## 4 Discussion
However, greater values were observed for football players with previous injury, which was in contrast to previous studies. Indeed, the current literature [41; 42; 43; 44] demonstrates an inability of those with prior unilateral hamstring strain injury to completely activate the involved limb, despite full voluntary exertion. Furthermore, it has been suggested that a failure to fully voluntarily activate the previously injured muscle may result in a limited stimulus for eccentric strength gain and fascicle lengthening during rehabilitation [45]. Therefore, the greater values on the RTD 0-50 ms in the present study, in football players with previously injured limb on BFlh vs. the contralateral limbs, could be possibly explained by a specific rehabilitation protocol especially for neural factors. Moreover, the difference between studies should be considered, especially the different times of screening, since in our case, this ranged between 2-19 months whereas previous studies ranged between 4 weeks-12 months [43], 1-18.2 months [41], and 2-18 months [42; 44]. Moreover, these differ studies in sample composition, with previous studies analyzing recreationally active males [41; 42; 43; 44] and the present study assessing professional football players. Furthermore, it should be noted that there are no differences in the rate of torque development and onset of muscle activity in football players with and without HSI in a prospective study, suggesting no association with injury risk [46]. In respect to muscle stiffness and tissue properties, [PERSON] et al. (2010) showed a significantly greater leg and hamstring musculotendinous stiffness, in the pre-season in Australian Rugby players after a lateral hopping test, in players that then sustained an HSI during the season, suggesting that those who recorded relatively high bilateral hamstring musculotendinous stiffness or leg stiffness values may have a higher risk of sustaining a noncontact, soft tissue hamstring injury during the season [47]. [PERSON] et al. (2021) reported an increase in the passive stiffness in football players' limbs with previous BF injury when compared to the healthy limbs using MyotonPRO\({}^{\text{\textregistered}}\) and the vibration disappearance threshold [48]. Our results contradict this finding since no differences were seen; however, it must be considered the different states of measurements (active vs. passive) and that SWE measurements provide a proxy of localized tissue properties [34] rather than a global measure of joint stiffness or the possible contribution of other muscles [48]. It should be noted that stiffness is a functional property and shear modulus is a tissue property. Therefore, these comparisons need to be considered with caution since two materials can have the same shear modulus but different stiffness. Indeed, a previous study reported only a moderate correlation between SWE and MyotonPRO\({}^{\text{\textregistered}}\) in the gastrocnemius muscle and Achilles tendon [49]. In contrast to previous results by [PERSON] et al. (2016, 2014), who quantified the T2 relaxometry during dynamic KFs and reported that players with a history of HSI had a decrease in the ST and an increase in the BF load during dynamic KFs [20; 21], and to [PERSON] et al. (2020), who showed a decrease in the active ST shear modulus with a lack of changes in the BFlh active shear modulus during an isometric knee contraction until failure, which led to an increase of BFlh/ST active shear modulus ratio, and consequently a higher BFlh and lower ST contribution in the fatigue task [29], our results showed no differences in the load sharing pattern in the active state. One of the possible explanations between the present study and previous studies [20,21,29] could be the specificity of the task. [PERSON]'s studies used dynamic KFs until exhaustion and [PERSON] used isometric contraction until exhaustion in healthy participants.