The main objective of the present study was to determine the acute changes in tendon thickness or CSA of the supraspinatus tendon in active young people after isoinertial training. The main results showed a significant reduction in volume (8.3%) and thickness (5%) after 10 repetitions of maximal isoinertial training.
The effect of load on tendons is well known [30–36]. Changes in tendon properties have been studied extensively mainly as an adaptation to long-term training programs.
The Achilles tendon, for instance, has been shown to increase CSA after a period of load training [30–33]. In another study where sectional changes in the Achilles tendon were analyzed after 9 months of training in sedentary individuals, the CSA of the tendon remained unchanged [34].
Thus, it has been suggested that the behavior of the tendon is associated with the magnitude of the mechanical load to which it is subjected, at least in terms of inducing morphological adaptations.
Additionally, several studies in humans have shown that resistance training for 12–14 weeks leading to muscle strength increases of up to 21% does not result in a concomitant increase in tendon CSA [35][36], but rather a marked alteration in tendon modulus, implying that there is a change in the composition of the structure rather than size.
However, analyzing the acute response of the tendon to external loads allows to establish what type of load may be more advisable to avoid exacerbating swelling. The study of acute responses has recently received kinetic interest but there seems to be no consensus on the behavior of the tendon under different types of mechanical loads.
In another study the authors demonstrated an acute and transient reduction in Achilles tendon CSA immediately after a single bout of resistance training using 4 concentric and eccentric exercises with different loads [between 100 and 150% of body weight] [36–39]. This reduction has been attributed to a positive morphological response of the tendon to mechanical stress.
In the study by Grigg et al [37], changes in the Achilles tendon were analyzed following eccentric and concentric loading in which both exercises (concentric and eccentric) used equivalent stress and duration and caused the CSA of the Achilles tendon to decrease by approximately 5 and 20%, respectively. The observed decrease in Achilles tendon CSA following concentric and eccentric exercise is consistent with previous in vivo research [38], in which a heel raises exercise combining concentric and eccentric muscle actions was shown to decrease ultrasound-determined Achilles tendon CSA by approximately 15%.
The acute response of the tendon will help us to establish what type of load may be of greater interest in order to avoid harmful loads that may jeopardize suitable tendon adjustment. In this regard, it has been suggested that tendon thickness may undergo an immediate and transient decrease after a single bout of resistance training from 20 to 250% of body weight, which is associated with an acute positive tendon response [37][39][40]. These acute changes appear to be associated with tendon dehydration, as well as fluid movement in the extracellular matrix of these structures [41], as a result of the alignment of collagen fibers following resistance exercise [38].
Therefore, the reduction in tendon thickness may represent an important marker in tendon adaptation. Also, this morphological change may be a positive response to inflammation, which may recorded based on structural changes in the tendon measured by ultrasound [41–43].
Increased levels of glycosaminoglycans and proteoglycans have been observed in aged and degenerated tendons, as well as in those with a high level of activity. Increased basal levels of tendon glycosaminoglycan and proteoglycan content lead to increased water binding, which could partly explain the changes in fluid metabolism and the consequent increase in tendon thickness [44].
Similarly in vitro models have shown that cyclic loading exudes water from the tendon, resulting in a decrease in tendon dimensions [45] and movement of water from the tendon core to the peritendinous space [46].
This extravasation of water has been attributed to the straightening of the curl and the realignment and stretching of collagen, which produces lateral compressive forces between the fibrils and a reduction of the interfibrillar space, resulting in positive hydrostatic pressure and, therefore, the movement of fluids out of the tendon, water being the major physiological component of the tendon, since it makes up 80% of the tendon. [47][48]. From all of the above, we know that the tendon adapts to the load and that it can vary in size with dehydration of the tendon through eccentric exercise.
Regarding isoinertial training, there are no previous studies that have analyzed the changes in the supraspinatus tendon after isoinertial loading, which currently represents an alternative of interest, since it combines concentric and eccentric loading in a single exercise. This was the main reason why we started to study the response of the supraspinatus tendon to this type of training.
The results of our study revealed significant differences (p < 0.01) in the ultrasound changes in the supraspinatus tendon CSA and in tendon volume (Table 2) after 10 repetitions of maximal force isoinertial training, so our initial hypothesis has been fulfilled.
The authors assume that the behavior of the tendon is similar to that previously described, however, surprisingly, in our study no association was found between the level of force exerted and the tendon response, which may imply a different behavior with respect to conventional training. It can be hypothesized that perhaps the variable of speed or eccentric overload provided by the isoinertial load may be more important variables than the force exerted.
Numerous authors recommend this type of isoinertial strength training since they emphasize that it has advantages over traditional strength training. Specifically, a single exercise mobilizes a concentric and eccentric load unlike traditional strength-gaining methods in which it is necessary to mobilize or lift a concentric contraction load [49][50]. Likewise, it has been proven that this type of training improves neuromuscular function, improving balance in older subjects [51].
Other authors recommend isoinertial training since it uses a fast and compact work protocol in which results are obtained in a short period of time, the young and trained athlete being the ideal subjects for this type of training. Likewise, it is a type of muscle training easily adapted by the athlete and simple to perform since the isoinertial equipment is easy to use [52], while ensuring improvements in strength, hypertrophy, muscle activation, muscle length, tendon stiffness, power and sports performance [20].
Since each subject could apply different mechanical loads depending on the applied force, a quartile analysis was performed (Table 3) to investigate the relationship between the applied force and the tendon response. No significant differences were found, this result being striking since previous studies with mechanical loads have suggested that load is a very important factor in tendon response. This fact may be attributed to the speed of execution particularly during the eccentric phase as reported by various authors [53–55] and the possibility that these devices may not provide as great an eccentric stimulus as expected and may not dehydrate the tendon as much [56].
After the isoinertial (concentric- eccentric) training of our study, we didn´t obtain significant results for the relationship between the applied force and the change in tendon CSA. As mentioned, many authors directly relate the increase in force to a greater acute response in the tendon, so in future studies, we should implement aspects such as the control of the speed of execution and the number of inertias (since our study only used one inertia).
Based on the foregoing, analyzing the acute response of the supraspinatus muscle tendon to isoinertial loads can provide relevant information for those using this technology and even a tool for the improvement of tendon health.