EFFECT OF ADDITIONAL ELECTRICAL STIMULATION OF MUSCLES ON MAXIMAL FORCE PRODUCTION

The effect of additional electrical stimulation (ES) of quadriceps femuris muscle on muscle force production was investigated during maximal voluntary isometric contraction (MVC). Eight sport students, all in good condition, participated in the study. ES consisted from the trains of impulses of different length (single impulse, 0.05 s, 0.10 s, 0.15 s, and 0.20 s) with the frequency of 100 Hz. After reaching the maximal voluntary force level, the ES was applied directly to the muscle. Results show the growing muscle force over the MVC level in all stimulating conditions reaching its plateau at 0.10 s of ES (24 % over the MVC). This suggests that the certain minimum time of the additional activation is needed to elicit the whole muscle force capacity. Results may help to elucidate the possible mechanical effects of the reflex activation of the muscles

TORQUE-JOINT ANGLE RELATIONSHIP FOR VASTUS LATERALIS MUSCLE

The aim of this work was to study a single muscle torque contribution during a knsee extension in different knee angles. Six subjects performed three tasks: maximum voluntary knee extension (MVC), supramaximal twitch of relaxed quadriceps femoris muscle, and contraction of relaxed vastus lateralis muscle, activated with a train of submaximal electrical impulses.~A ll contractions were performed at angles of 450, 600, 750, and 900 at the knee joint and in a supine position (hips extended). In both tasks with the electrical stimulation (ES) the same stimulation parameters were used in all angles. Results showed that relative torques (100% at 450) for MVC and the twitch were merely the same, exhibited a maximum (143.6% and 153.1%, respectively) at 750 and fell to 133.3% and 137.4% at 900. Vastus lateralis muscle attain its maximum (1 16.8%) at 600 and fell continuously to its minimum of 602 % at 900. It is possible to see that a single head of the quadriceps muscle behaves in a much different way as the whole quadriceps muscle. Establishing a torque joint angle relationship for single muscles or muscle groups may be interesting for muscles' dynamics modelling, used in a professional work

RECOVERY OF ELECTRICAL STIMULATION FORCE AFTER ONE-MINUTE OF MAXIMAL HOPPING AND CYCLING

INTRODUCTION Recovery of electrical stimulation force depends on a type of peripheral muscular fatigue. Low frequency force, designated to a low frequency fatigue, may recover in a few hours after the workout, while a recovery of a high frequency force, designated to a high frequency fatigue, ends in a few minutes after the workout (Cooper et aJ., 1988). The aim of the present study was to examine a recovery of electrical stim01ation force at 20 Hz and 100 Hz after one-minute of a 'maximal hopping on a force plate and cycling on a Monark ergometer. Eleven students of physical education (age 22. 9± 3.9 yrs, height 176.1 ± 4.1 cm, mass 71.8 ± 3.7 kg) performed both tests on separate weeks. Relaxed vastus lateralis museie (VL) was electrically stimulated before and 1., 2.,3.,4.,5.,6.,7., and 10. minute after the end of the workout and the isometric knee torque during stimulation was measured. Statistically significant changes to the pre-workout result were tested with a two-way student-test. Post-workout results were afterwards normalised to the preworkout torque. Asterisks denote a statistically significant change according to pre-test state (* -

A MODEL FOR THE SIMULATION OF VISCERAL MASS DISPLACENT IN DROP JUMPING

INTRODUCTION - Drop jumps may be mathematically modelled as a mass spring. damper system with a sufficient accuracy (Aurin and Zatsiorsky, 1984). It was stated that a mass of internal viscera does not affect a maximum jumping height and a frequency of jumping. Minetti and Belli (1994) found visceral mass, which presents 14% of the total body mass, oscillating in an opposite phase than a musculosketal mass during hopping and thus significantly influencing the jumping height and frequency of jumping, but also an energy consumption as well. The aim of the visceral mass on jumping height in a single drop jump by mathematical modelling. The model (Fig. 1) consisted of two masses connected by a spring and damper, where mass M2 presented the visceral mass. Elastic module K2 and damping module 82 defined an attachment of M2 to the other parts of the body. The model was described with two differential equations: M , * I , - B 2 * x , + K , * x , - K , * X ? =O Adz * I 2 - B, *X, - KZ *x, =O where MI presented the mass of the external container, XaIn d x2 vertical displa-cements of M1 and M2 from a position of equilibrium, B2 the dam-ping coefficient, K1 and K2 the stiffness coefficients. Numerical solution was performed by MATLAB (The Math works Inc.) The vertical displacement of the centre of gravity of both masses (CG) was calculated by varying K2 and 82 systematically at constant K1. The value for K1 was taken from aurin and Zatsiorsky, 1984. The maximum jumping height was defined as the apex of the trajectory of CG. RESULTS - Dependence of jumping height on K2 and B2 is presented in fig.2. CONCLUSION - Greater K2 and B2 resulted in higher maximum jumping height, indicating the importance of a control of visceral mass for maximising the result. In practice, the greater K2 and 62 can be achieved by increased abdominal pressure. REFERENCES - Aurin, A.S., Zatsiorsky, V.M. (1984) Biomechanical characteristics of human ankle-joint muscles. Eur.J.Appl. Physiology 52: 400406. Minetti, A.E.,Belli, G. (1994) A model for the estimation of visceral mass displacement in periodic movements. J. Biomechanics 27: 97-1 01

