Participants
Seventeen healthy young men (age, 21.4 ± 1.9 yrs.; height, 171.0 ± 5.9 cm; body mass, 62.3 ± 5.8 kg; mean ± standard deviation), with no history of a diagnosed neuromuscular disorder or lower limb injury, voluntarily participated in this study. This study was approved by the Ethics Committee of Ritsumeikan University. All participants provided prior written informed consent based on the guidelines of the Declaration of Helsinki.
Experimental procedure
In this study, all participants firstly attended morphological measurements (body height and body mass). Then, MRI measurements were conducted to obtain the foot and lower leg images. After the completion of morphological and MRI measurements, TFS was determined by using a toe grip dynamometer.
Measurements
Muscle size variables (ACSAmax and MV)
T1-weighted MR images of foot and lower leg were acquired using a 1.5 T (Signa HDxt, GE Healthcare UK Ltd., Buckinghamshire) and 3.0 T (Magnetom Skyra, Siemens Healthcare, Erlangen) MR system. Of all participants in this study, eleven participants were scanned by 1.5 T MRI, and the others (six participants) were scanned by 3.0 T MRI, due to the update of the MR system in Ritsumeikan university. To acquire the whole foot image, the participants lay in a supine position on the examination table of the MR systems with their dominant foot and ankle encased in the ankle coils (1.5 T MR system: HD Knee/Foot coil, GE Healthcare UK Ltd., Buckinghamshire; 3.0 T MR system: Foot/Ankle coil, Siemens Healthcare, Erlangen). To reduce motion artifacts during image acquisition, the foot and ankle were positioned in the coils and stabilized with Velcro straps so that the ankle was kept at an angle of 90 degrees of ankle plantarflexion (neutral position). Serial whole foot images were acquired from the sesamoids and calcaneal tuberosity of foot perpendicular to the plantar aspect of the foot, using a fast spin-echo sequence in 1.5 T MR system according to Chang et al. [18] (repetition time = 500 ms, echo time = 13 ms, slice thickness = 4 mm, gap between slices = 0 mm, field of view = 120 × 120 mm, flip angle = 90 degrees, matrix = 512 × 512) and 3.0 T MR system (repetition time = 700 ms, echo time = 12 ms, slice thickness = 3.5 mm, gap between slices = 0 mm, field of view =125 × 125 mm, flip angle =120 degrees, matrix = 1024 × 1024). The data acquisition time for foot was approximately 9 min. For the lower leg image, participants lay on the examination table and lower legs were placed parallel to the main magnetic field. Serial cross-sectional lower leg images were acquired from the knee cleft to just proximal to the malleoli (1.5 T MR system: repetition time = 600 ms, echo time = 7.7 ms, slice thickness = 10 mm, gap between slices = 0 mm, field of view = 360 × 360 mm, flip angle = 90 deg, matrix = 256 × 256; 3.0 T MR system: repetition time = 700 ms, echo time = 9.4 ms, slice thickness = 5 mm, gap between slices = 0 mm, field of view = 360 × 360 mm, flip angle = 120 deg, matrix = 1024 × 1024). The data acquisition time for each scan was approximately 3 min.
Whole foot and lower leg images were analyzed by using specially designed image analysis software (SliceOmatic 5.0Rev-3b, Tomovision Inc., Montreal). Seven plantar intrinsic foot muscles and two extrinsic foot muscles were separately segmented by one examiner (YK): FHB, FDB, ABH, adductor hallucis oblique head (ADDH-OH), adductor hallucis transverse head (ADDH-TH), abductor digiti minimi (ABDM) and quadratus plantae (QP), as plantar intrinsic foot muscles, and FHL and FDL as extrinsic foot muscles. The segmentation was manually performed in every image from the most proximal to the most distal image in which the muscle was visible (Fig. 1). Non-contractile tissues such as bone, tendon, fat, connective tissue, nerve tissue, and blood vessels were carefully excluded. Other plantar intrinsic foot muscles (e.g., lumbrical and flexor digiti minimi) were excluded from the analysis because these muscles could not be visually separated from each other.
