between eccentric and concentric actions or between the force levels in either type of contraction, although rather large variations were observed throughout the motions. In voluntary situations, the recruitment of fast motor units with higher muscle fiber CVs and the increased firing rate of the active units increases the muscle fiber CV as the activation level increases. Even though in conditions of supramaximal electrical nerve stimulation all motor units should be activated simultaneously, the duration of the M-wave in the present study decreased as the force level increased. Possible candidates for the change in the CV may be increased activation of the Na(+)/K(+) pump resulting from the activity in the muscle preceding the electrical stimulation and/or changes in the muscle fiber length between different force levels.

Adolescent↗

PubMed2001

Electromyogram power spectrum and features of the superimposed maximal M-wave during voluntary isometric actions in humans at different activation levels.

The frequency characteristics of the electromyogram (EMG) power spectrum, such as the median or the mean power frequency, as well as the duration of the muscle compound action potential response to a single supramaximal electrical stimulus (maximal M-wave) may both be related to the conduction velocity (CV) of the muscle fibre. To investigate this further, we studied in ten male subjects: the EMG of the vastus lateralis, vastus medialis and rectus femoris muscles during maximal isometric knee extensions at 40%, 60%, 80% and 100% of maximal voluntary contraction and also the maximal M-wave, elicited by a single supramaximal stimulus to the femoral nerve, of the same muscles at rest or superimposed on the same levels of voluntary contraction. The EMG was recorded during the constant force phase of the voluntary contractions, the duration of which was 2.5-4 s, with a 1.5 min pause between contractions. The average EMG (aEMG) and the median frequency (MF) were then calculated. The results indicated that as aEMG increased with increase in force, MF remained unchanged. However, while the amplitude of the M-wave was not affected, the duration of the M-wave was shorter as the force level increased. The duration of the M-wave may be affected by recruitment of faster motor units, by increased firing rate of the active units and by changes in the muscle fibre length. The shorter duration of the M-wave observed at higher force levels was not, however, accompanied by a corresponding increase in MF. The MF could not therefore be used as a parameter to reflect the changes in voluntary muscle activation and CV. When MF was calculated by overlapping the fast Fourier transform (FFT) windows (0.4 s long window moved data point by data point to the right) for 1 s periods of the isometric plateau phase at each force level, the difference between the lowest and the highest MF was quite substantial. These variations suggest that FFT window placement, i.e. which part of the signal is chosen for the analyses, may play an important role even in isometric situations.

Adult↗

PubMed2001

The effect of electrostimulation and high load exercises in patients with patellofemoral joint dysfunction. A preliminary report.

Traditional conservative treatment for patellar disorders is successful in about 80 percents of cases. We introduced two new conservative treatment protocols for patellar pathology in order to further improve the success rate. The first protocol consisted of high load/low repetition quadriceps femoris training (10 patients) while the second enclosed selective electrostimulation of vastus medialis muscle (7 patients). Results were evaluated clinically and neurophysiologically. High load/low repetition training resulted in significant increase of maximal voluntary contraction of quadriceps muscle (P < 0.001). Significant gain of Activity (P = 0.017) and Kujala scores (P = 0.07) was observed in group with high load/low repetition quadriceps training compared to patients with electrostimulation. There was no significant change in neurophysiological or clinical status between the beginning and the end of treatment with electrostimulation. Our results indicate that high load/low repetition quadriceps femoris training poses an important alternative to traditional conservative treatment protocol for patellar disorders.

Electric Stimulation↗

PubMed2001

EMG power spectrum and maximal M-wave during eccentric and concentric actions at different force levels.

The aim of the present study was to examine changes in the EMG power spectrum and in the maximal M-wave due to different activation levels of eccentric and concentric actions. Average EMG (aEMG) increased as the force increased but no consistent differences were observed in the amplitude of the maximal M-wave in any of the conditions. However, as the force level increased the duration of the M-wave decreased significantly (P<0.01) both in eccentric and concentric actions while no differences were observed between the action types. On the average, no major differences were observed in median frequency (MF) between any of the conditions, although rather large variations were observed throughout the motions. Increased muscle fiber conduction velocity (CV), as indicated by the decreased duration of the M-wave, could be due to recruitment of fast motor units with higher muscle fiber conduction velocity and increased firing rate of the active units. Also the changes in the muscle fiber length between the force levels may have affected CV.

