Discussion
The present study examined the variability of the BFlh proximal aponeurosis size in healthy recreationally active population and the relationships of the aponeurosis size with BFlh muscle size and knee flexor function. The main finding was that the proximal aponeurosis size was highly variable between individuals, and in contrast to our hypothesis, it was not related to muscle size or knee flexor maximal isometric or eccentric strength. The disproportion between aponeurosis size and muscle size/strength suggests that individuals with a relatively small aponeurosis will be subject to greater mechanical strain in the muscle tissue surrounding the aponeurosis, which may predispose them to hamstring strain injuries.
Despite the homogenous nature of the recruited cohort, there was a large 4.5-fold variability in BFlh proximal aponeurosis area between participants that was substantially greater than the variability in BFlh muscle ACSAmax (1.8-fold). Moreover, contrary to our hypothesis, these two variables were unrelated, and consequently, the aponeurosis/muscle area ratio in this study ranged from 0.53 to 3.09 exhibiting a sixfold range, and being 83% smaller in one individual than another even within this relatively homogenous cohort. Interestingly, the individuals with the lowest and highest aponeurosis area in this study (7.5 and 33.5 cm respectively) had similar BFlh ACSAmax (14.1 and 13.3 cm, Fig. 4), and consequently, their aponeurosis/muscle area ratios were 0.53 and 2.52. Similarly, from a functional perspective, our results showed that aponeurosis size was unrelated to knee flexor strength, whereas the sizes of the other components of the MTU (muscle and tendon area) were associated with muscle strength (Fig. 5).
The lack of relationship between aponeurosis size and muscle size may have important implications for the mechanical strain within the muscle tissue surrounding the aponeurosis. Based on modeling and in vivo measurements of mechanical strain, individuals with a relatively small aponeurosis/muscle size ratio would be expected to experience greater mechanical strain in the muscle tissue adjacent to the aponeurosis with a greater potential for injurious muscle strains. Therefore, our results combined with the fact that hamstring strain injuries typically occur near the BFlh proximal myotendinous junction suggest that individuals with a low aponeurosis/muscle size ratio may be at an increased risk of hamstring strain injury.
This study is the first to directly examine the relationship between BFlh muscle and aponeurosis size. The only similar data we are aware of examined a different muscle and found VL aponeurosis area in a cohort of elite weightlifters and recreationally active males to be strongly related to total quadriceps muscle volume (R = 0.85). Although the adaptation of tendon in response to resistance training has been shown with increases in size and stiffness, it remains unknown whether the aponeurosis shows similar plasticity. The greater VL aponeurosis area exhibited by the elite weightlifters compared to the recreationally active students in the study of Abe et al. suggests that this difference may be an adaptive response to resistance training. If this were the case, BFlh aponeurosis size may be modifiable through training and this could reduce the risk of strain injury. However, the effects of resistance training on aponeurosis size have yet to be investigated directly.
For comparative reasons, we measured aponeurosis width replicating the methods of a preliminary report, and the range of values obtained here (0.19–1.22 cm) was similar to their results. However, this method involved the measurement of aponeurosis width at an arbitrary point, which corresponded to different relative positions along both the aponeurosis and the muscle for each individual (Fig. 3A). This measurement was not at the point of peak muscle–aponeurosis contact interface distance. These limitations in the width measurement do not allow for any valid comparison with aponeurosis area or examination of the differences between individuals. Furthermore, the individuals exhibiting the lowest and highest aponeurosis area (more than fourfold difference) both had midrange aponeurosis widths (0.32 vs 0.63 cm, less than twofold difference). Therefore, the aponeurosis width measurement appeared to be a limited reflection of aponeurosis size and interindividual variability in this study.
Both aponeurosis and free tendon are considered to have a high safety factor (i.e., the ratio of failure stress to peak operating stress) such that the aponeurosis and tendon are capable of accommodating a range of loads well beyond the normal functional range with no risk of injury to these structures. This may partly explain the lack of relationship between muscle strength and aponeurosis size. Nevertheless, strain injuries typically occur within the muscle tissue adjacent to the aponeurosis rather than within the aponeurosis. Therefore, although a small aponeurosis may have a sufficient safety factor to preclude aponeurosis injury, it could make the adjacent muscle tissue vulnerable to injury.
An interesting observation made during the analysis of the MR images was that the BFlh aponeurosis extends not only longitudinally along the side of the muscle belly but also transversely into the muscle (Fig. 2A), in agreement with a previous report. Anecdotally, the proportion of the internal aponeurosis to the total aponeurosis area between our participants appeared highly variable. However, it is currently unknown how this aponeurosis morphology affects force transmission and stress distribution, and further study is needed to elucidate its relationship with muscle size and strength.
Despite the large number of studies examining possible risk factors for strain injuries, it is still unclear how to identify individuals at high risk of strain injury, especially those with no history of injury. The emerging evidence that aponeurosis size may be a risk factor for such injuries has significant implications. Establishment of such an anatomical feature as a risk factor would greatly help to distinguish at-risk individuals before an injury occurs. For that reason, a prospective study investigating aponeurosis area relative to muscle size and strength and recording which athletes go on to experience a strain injury would provide valuable information. Also, the possible interaction of aponeurosis area with other established risk factors (e.g., previous strain injury and strength imbalances) should be considered.
Some limitations of this study have to be considered. First, the knee joint axis of rotation was assumed to be passing through the knee joint space, which was identified using superficial anatomy. It was also assumed that knee flexor strength measurement in vivo reflected the force-generating capacity of BFlh muscle and the forces transmitted by the proximal aponeurosis.
In conclusion, the present study showed that the BFlh proximal aponeurosis size exhibits high variability within a relatively homogenous cohort of healthy young men, and it was not related to muscle size or knee flexor strength. Therefore, individuals with a relatively small aponeurosis may be at increased risk of hamstring strain injury.