INVERSE DYNAMICS OF TAKE-OFF ON SKI-JUMPING SIMULATOR

A special take-off simulator for ski-jumping was constructed to enable training of jumping skills. An inverse dynamics model was used to evaluate a take-off technique on that simulator. A 3D video based kinematical system was used to obtain body coordinates, which were then transformed to 2D sagittal plane. From a five link segment model the joint forces in horizontal and vertical direction as well as joint torques were calculated. Results showed sufficient sensitiveness of the method for a take-off assessment. A vibration effect, probably due to the low approaching velocity and thus a poorer balance, was observable, what was not a case in a normal snow conditions

Effects of acute fatigue on the volitional and magnetically-evoked electromechanical delay of the knee flexors in males and females

Neuromuscular performance capabilities, including those measured by evoked responses, may be adversely affected by fatigue; however, the capability of the neuromuscular system to initiate muscle force rapidly under these circumstances is yet to be established. Sex-differences in the acute responses of neuromuscular performance to exercise stress may be linked to evidence that females are much more vulnerable to ACL injury than males. Optimal functioning of the knee flexors is paramount to the dynamic stabilisation of the knee joint, therefore the aim of this investigation was to examine the effects of acute maximal intensity fatiguing exercise on the voluntary and magnetically-evoked electromechanical delay in the knee flexors of males and females. Knee flexor volitional and magnetically-evoked neuromuscular performance was assessed in seven male and nine females prior to and immediately after: (i) an intervention condition comprising a fatigue trial of 30-seconds maximal static exercise of the knee flexors, (ii) a control condition consisting of no exercise. The results showed that the fatigue intervention was associated with a substantive reduction in volitional peak force (PFV) that was greater in males compared to females (15.0%, 10.2%, respectively, p < 0.01) and impairment to volitional electromechanical delay (EMDV) in females exclusively (19.3%, p < 0.05). Similar improvements in magnetically-evoked electromechanical delay in males and females following fatigue (21%, p < 0.001), however, may suggest a vital facilitatory mechanism to overcome the effects of impaired voluntary capabilities, and a faster neuromuscular response that can be deployed during critical times to protect the joint system

Modulation in voluntary neural drive in relation to muscle soreness

The aim of this study was to investigate whether (1) spinal modulation would change after non-exhausting eccentric exercise of the plantar flexor muscles that produced muscle soreness and (2) central modulation of the motor command would be linked to the development of muscle soreness. Ten healthy subjects volunteered to perform a single bout of backward downhill walking exercise (duration 30 min, velocity 1 ms−1, negative grade −25%, load 12% of body weight). Neuromuscular test sessions [H-reflex, M-wave, maximal voluntary torque (MVT)] were performed before, immediately after, as well as 1–3 days after the exercise bout. Immediately after exercise there was a −15% decrease in MVT of the plantar flexors partly attributable to an alteration in contractile properties (−23% in electrically evoked mechanical twitch). However, MVT failed to recover before the third day whereas the contractile properties had significantly recovered within the first day. This delayed recovery of MVT was likely related to a decrement in voluntary muscle drive. The decrease in voluntary activation occurred in the absence of any variation in spinal modulation estimated from the H-reflex. Our findings suggest the development of a supraspinal modulation perhaps linked to the presence of muscle soreness

Topical Application of Activity-based Probes for Visualization of Brain Tumor Tissue

Several investigators have shown the utility of systemically delivered optical imaging probes to image tumors in small animal models of cancer. Here we demonstrate an innovative method for imaging tumors and tumor margins during surgery. Specifically, we show that optical imaging probes topically applied to tumors and surrounding normal tissue rapidly differentiate between tissues. In contrast to systemic delivery of optical imaging probes which label tumors uniformly over time, topical probe application results in rapid and robust probe activation that is detectable as early as 5 minutes following application. Importantly, labeling is primarily associated with peri-tumor spaces. This methodology provides a means for rapid visualization of tumor and potentially infiltrating tumor cells and has potential applications for directed surgical excision of tumor tissues. Furthermore, this technology could find use in surgical resections for any tumors having differential regulation of cysteine cathepsin activity

Effects of caffeine on neuromuscular fatigue and performance during high-intensity cycling exercise in moderate hypoxia

Purpose: To investigate the effects of caffeine on performance, neuromuscular fatigue and perception of effort during high-intensity cycling exercise in moderate hypoxia. Methods: Seven adult male participants firstly underwent an incremental exercise test on a cycle ergometer in conditions of acute normobaric hypoxia (fraction inspired oxygen = 0.15) to establish peak power output (PPO). In the following two visits, they performed a time to exhaustion test (78 ± 3% PPO) in the same hypoxic conditions after caffeine ingestion (4 mg kg$^{−1}$) and one after placebo ingestion in a double-blind, randomized, counterbalanced cross-over design. Results: Caffeine significantly improved time to exhaustion by 12%. A significant decrease in subjective fatigue was found after caffeine consumption. Perception of effort and surface electromyographic signal amplitude of the vastus lateralis were lower and heart rate was higher in the caffeine condition when compared to placebo. However, caffeine did not reduce the peripheral and central fatigue induced by high-intensity cycling exercise in moderate hypoxia. Conclusion: The caffeine-induced improvement in time to exhaustion during high-intensity cycling exercise in moderate hypoxia seems to be mediated by a reduction in perception of effort, which occurs despite no reduction in neuromuscular fatigue