In this study, the whole-body skeletal muscle metabolism during pitching motion was evaluated using PET. An accumulation of FDG was observed in the relatively small skeletal muscles of the fingers and toes, and asymmetric accumulation was observed in the thigh muscles. Interestingly, there was minimal accumulation of FDG in the rotator cuff and trunk muscles, which are considered important for the throwing motion.
EMG has been used in the evaluation of skeletal muscle activity during pitching [1, 6,7,8,9,10,11,12,13]. Since EMG has excellent time resolution, it was useful for investigating skeletal muscle activity in each throwing phase by synchronizing the time axis to motion analysis . However, when using EMG with surface electrodes, since it is necessary to attach the electrode beforehand to the muscle to be studied, it is only possible to measure the muscle that can be touched from the body surface and the muscles scheduled for investigation [4,5,6]. Therefore, it is difficult to observe the influence of exercise on skeletal muscle activity in deep-lying regions of the trunk and extremities . Alternatively, wire electrodes can be used for deeper skeletal muscles; however, they are invasive [8, 12]. In addition, the accuracy of the insertion site and the influence of noise artifacts similarly have a significant impact [8, 12]. Furthermore, in evaluation using EMG, the skeletal muscles of the whole-body are invisible, and the device’s cords and electrodes interfere with motion . Hence, it becomes impossible to study muscle activity in conditions that actually replicate the real-world pitching environment and performance . Therefore, only partial investigations on the upper limbs [6,7,8,9, 11, 12], lower limbs [4, 10, 13], or the trunk  have been performed in pitchers so far.
Some studies have utilized FDG-PET to display cumulative muscle metabolism during exercise [15, 16, 18,19,20]. FDG taken up by muscle cells is not metabolized and remains in the cells as FDG-6-phosphate after phosphorylation . Thus, FDG accumulation in the muscle can be used as a parameter of glucose uptake by the muscle and is an indicator of muscle activity . Glucose metabolism measured by FDG-PET demonstrates a high correlation with the intensity of muscle activity, and its reliability as an index for measuring muscle activity has been confirmed . Fujimoto et al.  and Tashiro et al.  used PET in the evaluation of muscle metabolism during running (one of the first studies). Other studies have investigated tissue glucose uptake with PET during tasks such as isometric muscle contractions  and dynamic strength exercises , and during more complex tasks requiring endurance, such as walking , running , and double poling . In a previous study, whole-body PET-CT was used to examine where glucose uptake occurs in the skeletal muscles in a single performance of Part 2 of the Fédération Internationale de Football Association’s 11+ injury prevention program  and following its 4-week routine performance . Bojsen-Møller et al.  proposed that PET imaging might be a promising adjunct modality or an alternative to more traditional methods of investigating muscle activity during complex human movements. Rudroff et al.  mentioned that PET has the potential of being the “gold standard” of metabolic imaging for exercise physiology.
In this study, it was observed that the accumulation of FDG in the relatively small skeletal muscles of the fingers, especially in the muscles that have their origin and insertion within the hand (intrinsic muscles). The fingers control, which is the end of the kinetic chain of the pitching motion, is extremely precise and important . The intrinsic muscles are known to play a vital role in the flexing and stabilization of the metacarpophalangeal (MCP) joint. Ketchum et al.  reported that the intrinsic muscles contribute more than 70% of the overall moment of the flexion at the MCP joint. However, studies on the intrinsic finger muscles activity during pitching have not been conducted. This is presumably because the evaluation of intrinsic muscles is difficult with EMG. Using another approach, Kinoshita et al.  measured finger force for fastball pitching; a linear relationship was observed between peak forces and ball velocity, and the peak ball reaction force for fastballs exceeded 80% of maximum finger strength. According to these studies, the training of finger muscles is important for enhancing pitching performance [26,27,28]. Similarly, FDG accumulation increased in multiple intrinsic foot muscles in this study. The control of the intrinsic foot muscles is equally important for the first step in the kinetic chain of the pitching motion . However, the intrinsic foot muscles activity has not attracted significant attention until now due to the difficulty to be evaluated with EMG [4, 10].
