Maximum toe flexor muscle strength and quantitative analysis of human plantar intrinsic and extrinsic muscles by a magnetic resonance imaging technique

Twenty six young healthy sedentary volunteers (14 men, 12 women; age, 20.4 ± 1.6 yrs; height, 167.5 ± 7.5 cm; body mass, 60.7 ± 8.6 kg; means ± standard deviation [S.D.]) participated in the study. None of them reported any history of diagnosed neuromuscular disorder or lower limb injury. In addition, the participants had no visible symptoms of hallux valgus or toe deformities. Foot anthropological measurements, i.e. foot length (FL) and arch height (AH), were measured using a ruler. FL was determined as the length between the most posterior aspect of the calcaneus and the tip of the longest toe, and AH was defined as the perpendicular distance between the navicular tuberosity and the floor [17]. Next, the arch index (AI) was calculated as the ratio of AH to FL.

The methods and procedures used in this study were in accordance with the current local guidelines and the Declaration of Helsinki, and were approved by the Research Ethics Committee involving Living Human Participants at Ritsumeikan University. All the participants were informed about the study requirements, benefits, and risks prior to study onset. Written consent was obtained from each participant.

The maximum voluntary isometric TFS was measured using a specifically designed dynamometer (T.K.K. 3361, Takei Scientific Instrument Co., Niigata), which has a range of force between 1 and 400 N. The experimental setup for maximum TFS measurements is shown in Figure 1. TFS was measured in the sitting condition with 90 degrees of their hip and knee joint angles with the ankle in a neutral position in order to reduce the plantar fascia tension by truss effect. Under vertical loading or body mass on the foot, the plantar aponeurosis was extended by the truss structure of the foot [18]. Participants were instructed to place their test foot within the heel stopper and optimally gripped the grip-bar with their toes on the dynamometer. During the measurements, participants placed their arms in front of their chest and were instructed to perform the task without moving their trunk from the seat or pulling the grip-bar by flexing the knee. Participants first performed a few test contractions as submaximum efforts to familiarize themselves with the measurement. Then, they performed voluntary isometric contractions as explosively as possible and attempted to maintain the maximum force for 3 seconds. Measurements were repeated three times with at least a one-minute rest period between bouts. Both right and left feet were measured in a randomized order. The largest value among the three trials was used for further analysis.

The participants lay in a supine position on the examination table of a 1.5-T MR system (Signa HDxt, GE Healthcare UK Ltd., Buckinghamshire). Whole foot images were acquired using an ankle coil (HD Knee/Foot coil, GE Healthcare UK Ltd., Buckinghamshire) positioned in the center of magnet. To reduce motion artifacts during image acquisition, the foot and ankle were encased in the coil and stabilized with cushions and Velcro straps so that the ankle was at an angle of 15 degrees of ankle plantarflexion. Serial T1-weighted MR images were acquired from the longitudinal distance between sesamoids (0% foot length) and calcaneal tuberosity (100% foot length) of foot perpendicular to the plantar aspect of the foot, using a fast spin-echo sequence (repetition time = 500 ms, echo time = 16 ms, averages = 3, slice thickness = 4 mm, gap between slices = 0 mm, field of view = 120 × 120 mm, flip angle = 90 degrees, matrix = 512 × 512), according to Chang et al. [7]. The data acquisition time for each foot was approximately 9 min. For lower leg images, legs were placed to parallel to the main magnetic field. Serial T1-weighted axial MR images were acquired from the knee cleft just proximal to the malleoli (repetition time = 600 ms, echo time = 7.7 ms, averages = 2, slice thickness = 10 mm, gap between slices = 0 mm, field of view = 360 × 360 mm, flip angle = 90 deg, matrix = 256 × 256). Due to the length of the legs, images acquisition required two passes. The data acquisition time for each scan was approximately 3 min.

To determine the location for measuring the CSA, we selected the image at the MTP joint that was near 20% of the longitudinal foot length. ACSA of plantar intrinsic muscle is the largest at the location of 20% of the longitudinal length [7]. Examining each image, we excluded non-contractile tissues such as bone, tendon, fat, connective tissue, nerve tissue and blood vessels, wherever possible. To identify an accurate segmentation of each individual muscle was not possible for the smaller muscles, but anatomical groups of the intrinsic muscles could be identified. Green and Briggs [10] have divided the plantar intrinsic muscles into five groups: medial (abductor hallucis and flexor hallucis brevis), adductor (adductor hallucis), central (flexor digitorum brevis, quadratus plantae and lumbricals), interosseous (dorsal and plantarinterosseous), and lateral (abductor digiti minimi and flexor digit minimi brevis). In this study, the plantar intrinsic muscles were divided into three muscle groups as follows; MED: flexor hallucis brevis, flexor digitorum brevis, quadratus plantae, lumbricals and abductor hallucis, ADH: adductor hallucis, and LAT: abductor digiti minimi, flexor digiti minimi brevis, dorsal and plantar interosseous muscles (Figure 2), because the boundaries of muscle groups were not easily separable to five groups in all subject. For extrinsic foot muscles, FHL and FDL were identified in each image along with the lower limb, and maximum ACSA of each muscle was determined. All measurements and calculations were conducted by the two investigators (TK, NT) using specially designed image analysis software (SliceOmatic 4.3, Tomovision Inc., Montreal).

Using the measured CSA of each muscle, the estimated maximum muscle strength (Fmax) was also calculated from the specific tension. The values of the specific tension were used 15–25 N/cm² for lower and upper estimations of maximum muscle strength [15, 16].

All data were presented in the form of mean ± S.D. The intra-rater reliabilities of TFS measurement were assessed by intra-class correlation coefficient (ICC). A linear regression analysis was performed for the relationship between TFS and CSA of each muscle, and we also examined whether the y-intercept for the regression line was different from zero. Stepwise multiple linear regression models were used to calculate the dependent variables of TFS among test parameters (FL, AH, AI and CSA of each muscle). An unpaired t-test was used to examining the differences between genders. The level of significance was set at p < 0.05.