# Effect of Different Local Vibration Durations on Knee Extensors' Maximal Isometric Strength
## 1 Introduction  
Local vibration (LV) is a powerful tool for stimulating numerous receptors (e.g., cutaneous receptors, Golgi tendon organ, muscle spindles) by passively stretching the entire vibrated musculotendinous system [1; 2; 3; 4]. When applied at high frequency (a range between 80 and 100 Hz) and low amplitude (<0.5 mm) to a tendon or relaxed muscle, it mainly stimulates primary inferences (Ia) [2; 5; 6], the latter projected onto motor neurons via a monosynaptic reflex circuit, as well as a more complex poly-synaptic circuits, which can result in a tonic vibratory reflex contraction (TVR) [7; 8; 9]. At rest, prolonged exposure (20-30 min) to LV on the plantar flexor [10; 11; 12] or knee extensor muscles [13; 14; 15; 16; 17; 18], in most cases, leads to a loss of strength (for a review, refer to [19]), even though several studies have failed to demonstrate this effect [20; 21; 22; 23]. The primary cause of this fatigue could be an intramuscular (peripheral) alteration beyond the neuromuscular junction [24; 25]. However, this is still debatable, as in many cases,markers of peripheral fatigue (e.g., the maximal M-wave response; the mechanical twitch obtained using peripheral electrical stimulation or the characteristics of this twitch) were not altered by the prolonged LV protocols [10; 11; 20; 21; 23]. The second cause could be central (neural) alteration upstream of the neuromuscular junction. The magnitude of exercise-induced central fatigue is commonly evaluated by the level of voluntary activation through the use of interpolation twitch technique (ITT), electromyographic activity during maximal contraction, and early (less than the first 100 ms from the onset of muscle contraction) rate of force development evaluations [26; 27; 28]. The ITT technique with peripheral nerve stimulation [10; 21] or corticospinal stimulation [14; 23] was used in only four studies, and the results were never influenced by the LV protocol, regardless of strength loss. It seems that ITT, either with peripheral nerve stimulation [10; 21] or corticospinal stimulation, is not affected after the application of different LV protocols, even if they lead to a significant reduction in maximum strength. However, a decrease in electromyographic activity [10; 11; 12; 15; 16; 17] or a reduction in the rate of torque development (RTD) [15; 16; 29] was observed as soon as a loss of strength was observed. Concomitant with the observed reduction in maximum strength after prolonged LV applications, an inactivation of \(\alpha\)-motor units from the Ia afferent nerves can also be observed, as assessed through H-reflex evaluation [11; 20; 30]. This reduction in the H-reflex has long been attributed to presynaptic causes and is suspected to be involved in the observed reduction in muscle strength [3; 31; 32; 33]. However, several studies have recently highlighted a decrease in post-synaptic excitability (i.e., motor neuron excitability) following an acute LV protocol. This is reflected in a reduction in the thoracic motor-evoked potential amplitude (TMEP) [14; 29; 34], a motor-unit-type-dependent change in persistent inward current (i.e., an increase for the lower-threshold and a decrease for the higher-threshold motor units) [35] and a reduction in motor unit discharge frequency [13]. Note that a decrease in post-LV motor neuron excitability appears to be accompanied by increased cortical excitability, estimated using the ratio of corticospinal tract excitability (measured with transcutaneous magnetic stimulation) to motor neuron excitability [14; 29]. Nevertheless, this increase in cortical excitability does not appear to be sufficient to counteract the negative effects of LV on maximal force production. As a result, the origin of this loss of strength could come from a potential increase in motor neuron recruitment thresholds and a decrease in firing rates, particularly in fast motor units, resulting in a decrease in the activation of the y-loop. The majority of the results presented above were obtained for prolonged LV protocols (>20 min).