Some skeletal muscles of the femoral region demonstrated unilateral accumulation of FDG. This might reflect the asymmetric movements performed during pitching. The pitching motion has been classified into six phases . Specifically, it was suggested that the iliacus on the non-throwing side is involved in the lifting movement of the lower limbs during the wind-up phase [4, 10, 13, 29, 30]. On the contrary, the medial hamstrings, vastus medialis, and adductor muscle on the throwing side are responsible for the translational motion during the stride phase to the acceleration phase and the raising motion of the lower extremity during the deceleration phase [4, 10, 13, 29, 30]. The sartorius and gracilis muscles on the non-throwing side are involved in the process of standing up on one leg during the acceleration to follow-through phase [4, 10, 13, 29, 30]. In a previous EMG study, Yamanouchi  investigated the abductor, adductor, quadriceps, biceps, tibialis anterior, and gastrocnemius muscles activities; it was concluded that the abductor and adductor muscles play important roles. Similarly, activation of the adductor muscle was observed in this study. In the investigation of the vastus medialis, biceps femoris, rectus femoris, gluteus maximus, and gastrocnemius muscles activities by Campbell et al. , it was concluded that stride foot contact on ball release was the most demanding phase of the pitching motion with a very high bilateral muscular activation. The iliacus and sartorius muscles were excluded from the investigation . Since the iliac muscle is in a deep position, it would be difficult to investigate its activity using EMG. Regarding the biceps femoris muscle on the throwing side, reports equally indicated a high activity [4, 10, 13]. In addition, Erickson et al.  conducted EMG studies and reported a higher hamstring activity in the throwing side than the non-throwing side, which is consistent with this study.
Interestingly, minimal accumulation of FDG was observed in the rotator cuff and trunk muscles in this study, suggesting the possibility that these muscles contribute less to the throwing movement. The shoulder is an integral part of the kinetic chain in the throwing motion. Rotator cuff muscles act as dynamic stabilizers of the glenohumeral joint during throwing [6, 7]. An initial report of shoulder muscle activity during baseball pitching evaluated by EMG was published in 1983 by Jobe et al. . Since then, considerable research using EMG for the muscles around the shoulder joint during pitching has been performed [6,7,8,9, 11, 12]. According to these studies, the rotator cuff demonstrated high activity during the early cocking to deceleration phases [6,7,8,9, 11, 12]. This study showed no increase in glucose metabolism in the rotator cuffs. Around the shoulder joint, increased metabolism was observed in the posterior aspect of the deltoid muscles on both sides, the trapezius muscle, teres major, latissimus dorsi on the pitching side, and the levator scapulae on the non-throwing side in this study. Similarly, increased metabolism in both sides of the biceps and brachial muscles was observed in this study.
There is only one report on muscle activity of the trunk during pitching measured by EMG . In the investigation of the muscle activity of the abdominal rectus, abdominal oblique, lumbar paraspinous, and gluteus maximus muscles by Watkins et al. , it was reported that the abdominal rectus and abdominal oblique muscles on the non-throwing side demonstrated an increase in the activity level. In this study, no increase in glucose metabolism was observed in the trunk muscles; however, increased metabolism was observed only in the gluteus maximus muscle on the throwing side.
Understanding the patterns of whole-body muscle metabolism in baseball pitching is important to enhance performance and decrease injury potential . For practical application, data from this study provide strength and conditioning professionals more definitive evidence on the importance of finger and foot muscular strengths and endurance training for pitchers. Furthermore, training regimens promoting both bilateral and unilateral lower extremity muscular strengths and endurance in multiple planes (similar to the movements in pitching) are critical to address the specific demands of the pitching motion [1, 3, 4, 30, 31].
This study had some limitations. First, the number of throws that is optimal for measuring FDG accumulation is unclear. Based on previous studies, it was better to exercise for 20 min before and after the FDG injection. Thus, the participants threw 40 balls in 20 min before and after FDG injection. However, since the pitching motion was completed in approximately 2 s, the appropriateness of the exercise load in this study is questionable. In addition, it is possible that the motion and the pattern of muscle metabolism changed with fatigue . We are planning to examine these issues by increasing the number of throws in future studies. Second, the method of FDG-PET only accounted for muscle glucose uptake. Other substrates, such as free fatty acids, muscle glycogen, and lactate are metabolized in active muscle cells. Nonetheless, glucose oxidation increases with exercise intensity, and glucose uptake increases fairly, in proportion to glycogen utilization with an increase in the intensity of exercise . Third, the method used to define ROI. As FDG uptake was measured at an arbitrary site on the target muscle, it did not reflect the uptake of the entire muscle volume; therefore, further investigation of muscle metabolism using PET is required. Fourth, the sample size was limited due to concerns over radiation exposure. The effect size of the mean SUV of the peroneus brevis was 2.33 on the throwing side, which was greater than the expected level of 2.0; this was the basis for calculating the sample size before the investigation. Thus, the sample size of the pitcher and control groups (n = 10 each) in this investigation was appropriate. Despite these limitations, no study has employed PET-CT in the examination of the whole-body muscle metabolism (during pitching).
To the best of our knowledge, this is the first study to evaluate whole-body skeletal muscle metabolism during pitching. PET is a useful tool for measuring the skeletal muscle metabolism of the whole body in an environment that accurately replicates actual pitching and is, therefore, useful for assessing other dynamic activities. As EMG research can only measure specific parts of the body, it was necessary to integrate several different studies of various participants and methods to consider and devise a novel whole-body training method.