Adult↗

PubMed2000

Fatigue after submaximal intensive stretch-shortening cycle exercise.

OBJECTIVE: The aim of the present study was to examine some sites of neuromuscular fatigue after submaximal intensity stretch-shortening cycle exercise. METHODS: Twelve male subjects performed consecutive sledge jumps at 60% of maximal height until exhaustion (mean duration 443.7 s +/- 304.9 s, mean +/- SD). RESULTS: During the exercise, the blood lactate increased from 1.8 +/- 0.6 mmol x L(-1) (before exercise) to 6.1 +/- 1.7 mmol x L(-1) (P < 0.001) and serum creatine-kinase from 248 +/- 142 IU x L(-1) to 584 +/- 344 IU x L(-1) (P < 0.001). Electrical stimulation of the vastus lateralis and quadriceps femoris muscles to induce isometric knee extension resulted in decreased peak torque during single and double twitch after workout (from 22.1 +/- 6.3 Nm to 17.3 +/- 8.0 Nm, P < 0.05, and from 96.6 +/- 15.4 Nm to 76.2 x 19.8 Nm, P < 0.001, respectively), whereas there were no significant changes in contraction and relaxation times. Torque during 20-Hz stimulation decreased significantly (from 23.7 +/- 9.2 to 16.1 +/- 7.8 Nm, P < 0.01) but not at 100-Hz stimulation. During maximal voluntary isometric knee extensions, the rate of torque development was significantly (P < 0.01) more impaired than maximal torque (from 1619 +/- 390 Nm x s(-1) to 1,004 +/- 360 Nm x s(-1) and from 185 +/- 30.7 Nm to 151 +/- 32.3 Nm, respectively, both P < 0.001). At the same time, the muscle activation level increased by 15.8 +/- 24.1% (P < 0.05). The mean EMG amplitude of vastus lateralis during MVC increased by 34.9 +/- 39.2% (P > 0.05). CONCLUSION: It was concluded that after submaximal stretch-shortening exercise, the low-frequency fatigue occurred, very likely caused by lower Ca2+ release per single action potential.

Action Potentials↗

PubMed1998

Neuromuscular fatigue after maximal stretch-shortening cycle exercise.

To examine some possible sites of fatigue during short-lasting maximally intensive stretch-shortening cycle exercise, drop jumps on an inclined sledge apparatus were analyzed. Twelve healthy volunteers performed jumps until they were unable to maintain jumping height > 90% of their maximum. After the workout, the increases in the blood lactate concentration and serum creatine kinase activation were statistically significant (P < 0.001 and P < 0.05, respectively) but rather small in physiological terms. The major changes after the workout were as follows: the single twitch was characterized by smaller peak torque (P < 0.05) and shorter time to peak (P < 0.05) and half-relaxation time (P < 0.01). The double-twitch torque remained at the same level (P > 0.05), but with a steeper maximal slope of torque rise (P < 0.05); during 20- and 100-Hz stimulation the torque declined (both P < 0.01) and the maximal voluntary torque changed nonsignificantly but with a smaller maximal slope of torque rise (P < 0.01) and a higher activation level (P < 0.05), accompanied by an increased electromyogram amplitude. These findings indicate that the muscle response after the short-lasting consecutive maximum jumps on the sledge apparatus may involve two distinct mechanisms acting in opposite directions: 1) The contractile mechanism seems to be potentiated through a shorter Ca2+ transient and faster cross-bridge cycling, as implied by twitch changes. 2) High-frequency action potential propagation shows an impairment, which is suggested as the possible dominant reason for fatigue in exercise of this type.

Action Potentials↗

PubMed1998

The effects of superimposed electrical stimulation of the quadriceps muscles on performance in different motor tasks.

