Using first bout effect to study the mechanisms underlying eccentric exercise induced force loss.

INTRODUCTION: The first bout of eccentric exercise is known to have a protective effect on the consequent bouts. This effect is still disputable as it is not known whether it protects muscle damage by reducing force production or by improving force recovery in the healing process. The underlying mechanisms of this protective effect have not been fully understood. OBJECTIVES: To determine the mechanisms of this protective effect, three different loads were used for the first eccentric bout. This was done to investigate whether the protective effect is related to the size of the load in the first bout. To determine the neural adaptations, voluntary activation was assessed and to determine the muscular adaptations, the resting twitch was measured. METHOD: Thirty healthy participants were selectively allocated into three groups (low-, moderate- and high-load group) to match for maximal voluntary contraction (MVC) (n = 10 per group). Participants in each group performed only one of the three sets of ten eccentric (ECC) exercises of the elbow flexors (10%, 20% and 40% of MVC) as their first eccentric bout. The second bout of eccentric exercise was performed two weeks later and was identical for all the three groups, i.e., 40% ECC. RESULTS: The results showed that for the first bout, MVC, voluntary activation and the resting twitch displayed significant (p < 0.0001) interaction (group x time). This was not the case however for the second bout as there was no significant (group x time) interaction in all outcome variables immediately after exercise. When the first and second bouts were compared, it was found that the high-load group had faster recovery in MVC at day 1 and 4 corresponding to voluntary activation and only at day 4 corresponding to the resting twitch. CONCLUSIONS: In this study, it was found that high-load exercise aids fast recovery either via neural or muscular adaptations.

Effectiveness of roundhouse kick in elite Taekwondo athletes.

The roundhouse kick is a powerful attack in Taekwondo. Most athletes intently perform this kick for scoring in competition. Therefore, kinematic and kinetic analyzes of this kick were the topics of interest; however, they were separately investigated and rarely recorded for impact force. Our objectives were to investigate knee and ankle joint kinematics and electromyographic (EMG) activity of leg muscle and compare them between high-impact (HI) and low-impact (LO) kicks. Sixteen male black-belt Taekwondo athletes performed five roundhouse kicks at their maximal effort. Electrogoniometer sensors measured angular motions of ankle and knee joints. Surface EMG activities were recorded for tibialis anterior, gastrocnemius medialis, rectus femoris, and biceps femoris muscles. Based on maximal impact forces, the athletes were classified into HI and LO groups. All athletes in both groups showed greater activation of rectus femoris than other muscles. The HI group only showed significantly less plantarflexion angles than the LO group during preimpact and impact phases (P<0.05). During the impact phase, the HI group demonstrated significantly greater biceps femoris activation than the LO group (P<0.05). In conclusion, rectus femoris activation could predominantly contribute to the powerful roundhouse kicks. Moreover, high biceps femoris co-activation and optimal angle of ankle plantarflexion of about 35° could help achieve the high impact force.

Responses of human soleus motor units to low-threshold stimulation of the tibial nerve.

The peristimulus frequencygram (PSF) has recently been shown to illustrate postsynaptic potentials of motoneurones much more reliably than the peristimulus time histogram (PSTH). The aim of this investigation was to examine the profile of the postsynaptic potential (PSP) in soleus motoneurones in response to an H-reflex with and without accompanying M waves of different magnitude by using PSTH and PSF profiles of single motor units. Nine men and five women healthy subjects participated in this study. Electrical stimuli were delivered to the tibial nerve in the popliteal fossa. The reflex response of the soleus muscle was recorded using both surface electromyogram and single motor unit potentials. The PSTH analysis demonstrated that there were four different synaptic events following low-intensity stimulation of the tibial nerve: primary enhancement in firing probability (H-reflex or E1), primary reduction in firing probability (primary silent period or SP1), secondary reduction in firing probability (secondary silent period or SP2), and secondary enhancement in firing probability (E2). On the other hand, the PSF analysis indicated only two reflex responses, long-lasting enhancement in discharge rate including the H-reflex (LLE) and long-lasting decrease in discharge rate (LLD). The results of the two analyses methods are compared and contrasted. While the PSTH demonstrated that there was a silent period (SP1) immediately following the H-reflex, the PSF indicated an increase in discharge rate during the same period. The PSF also indicated that, during SP2 and E2, the discharge rate actually decreased (LLD). It was therefore suggested that LLD involved activation of several inhibitory pathways including the autogenic inhibition of units via the Golgi tendon organs. It was concluded that the PSF could indicate the details of the postsynaptic potentials and is very useful for bringing out previously unknown effects of electrical stimulation of muscle nerves.

A study of synaptic connection between low threshold afferent fibres in common peroneal nerve and motoneurones in human tibialis anterior.

We have induced H-reflex responses in human tibialis anterior motor units and analysed the results using the classical technique, peristimulus time histogram (PSTH), and a new technique, peristimulus frequencygram (PSF). The PSF has recently been shown to be more reliable than the PSTH for indicating the synaptic connections on motoneurones, and therefore we wished to examine the differences between the two analysis methods. Experiments were conducted on eleven healthy subjects (7 males and 4 females) who did not have any known neurological disorder. The subject sat comfortably on a dental chair and the common peroneal nerve was stimulated. In each experiment, about 600 electrical stimuli were applied to the nerve randomly between 1 and 2 s. The recordings were taken with both by surface electromyogram (SEMG) and as single motor unit potentials. We found that, when a stimulus induces an H-reflex, it also generates a period of reduced activity (silent period) and a long latency excitation in the PSTH. However, the PSF records in general do not match the indications of the PSTH records. For example, when the PSTH indicated existence of a silent period immediately following the H-reflex response, the discharge rate of the unit was in fact higher than the prestimulus rate. On the contrary, during the PSTH illustrated long latency excitatory response, the discharge rate was lower than the prestimulus rate. Our findings suggest that PSF gives significantly different results compared with the PSTH in determining the synaptic connection of the low threshold muscle afferents to the motoneurones. While PSTH indicated that there was a silent period immediately after the H-reflex, the PSF demonstrated that the silent period was actually a continuation of the net excitatory effect and not a genuine inhibition since the small number of action potentials occured during this period displayed higher discharge rates than the prestimulus level. Furthermore, the long latency excitation, as it was indicated in the PSTH; was actually a net inhibitory effect since the large number of spikes that occured during that period had lower discharge rates than the prestimulus average. In the lights of the recent brain slice findings and completely different results obtained using the two analysis techniques, we suggest that the PSF analysis should be used along with the PSTH to illustrate the net synaptic connection between peripheral receptors and motoneurones in the human nervous system.