## 4 Discussion
### Effects of Strength Training on Hamstring Injury Incidence  
The contribution of the hamstring to sprint speed increases with running velocity [2]. In sprinting, the function of the hamstring in the late swing phase is that of the hip extensor concentrically acting to quickly swing the thigh back [2,4] while also acting as a knee flexor to eccentrically decelerate the forward swing of the lower leg [2,4,31]. In the early contact phase, the hamstring function minimizes the loss of running speed by performing a concentric action as the knee flexor and hip extensor muscles, enabling the body's center of gravity to shift forward smoothly [4,31]. The hamstrings must generate a large amount of power during these phases to maximize running speed during sprinting [2, 4, 3]. The relative iEMG of the biceps femoris is significantly greater in the late swing phase during supra-maximal running than it is in running at maximum speed [5]. The power demonstrated by the eccentric contraction of the knee flexors (hamstrings) and that demonstrated by the concentric contraction of the hip extensor in the late part of the swing phase are reported to increase during supramaximal running [5]. Supramaximal running, wherein a sprinter sprints at a supramaximal level by artificial means, forces the hamstring to perform activity at extremely intense levels [5,6]. In sprinters, these generated high forces are postulated to be related to the hamstring injuries during supramaximal running.  
A relationship between an insufficient hamstring strength in eccentric contraction and hamstring injury occurrence has been reported [15,25,32,35]. [PERSON] et al. [15] suggested that hamstring injury in elite sprinters was associated with weakness of the hamstrings in eccentric action and weakness of the hip extensors in concentric action. Therefore, strengthening training was undertaken to increase muscle strength so much so that it can respond to the high-intensity (force) activity demonstrated by the hamstring in supramaximal running through the exercise of contracting concentrically and eccentrically.  
In the 2000s, several studies have demonstrated the effectiveness of eccentric strength training on hamstring muscles [20,21,36,37]. Systematic reviews [38] have recommended that Nordic hamstring exercises is effective to prevent hamstring muscle injury. Nordic hamstring exercises can put the maximum load on the hamstring during the eccentric phase. The primary injury mechanism is high-speed running; more specifically, it occurs during the swing phase, when the hamstrings are required to contract forcefully while lengthening to decelerate the extending knee and flexing hip [39]. Therefore, the low level of hamstring eccentric strength [15,25,32,35] has been cited as the primary risk factor for hamstring injury. The Nordic hamstring exercise can maximize loading on the hamstring in the eccentric phase [21]. During the execution of the Nordic hamstring exercise, a progressive resistant torque imposed by the trunk position leads to increased neuromuscular hamstring activation at longer muscle lengths [40,41], precisely at the position most vulnerable to injury [42]. The lumbopelvic region muscles effect hamstring injury risk more than the distal muscles of the knee and ankle do [15].  
The practice of strength training applies an eccentric load to the hamstring through the trunk position at the injury position. Eccentric strength training worked well against load on hamstring during supramaximal training. In this study, the number of hamstring injuries during the intervention period with eccentric strength training decreased relative to the baseline period. All sprinters strengthened their hamstrings with leg curls, hip extensions, and two types of modified Nordic hamstring exercises. This likely decreased the number of hamstring injury cases during supramaximal running.