## 1 Introduction  
When rapid isometric contractions of different submaximal intensities are performed, a strong linear relationship is observed between peak force/torque and peak rate of force/torque development. The slope of this relationship, named the rate of force development scaling factor (RFD-SF) or rate of torque development factor (RTD-SF), has been proposed as a measure of neuromuscular quickness of submaximal contractions [1, 2]. Some claim that RTD variables may be more effective to evaluate training-related adaptations, to follow up recovery from an injury, or to distinguish between various populationsthan the maximal torque (Tmax) exerted during maximal voluntary isometric contraction (MVC) [3; 4; 5; 6]. Thus it is not surprising that the RTD-SF has been lately performed more frequently and reported as an indicator of rapid torque generation [1; 6; 7; 8]. Since being verified as reliable [1; 9; 10; 11], the RTD-SF protocol has been used for isometric neuromuscular assessment of upper and lower limb muscle groups [6; 7; 9; 10; 12], to explore the effects of sex [13], aging [13; 14], fatigue [15], and health conditions such as osteoporosis or multiple sclerosis [6; 8; 14], as well as in studies exploring inter-limb asymmetries [7; 11; 15]. In addition, [PERSON] et al. [12] investigated the effects of high-speed cycling on rate-dependent mobility in active older adults. Although [PERSON] et al. [16] recently studied the effect of cognitive task-induced mental fatigue on hand force production capacities including RTD-SF, the study of [PERSON] et al. [12] remains the only one that explored the sensitivity of the RTD-SF protocol to an exercise intervention. Thus, it would be of interest to study how other types of training, for example, isometric, affect RTD-SF variables. Knee muscle strength is of paramount importance for daily life activities and athletic performance. It has been shown that quadriceps femoris (QF) strength has significant implications for fall and injury prevention as well as for desirable outcomes of rehabilitation following injuries and surgical interventions [3; 17]. Both voluntary isometric contraction (VOL) and electromyostimulation (EMS) have been used in strength training for improving muscular strength [18; 19; 20] and/or fighting deficits in muscular activation and to improve strength (Tmax and RTD) [17; 21]. In line with the specificity of training effects, specific isometric strength improvements have been observed [22; 23; 24]. Many authors have attempted to determine whether EMS or VOL exercise provides a more efficient stimulus to improve muscle strength [22; 24; 25; 26]. However, the training effects were greatly dependent on training intensity, type of muscle action, or participants' training history. In addition to gains in maximum strength, resistance training can evoke significant increases in RTD particularly when exercises are performed with intention to produce rapid torque irrespective of actual movement speed, while the similarity between training and testing movement patterns evokes the greatest improvements [5]. However, increases in RTD have also been observed in studies using high load resistance (i.e., strength) training without the intention for fast movement speeds [5; 27; 28]. Even so, studies that investigated the influence of EMS aiming at maximum strength development for rapid torque production are scarce and less clear. In line with that, the effects of the strength training modality on the development of RTD-SF (i.e., neuromuscular quickness) have not been investigated so far. It therefore remains unclear how strength training that includes EMS and VOL exercises affects rapid strength production under isometric conditions. Under certain circumstances, neural adaptation to unilateral training goes beyond the muscles directly involved in the exercise and results in increased voluntary activation and strength gains in the contralateral homologous muscle, i.e., non-trained limbs [29].