Effects of Strength Training at Different Intensities on the Plantar Flexors' Functional, Neuromuscular, and Tendon Properties After Achilles Tendon Rupture

Among the injuries that affect the Achilles tendon, rupture is one of the most frequent. This injury can generate functional, neuromuscular, and tendon deficits that can last for long periods or even be permanent. In the long term (i.e., more than a year after the injury), individuals present functional impairments related to the lower limb, deficits in the capacity to produce muscle force, as well as higher levels of muscle activation (as a compensatory response). Such changes may be due to injury adaptations in the plantar flexor muscles' architecture, which may have shorter, more pennate fibers, leading to reduced muscle thickness. Furthermore, the Achilles tendon may be elongated, with a greater cross-sectional area, presenting lower stiffness and quality (i.e., lower Young's modulus). Strength training can play an important role in recovering from Achilles tendon ruptures, as it promotes functional, neuromuscular, and tendon adaptations that can minimize deficits caused by the injury. However, there is a gap in the literature regarding strength training, as well as the dose vs response relationship, regarding functional, neuromuscular, and tendon adaptations after Achilles tendon rupture. Thus, the aim of the present study is to compare the effects of a strength training program of different intensities on plantar flexors' functional, neuromuscular, and tendon outcomes after Achilles tendon rupture. Men aged between 20 and 50 years old who suffered a total unilateral rupture of the Achilles tendon between one and five years after the rupture will be included in the study, as well as healthy men who did not suffer the rupture (control group). Participants who have suffered an Achilles tendon rupture will undergo a progressive lower limb strength training program twice a week for a total duration of 12 weeks, which will be randomized between two groups: low intensity (G55 - 55% of 1-RM) and moderate intensity (G70 - 70% of 1-RM). The following outcomes will be evaluated: a) Achilles tendon's morphological (length and cross-sectional area), mechanical (force-elongation relationship and stiffness) and material (stress-strain relationship and Young's modulus) properties; b) triceps surae muscles' morphological (architecture [fascicle length, pennation angle and thickness] and quality [measured by echointensity and specific tension]) properties; c) ankle functionality (maximum height in the heel raise test); d) the plantar flexors' force production capacity (peak and rate of torque development in different joint positions); e) the plantar flexors' muscle voluntary activation; and f) triceps suraes' neuromuscular capacity (i.e., recruitment curves). Assessments will be performed at two times (pre-training; and after 12 weeks of training [post-12]) through functional tests, ultrasound techniques, isokinetic dynamometry, electromyography, and percutaneous electrical stimulation. An intraclass correlation coefficient will be used to verify the test-retest reproducibility of ultrasound measurements. The Chi-Square test will be used to compare the level of physical activity (pre-training) between the groups. The results of the intervention will be expressed using descriptive statistics (mean, standard deviation, and standard error). The normality and sphericity of the data will be tested using the Shapiro-Wilk and Mauchly tests, respectively. A generalized estimating equation, followed by Bonferroni post-hoc, will be used to compare the effects of groups (G55 and G70) and times (pre-training, Post-6, and Post-12). A one-way ANOVA, followed by a Bonferroni post-hoc, will be used to compare the control group participants' limbs with the healthy and injured limbs from both intervention groups (G55 and G70) in the Pre and Post-12 times. The effect size will be estimated for each outcome. All statistical analyzes will be performed using SPSS software.