This is the first study to apply FDG-PET to investigate comprehensive lower-body skeletal muscle activity during foot exercise. The most important findings of this study were that the foot rock-paper-scissors exercise affects the medial longitudinal arch of the foot, while the towel-gathering exercise affects the muscles involved in the grip of the toes. As these findings indicate that the two exercises target different skeletal muscles, it is necessary to train them according to their respective purposes.
Glucose is one of the energy sources for skeletal muscle. Like glucose, 18F-FDG is taken up by myocytes, although it is not metabolized and remains in myocytes as FDG-6-phosphate (“metabolic trapping”) [16, 17, 19]. Since metabolic trapping is maintained for approximately 2 h after injection [21], FDG-PET reflects skeletal muscle glucose metabolism during exercise. Fujimoto et al. used PET to evaluate muscle activity during exercise in one of the first PET-based studies on muscle activity during running [16]. Other studies have investigated PET during more complex tasks requiring endurance, such as running [22] and double-poling [23]. In a previous study, our group applied FDG-PET to the FIFA 11+ training program and reported on changes in muscle activity during training [20, 24]. We have also evaluated muscle activity in the lower limbs using a belt-electrode skeletal muscle electrical stimulation system to demonstrate the effectiveness of FDG-PET in passive exercise [25]. These findings provide a rationale for assessing skeletal muscle activity using FDG-PET.
For isometric strengthening of the intrinsic foot muscle, the most recognized exercise is the short foot exercise [26], where volitional control of the intrinsic foot muscles elevates the foot arches and shortens the foot. This exercise is described as part of the core paradigm introduced by McKeon et al. [26]. The short foot exercise is typically challenging to teach and learn; therefore, three gradual training steps have been recommended. Foot rock-paper-scissors is a complex exercise that consists of toe clenching, spreading the toes out, and extension of the first toe in succession, but it is easy to learn. When the toes are spread out, because of the circumferential motion that occurs at the first and fifth toes, activation of the muscles extends and abducts these digits.
The results of the present study showed that, as in previous investigations [5–7, 9], foot rock-paper-scissors produced significant skeletal muscle activity in the extensor hallucis brevis, flexor digitorum longus, and quadratus plantaris muscles. During towel gathering, significant skeletal muscle activity was observed in the tibialis anterior, peroneus brevis, extensor hallucis brevis, and abductor hallucis muscles. Significantly more activity in the extensor hallucis longus muscle was observed during towel gathering than during foot rock-paper-scissors (P=0.046). There was also a tendency for skeletal muscle activity to occur in the tibialis anterior, extensor hallucis longus, extensor digitorum brevis, flexor hallucis brevis, adductor hallucis, and abductor digiti minimi muscles in the foot rock-paper-scissors group. However, there was large variation among individuals, and no significant difference was observed. In the towel gathering group, skeletal muscle activity tended to occur in the flexor digitorum longus, flexor hallucis longus, abductor hallucis, flexor digitorum brevis, quadratus plantae, lumbrical, and interosseous muscles. Towel gathering involved effective activity not only in the flexors of the toes but also in the extensors. During towel gathering, all toes are flexed via contraction of the flexor digitorum longus and flexor hallucis longus muscles to grasp the towel. Next, the tibialis posterior and tibialis anterior muscles contract by dorsiflexing the ankle joint while grasping the towel. Finally, when releasing the towel, the toes are extended and abducted. Although we speculated that the abductor hallucis longus muscle was used, we observed that the extensor muscle group contracted at the same time. Previous studies using EMG focused only on the flexor muscle group because of its characteristics, which remain to be clarified [10].
The present results indicate that foot rock-paper-scissors is effective for exercising the medial longitudinal arch, while towel gathering is effective for improving the toe grip force. Furthermore, Kelly et al. [27] reported that there is a positive correlation between the muscle activities of the abductor hallucis, flexor digitorum brevis, and quadratus plantae, which are clearly important in postural control and are recruited in a highly coordinated manner for stabilization of the foot and maintenance of balance in the mediolateral direction, particularly during single-leg positions. This suggests that continued towel gathering may also be effective in reducing the sway of the center of gravity.
This study had several limitations. First, the FDG-PET method captures only muscle glucose uptake. Although other substrates such as free fatty acids, muscle glycogen, and lactate are also metabolized in active myocytes, glucose oxidation increases with exercise intensity, and glucose uptake increases in proportion to glycogen utilization when exercise intensity rises [17]. In addition, previous reports have shown that FDG uptake is higher in muscles composed of type I fibers than in muscles composed of type II fibers [28]. Therefore, this result may not completely reflect all skeletal muscle activity. Second, the study included a single session only, preventing us from examining continuous effects. Future studies should aim to evaluate the effects of continuous training sessions on skeletal muscle activity, changes in toe muscle strength, and improvements in static balance ability. Indeed, the comprehensive patterns of skeletal muscle activity for commonly performed foot exercises are unknown, and the skeletal muscles related to the control of static balance and extent of their contributions remain to be clarified. Third, since a manual method was used to measure the SUV, the ROI range may not be accurate. Fourth, the measurement was performed in one slice using the landmark as an index, meaning that it does not reflect the activity of the entire muscle. However, data from our previous studies suggest that the difference is not significant [20, 24]. Lastly, this study was conducted on trained athletes. The results of this study may not be generalizable to patients with lower extremity disorders.