Foot length (FL) was determined as the distance between the medial calcaneal tuberosity and sesamoids bone of the first metatarsal in MR images [18]. The positions of all ACSAs of each intrinsic foot muscle were expressed relative to FL (0% FL: medial calcaneal tuberosity, 100% FL: sesamoids bone of the first metatarsal). Furthermore, lower leg length (LL) was determined as the distance between the most prominent point of the medial malleolus at the tibia and intercondylar eminence of tibia in MR images. The ACSAs of each extrinsic foot muscle were expressed relative to LL (0% LL: most prominent point of medial malleolus, 100% LL: intercondylar eminence of tibia).
Two variables, ACSAmax and MV, were adopted as representing the size of each muscle and muscle groups (explained below). ACSAmax was defined as the maximal ACSA along the FL (or LL), and muscle volume (MV) was calculated by summing all the ACSAs for each muscle multiplied by the slice thickness. Intra-rater repeatability for measuring ACSAmax and MV of four participants in this study was assessed by intra-class correlation coefficients (ICC). The ICC (1, 3) values of ACSAmax and MV of individual muscles were 0.869–0.999 and 0.842–0.996, respectively, and good to excellent repeatability was confirmed [21]. In addition, ACSAmax and MV of four muscle groups were analyzed: whole plantar intrinsic foot muscles (all analyzed plantar intrinsic foot muscles), extrinsic toe flexors (FHL and FDL), intrinsic great toe flexors (FHB, ABH, ADDH-OH, and ADDH-TH), and intrinsic lesser toes flexors (QP, FDB, and ABDM).
TFS
In accordance with the procedure adopted in a previous study [22], the maximum voluntary isometric TFS was measured using a commercially available isometric dynamometer (T.K.K. 3361, Takei Scientific Instrument Co., Niigata). The participants were seated in a chair and positioned the hip and knee joints at 90 degrees of flexion with the ankle joint at 90 degrees of dorsiflexion (neutral position) for generating TFS. The participant’s foot was placed on the dynamometer with the posterior heel adjusted at the heel stopper, and the first proximal phalangeal gripped the grip bar. During the measurement, the participants were instructed to cross their arms in front of their chest and gripped the grip bar using all toes as much as possible without any extraneous movements. Familiarization trials for 2–3 times with submaximal force outputs were conducted before the actual measurements. After participants completed the familiarization trials and a rest period of three minutes, the participants performed the task with maximal effort for at least 3 s. The maximal trial was repeated twice with at least one-minute rest, and the larger value of the two measurements was used for further analysis. The ICC for the two measurements was 0.869.
Statistical analysis
Descriptive data are presented as means ± SDs. The ACSAs for each constituent muscle of plantar intrinsic and extrinsic foot muscles along the FL and LL, respectively, were calculated at 5% intervals by spread sheet software (Microsoft Excel, Microsoft Corp., Redmond, WA) and expressed relative to the FL and LL, respectively. Normality of measured variables was assessed by the Shapiro-Wilk test. MV of the ABDM was the only variable that was not normally distributed, and it was log-transformed for further analysis. All subsequent analysis was conducted by using parametric statistical tests. Pearson’s correlation coefficients were computed to examine the relationship between muscle size and TFS. When either or both of ACSAmax and MV had a significant correlation with TFS, the differences between these correlation coefficients for TFS in that muscle were statistically assessed by an online resource (http://comparingcorrelations.org [23] was used to implement Meng et al.’s [24] z test (two dependent groups, overlapping, and two-tailed test)). Stepwise multiple liner regression analysis was conducted by using TFS as a dependent variable, with ACSAmax or MV of the muscles that were significantly correlated with TFS as independent variables. The level of significance was set at p < 0.05. All data were analyzed using statistical software (SPSS 27.0, IBM Co., USA) unless otherwise stated.