BACKGROUND: Some examples are presented to show how superimposed electrical stimulation of muscles during maximum voluntary effort can be used to enable subjects to achieve greater joint torque, rate of joint torque development, and the joint angular velocity as when performing only voluntarily with maximum effort. METHODS PARTICIPANTS: Ten students of physical education not involved into a regular sport training participated the study. INTERVENTIONS: The subjects performed isometric and concentric knee extensions and squat jumps, without and with superimposed electrical stimulation of the quadriceps muscles. MEASURES: A knee torque and angular displacement were observed during isometric and concentric knee extension. During the jumping, a vertical component of the ground reaction force was measured. RESULTS: Results showed that the maximal knee torque, as well as a torque rise, were higher in isometric knee extension trials with superimposed electrical stimulation. The same was observed in concentric knee extensions, where greater knee torque during superimposed electrical stimulation also resulted in higher angular velocity and a more extended knee at the end of the movement. In squat jumps, the application of the superimposed electrical stimulation resulted in poorer performance. CONCLUSIONS: It was concluded that superimposed electrical stimulation was effective in some simple motor tasks, but not in more complex tasks where a co-ordination of multiple muscles of different groups was necessary for improved performance.

Acceleration↗

PubMed1995

Muscle activation level during maximal voluntary effort.

To assess the extent of the level of muscle activation during maximal voluntary effort, a method of superimposed trains of electrical impulses delivered at 100 Hz was employed. During a maximal voluntary contraction (MVC) in isometric knee extension, a submaximal superimposed electrical stimulation (ES) of differing train durations was induced to the quadriceps muscle, when maximal voluntary torque was achieved. For all train durations the force increased during ES. During 100 ms and longer trains, the additional torque reached a plateau. The same principle of submaximal electrical stimulation superimposed over MVC was used in explosive isometric knee extensions, comparing the rate of force growth in trials with and without ES. In all subjects ES had an augmenting effect during the increase in force and up to the maximal force. It was concluded that the subjects were not able voluntarily to activate fully their quadriceps muscle. This was true both for maximal force and for the increase in force. It seems that the existence of an activation deficit is a fact, the question is how to quantify it in a reliable and valid way.

Adult↗

PubMed1995

Cross-section areas of calf muscles in athletes of different sports.

Differences in cross-section areas of calf structures were studied in different sportsmen groups and controls: sprinters, long-distance runners, free-climbers, and sport non active persons. The cross-section areas of calf tissues were established by magnetic resonance imaging. There were no statistically significant differences between the groups in single muscle areas. The only statistically significant difference was found in subcutaneous fat between sportsmen and non active persons. Results show a general adaptive tendency of the muscle to greater activity, without exhibiting sport-group specificity differences.

Adaptation, Physiological↗

PubMed1992

Behaviour of triceps surae muscle-tendon complex in different jump conditions.

The force-length relationship of the human muscle-tendon complex (MTC) of the triceps surae and the achilles tendon was investigated in various stretch load conditions. Six male subjects performed various vertical jumps with maximal effort: squat jumps (SJ), counter movement jumps (CMJ) and drop jumps (DJ) from a height of 24 cm, 40 cm and 56 cm. The force-length relationship was calculated from the signals of the components of the ground reaction forces and the kinematic data obtained from the high-speed film records. Surface electromyograms (EMG) of the soleus, gastrocnemius and tibialis anterior muscles were also recorded. The force-length diagrams showed individually high sensitivity to the imposed stretch load. In conditions with relatively low stretch load requirements there was a counter-clockwise direction observable, indicating that the energy absorbed during the eccentric, or lengthening phase was lower than the energy delivered during the concentric, or shortening phase. In high load conditions this relationship was reversed indicating a negative energy balance. The EMG-length diagrams of SJ and CMJ consisted of an initial isometric loading of the muscle, followed by a shortening phase with only slightly reduced EMG amplitudes. In DJ, however, the diagrams showed an initial lengthening of the MTC with fairly constant activation amplitudes. After 40 ms an isometric loading of the muscle, lasting for approximately 80 ms, was followed by a shortening phase. It was concluded that segmental stretch reflex activation represented the predominant activation process during the isometric loading phase, to meet the adequate stiffness properties of the MTC.

Achilles Tendon↗