This study used chemical shift encoding-based water-fat MRI at the thigh in healthy volunteers to extract the PDFF and relCSA of quadriceps and ischiocrural muscles bilaterally and a rotational dynamometer to assess relMVIC in extension and flexion. Muscle PDFF was a better predictor of the relMVIC than the relCSA. We observed significant differences in the relCSA of ischiocrural and quadriceps muscles, but also in the PDFF of ischiocrural muscles between males and females.
Grimm et al. [3] recently reported averaged PDFF values between 5.6 and 6.9% in healthy young males; however, in their investigation, PDFF measurements were derived from all thigh muscles and without distinct exclusion of intermuscular tissue. Specifically for the quadriceps muscles, a previous study [22] reported a mean intramuscular PDFF of 4.02% in a cohort of healthy males. Schlaeger et al. [2] performed the segmentation of individual thigh muscles as part of the MyoSegmenTUM_thigh database, with average PDFF values of 3.71% and 3.93% for the right and left quadriceps muscle as well as 4.38% and 4.44% for the right and left ischiocrural muscles by also focusing on healthy males and females. However, a distinction between genders has not been achieved in their study because only three healthy females were included in total [2]. Thus, the PDFF values obtained in the present study seem to be principally in the range of previously reported values.
Alterations in fat composition of thigh muscles have been shown in the context of diseases such as musculoskeletal disorders, metabolic diseases, and NMD [1,2,3,4,5,6,7,8,9]. Specifically, patients suffering from different types of muscular dystrophy, Pompe disease, sarcopenia, osteoarthritis, or type 2 diabetes mellitus showed increased PDFF or intramuscular fat when compared to healthy controls [1,2,3,4,5,6,7,8,9]. Although it has been hypothesised that increased fatty infiltration or transformation of thigh muscles is related to decrease in muscle strength [21, 23], studies correlating parameters such as the PDFF or absolute or relative CSA of thigh muscles with objective measurements of muscle strength are rare. An inverse relationship was revealed between the PDFF and strength at the thigh in patients with NMD [1, 8]. In these studies, the obtained strength measurements were acquired with a handheld myometer [1, 8]; however, the usage of more objective devices to test muscle strength, such as isokinetic dynamometers that enable robust assessments of the muscle strength of specific functional muscle groups, is still largely missing to evaluate the association between fat components and thigh muscle function. One previous study [22] focusing on the fat-strength interaction of quadriceps muscles by only testing a small cohort of healthy subjects used an isokinetic dynamometer for isometric strength measurement in knee extension and indicated that the quadriceps intermuscular adipose tissue fraction and intramuscular PDFF correlate significantly with physical strength, represented by the MVIC.
The present study confirms this first evidence of an interaction between muscular fat and strength by examining the associations between the PDFF and relMVIC at the thigh in healthy subjects and by showing not only significant correlations for the PDFF of the quadriceps muscles but also for ischiocrural muscles. Moreover, we were able to show that, in contrast to the relCSA, the PDFF of the quadriceps and ischiocrural muscles were significantly associated bilaterally with relMVIC in knee extension and flexion.
These new insights in the interactions of fat and strength parameters in human muscles demonstrate the importance of muscle quality, consisting of the individual number of muscular contractile elements, the specific fatty infiltration as well as the strength capacity [21]. The muscle quality might be able to predict and flesh out muscle (dys-)function a lot better than the relCSA does. Of note, there is no significant correlation between the relMVIC in flexion and the relCSA of the ischiocrural muscles for the left and right side. The anatomical difference (quadriceps muscles: one-joint muscles; ischiocrural muscles: multi-joint muscles) as well as the variations concerning the muscular structure and activation should be considered when interpreting this finding [35]. To specify the knowledge of muscle quality and (dys-)function in the future, further studies are needed to include and prove the meaning of the muscle volume and in particular the physiological CSA, the probably most powerful predictors concerning muscular strength [36,37,38].
Our observations seem to be in accordance with the finding of paraspinal PDFF being significantly correlated with the relMVIC in extension and flexion of the trunk, whereas paraspinal mean CSA only showed significant correlations with relMVIC in flexion in a previous study [39]. Furthermore, the suggested superiority of PDFF over relCSA for the prediction of muscle strength seems to complement previous work at the thigh region derived from patients with pathology, demonstrating negative correlations between the PDFF and muscle strength and indicating that muscle fat composition rather than muscle size correlates with knee extensor strength [5, 7,8,9, 40, 41].
