# Effect of Different Local Vibration Durations on Knee Extensors' Maximal Isometric Strength
## 4 Discussion  
The aim of the present study was to identify the shortest duration that would induce a reduction in the knee extensor muscles' force and confirm the central origin of such losses. The main finding of the present study was that the application of LV, lasting either 1, 3, or 6 min, does not negatively affect the quadriceps muscles' force production, not inducing any central and/or peripheral fatigue. Until now, the majority of the studies reporting significant reductions in maximum strength (from 4.3 [15] to 17.4% [17] for knee extension and from 5 [10] to 19% [12] for plantar flexion) have used long-term protocols, lasting between 20 and 30 min [13; 14; 15; 16; 17; 18]. Considering that, in the above studies, as well as in the present study, the frequencies and amplitudes of the LV protocols were similar (frequency: 70-120, amplitude: 0.2-2 mm) [10; 11; 12; 14; 15; 18; 29], it can be concluded that the observation of no significant force reduction and the absence of central and peripherical fatigue, as they were evaluated in the present study, should be the outcome of the very short LV duration used in this research. We found no changes in markers of peripheral fatigue independent of experimental conditions (i.e.,  
I muscle fibers [11; 12; 15]. This reduction in muscle EMG activity was accompanied by a significant reduction in muscle force production [20; 21; 22; 23]. More surprisingly, the level of voluntary activation remained unchanged when investigated despite the presence of a loss of strength and reduced EMG activity [10; 14]. For instance, [PERSON]'s study used transcranial magnetic stimulation to assess the level of voluntary activation. This method has several limitations, such as (i) using an estimation of the amplitude of potentiated stimulation to calculate the VAL, (ii) lower reproducibility, and (iii) a lower target specificity and stimulation intensity [27]. These biases, inherent in the method used, may have led to the absence of any significant interaction between the two conditions (CONT vs. LV) in their work, despite the loss of force. Although not significant in the [PERSON] et al. study, a reduction in VAL was obtained in 10 of the 15 participants [10]. Conversely, a reduction in RTD is regularly observed in association with a loss of strength [15; 16; 29]. This result confirms the central origin of the loss of strength after LV and the difficulty in recruiting fast fibers in producing an explosive contraction [28]. In our case, the preservation of EMG activity levels, the voluntary activation level, and the RTD is consistent with the lack of force loss observed in our study. The effect of LV on central nervous system excitability has been increasingly well established [34; 46]. A reduction in motor neuron excitability assessed at rest or during weak contractions by means of thoracic or cervicomedullary stimulation corroborates the loss of force for durations of 30 min [14; 29]. Moreover, this reduction in motoneuronal excitability appears to be partly responsible for a reduction in spinal loop excitability assessed by the H reflex [34; 36]. The maintenance of maximal force production and the level of voluntary activation in our study might imply that motoneuron excitability and, consequently, the spinal loop remain intact. In addition, only 30 s of LV applied to the soleus muscle tendon is sufficient to induce a reduction in the H-reflex. However, no loss of strength was found despite reduced H-reflex assessed in the study by [PERSON] and [PERSON] on the vastus medialis muscle after 30 min of LV [20]. The same result was also observed for muscles involved in plantar flexion [22]. These results weaken the hypothesis of a direct relationship between vibration-induced altered spinal loop excitability and the ability to produce maximum force.