The present experimental study aims to evaluate the effectiveness and tolerability of isoinertial strength training of the hamstrings using machines in patients with ACL-R during the intermediate post-intervention phases.

A proper post-operative rehabilitation program is essential for a positive outcome after anterior cruciate ligament (ACL) reconstruction surgery, a procedure known to cause morpho-functional issues in the short, medium, and long term in individuals who undergo it. Generally, after an initial rehabilitation phase focused on protecting the healing process of the new ligament and regaining range of motion, the rehabilitation path focuses on muscular strengthening and neuromuscular control of both lower limbs. It has been demonstrated that deficits in extensor and flexor muscle strength, as well as their strength ratios, increase the risk of re-injury. Specifically, in athletes, the rate of recurrence, defined as a new injury to the same or opposite side ACL, is considerably high.

High-intensity resistance training of the operated limb has often been contraindicated in the early post-operative period, as it was considered detrimental to the new ligament, joint cartilage, and surrounding soft tissues. However, recent studies suggest that the early application of progressive eccentric-focused exercises, even with high loads, can be safely used to increase muscle volume and strength levels in individuals undergoing ACL reconstruction (ACL-R). Therefore, eccentric reinforcement can be a valid alternative to traditional concentric work for improving lower limb muscle strength after ACL-R.

Recently, it has been demonstrated that strength training using isoinertial machines in healthy individuals leads to positive chronic adaptations in terms of strength recovery, power, and functional capacities such as sprinting, change of direction, and agility. Isoinertial training allows for exercises in both closed and open kinetic chains, as well as multiplanar motor activities. The combination of concentric phases with subsequent eccentric overload allows for the development of high levels of strength and power with low energy cost, promotes intermuscular coordination, preferentially recruits motor units with high activation thresholds, and increases cortical activity. Increased muscle activation, particularly during eccentric muscle actions, indicates a higher mechanical load and a greater training stimulus, leading to better and earlier protein synthesis and subsequent muscle hypertrophy compared to traditional resistance training. Furthermore, the metabolic cost required for an eccentric contraction is about a quarter of that for a concentric contraction at the same external load. Therefore, at the same contraction velocity, eccentric contractions allow for a higher force expression.

While the use of isoinertial equipment is well-established in healthy individuals, scientific evidence regarding its use in rehabilitation protocols is currently limited. The use of eccentric overload, in addition to improving parameters related to muscle strength and power, seems to promote tendon remodeling due to an increased production of fibroblasts and collagen, generated by the higher mechanical load compared to traditional concentric exercise. In lower limb pathologies, isoinertial rehabilitation following injury can be a valid alternative to traditional strength training for the development of muscle hypertrophy and function. However, specific studies investigating the effects of isoinertial rehabilitation in the intermediate stages of recovery following ACL-R and monitoring the tolerability of the treatment are currently lacking.

Inclusion Criteria:

• having undergone ACL-R surgery following a unilateral rupture
• with reconstructive surgical technique using autograft tendon from the semitendinosus and gracilis muscles (ST-GR), also in the presence of meniscal injuries treated with selective meniscectomy or meniscal suturing.
• Participants must report a level of physical activity between 5 and 10 assessed using the Tegner scale, and have completed the third month of post-operative rehabilitation

Exclusion Criteria:

• concurrent injuries involving other ligaments of the knee, cartilage injuries requiring surgery to repair the damage
• previous ACL reconstruction (re-rupture with a new ACL reconstruction) on the knee affected by the new ACL injury.
• Additionally, if the level of pain prevents the subject from performing the functional tests, they will not be included in the study