Rehabilitation protocols designed for muscle injuries should be built on the scientific knowledge about the injury and the therapeutic options we have to treat them. The knowledge about muscle injuries would include muscle injury biology; muscle group anatomy, structure, histology and function; types and mechanisms of injury; injury risk factors; reinjury risk factors; etc. The biology of healing a muscle injury is a reparative process [7] with the formation of a scar [8]. The healing process of a skeletal muscle injury is divided into three phases: destruction, repair and remodelling, ending with a new myotendinous junction (MTJ) between the repaired myofibers [9, 10]. The optimal healing process is characterised by stimulating regeneration and minimising reparation, so the smallest scar possible.

There is no clear evidence regarding the use of medications [11], cooling [12] or platelet-rich plasma [13] with limited impact on return to play (RTP) or reinjury statistics. Several protocols have been published to treat hamstring muscle injuries (HMIs) using stretching exercises [14], balance [15], eccentric exercises [16, 17] and different combinations [18–20]. Although the aetiology of HMIs is multifactorial, most rehabilitation programmes focus on just one or two factors [21]. Our purpose is to design and develop a criteria-based rehabilitation programme with clear objective progression criterion for each phase and RTP. The main of knowledge about muscle injuries has been reached about hamstring and rectus femoris injuries, so the process for a rehabilitation protocol for HMIs has been described (Fig. 35.1).

Many exercises to achieve a global approach to the injury have been used, with the aim of correcting and reducing the biomechanical disorders, which influence the progression of injury [22, 23]. We will include exercises or programmes showing effectiveness as treatments for HMIs or reducing the risk of injury to lower extremities such as proprioception or neuromuscular exercises [24]. To design an exercise standard offers the option of adapting the protocol to the patient’s physical condition, sport or the available equipment. Exercises will progress from single, basic, low demanding to more complex and combined, until accomplished exercises reproduce sport movements.

The design of exercises will be completed keeping in mind hamstring anatomy and function, injury mechanism, etc. in order to target the muscle and location we need to treat [25, 26]. It is also important to design the exercises focusing not only on contraction type and load; ROM, uni-bilateral exercises in open-closed kinetic chain, hip- or knee-dominant and multi-joint movement exercise and length of the movement need to be taken into account [27, 28] (Fig. 35.2).

The concept of elongation stress on hamstrings (ESH) has recently been introduced and aims to assess hamstring elongation. This is achieved by subtracting the knee flexion angle from the hip flexion angle [29]. The more positive the ESH is, the more stress on hamstrings and the opposite. Therefore, we can use the ESH as a criterion to objectively monitor hamstring stretch progression during.

About the strength, the load for an exercise is a key point in a training programme, and as muscle injury rehabilitation is a training programme, if loads are not appropriated correctly, we will not be able to achieve our goals. Hamstrings’ peak torque (PT) angle shifts to longer muscle length after eccentric training, and as more elongate is the muscle during eccentric work, the higher is the shift in the PT angle [30]. This shift in hamstrings’ PT angle has been also reported after concentric exercises, but only when performed at long lengths [31]. Eccentric lengthening exercises have shown good results in HMIs rehabilitation; we purpose to perform all strength exercises at longer length possible in order to correct PT angle during the whole rehabilitation process. About the quantification and progression of strength exercises, isometric, concentric and eccentric exercises will overlap during the protocol, with part of the strength work performed at long lengths [32, 33] (Table 35.2).