## 1 Introduction  
Endurance training leads to vascular and muscular adaptions, favoring the improvement of endurance performance. [1]. More recently, it has been shown that exercise economy is one of the factors determining performance in endurance sports [2]. Exercise economy is defined as the oxygen uptake required at a given submaximal speed [1] and this concept is frequently associated with the work economy that is expressed as aerobic metabolic rate or oxygen uptake at a given speed [3].  
Many sports require a combination of strength and endurance training for successful performance and it is known that the inclusion of strength training in an endurance season training period can represent the key strategy to enhance the endurance performance [4]. Strength training increases muscle strength and power. Indeed, in well-trained athletes, the capacity to sustain a high intensity in endurance performances, without the influence of aerobic capacity, could be associated to the strength development [3]. The effectiveness of concurrent training--such as endurance and strength training in the same training program--has long been recognized, in fact, expert trainers believe that the addition of non sport-specific strength training (NST) to sports-specific training improves sports performance [4]. NST is every strength exercise with non-specific sport movements (i.e., squat, deadlift, leg press, leg extension machine, etc.).  
Several studies showed that sport-specific strength training improves endurance performance. For example, the running economy increased when incorporating uphill-sprint interval training in the endurance training programs [5]. However, the inclusion of NST in workout sessions leads to similar results: for example, maximal strength exercise (i.e., squatting with maximal external load) before cycling endurance performance improves the subsequent 20 km time trial testing with a better cycling economy [6]. The effect of NST on endurance performance [7; 8; 9] highlighted that a strength training per se could help endurance athletes to be more economical, in terms of submaximal forces developed during performance (stride during running or pedal during cycling) [1]. For a better yield, strength-training load should be of mixed intensity, frequency and volume [8] and exercises should have kinetics and patterns (rate of force development--RFD, peak force, acceleration, etc.) similar to the athletic requirements [9] (i.e., high exercise speed during squat, lunge and calf if the sport needs lower body fast movements).  
Although there is evidence that NST improves endurance performance, it is still not clear which type of NST (maximal, pylemetric, explosive or resistance strength training) is the most effective for specific endurance sport disciplines. Training strength with different types of load movement--dynamic or isometric, maximal or resistance, explosive or pylemetric--might be more effective for some sports, but several opinions appear controversial. Some studies on swimming provided evidence in favor of pylemetric strength training [10; 11], others in favor of maximal strength training [12]. In running disciplines explosive strength training seems to work better, as it decreases the muscle activation time, while heavy strength training resulted less efficient [13].  
Given this background, the purpose of this review and meta-analysis was to assess which type of NST intervention has proven more efficient to improve the performance, in four different endurance sports: running, cycling, triathlon and cross-country skiing.