It is important to now have evidence of correlations and predictions of PDFF and muscle strength also in healthy subjects as they clearly differ from patients with musculoskeletal disorders, metabolic diseases, or NMD regarding the ranges of PDFF measurements. PDFF values are generally much lower, and the produced strength values are higher in healthy subjects [1,2,3,4,5,6,7,8,9]. Knowledge about such correlations and our prediction model in muscles that are not or not yet pathologically fatty infiltrated allows the PDFF to become a biomarker and to potentially facilitate early treatment protocols or to arrange counteracting individual interventions such as changes in lifestyle or specific training programmes with individual adapted physical activities in order to maintain or improve muscle function. Former studies dealt with the clinical evaluation of the knee joint agonist-antagonist relationship [35, 37, 42,43,44]. Concerning knee dysfunctions and knee joint injuries, like ruptures of the anterior cruciate ligament, there is a high importance of the quadriceps-ischiocrural ratio. In our study, the connection of PDFF, relCSA, and relMVIC was as follows: in both genders, ischiocrural muscles were more infiltrated by fat than quadriceps muscles, had only about one third of the quadriceps relCSA, and produced about half of the quadriceps relMVIC. These insights pave the way for further research to prove the role of PDFF concerning muscle function and its role as an indicator to analyse and define muscle quality and fat-induced loss of muscle function as well as to develop corresponding training programmes dealing with these ratios. As a perspective, the role of physiological CSA, muscular pennation angle, and muscle volume could complement and specify muscle quality [32, 35].
Concerning methodology, chemical shift encoding-based water-fat MRI is confirmed as a fast method which can be added to routine MRI protocols of the thigh region to visualise and quantify muscle quality and possibly forecast deficits in muscle strength and function, leading over to subsequent specific training programmes. In this context, PDFF derived from chemical shift encoding-based water-fat MRI is a more robust and objective approach compared to the semi-quantitative, post-acquisitional analysis of conventional images [13]. However, the method requires compensation for confounding factors, reflected by T2* decay, a potential quantification bias due to multiple spectrum peaks, influence of eddy currents, and the T1 difference between fat and water compartments [28,29,30, 45]. Current sequences, such as the sequence used in this study, take these influencing factors into account. Regarding the muscle strength measurements, there is a need of familiarisation sessions prior to the testing visit to ensure to collect valid data. The rotational dynamometer testing procedure as well as the human ability to develop the real maximum of isometric strength are complex so that more visits are necessary to gain stable values of MVIC. Therefore, all participants performed at least one initial training session.
There are limitations to this study that we acknowledge. First, the comparatively small sample size. Future studies may enrol a larger sample size, which can be particularly realised by multicentre approaches using pre-existing and state-of-the-art imaging collected in joint databases [2]. Second, future studies may add magnetic resonance spectroscopy to explore the distribution of lipids within thigh muscles and distinctly quantify the intra- and extra-myocellular lipid levels [13]. Third, our approach of ROI placement was restricted to segmenting the ten most central slices whilst previous investigations used larger extension or semi-automated algorithms [2, 22, 46,47,48]. However, since we only included healthy subjects who were characterised by rather homogeneous fat distributions, a manual segmentation approach of only representative slices seems to be justified [22, 46]. Fourth, concerning the methodical way of strength measurement, there is a need of including physiological CSA, angle of pennation, and muscle volume in future studies as conclusive, modifying strength predictors and to integrate these values in prediction models [36, 49]. Fifth, the present study did not enrol patients suffering from musculoskeletal disorders, metabolic diseases, or NMD whilst insights in varying muscle quality of different top-performing athletes are still missing.
In conclusion, we observed correlations between the PDFF and relCSA and relMVIC at the thigh by quantitative MRI and precise measurements by a rotational dynamometer. In contrast to relCSA, the PDFF of the quadriceps and ischiocrural muscles was significantly associated bilaterally with the relMVIC in extension and flexion. Thus, chemical shift encoding-based water-fat MRI can provide important information and may potentially track early changes in muscles not pathologically fatty infiltrated. This might help to initiate early, individualised training protocols with specific physical activities in order to maintain or improve muscle function.