This paper describes a process through which students can begin to understand skeletal muscle function. Muscles shorten when they contract, which results in the insertion being pulled towards the origin. Since muscle contractions induce movement(s) at the joint(s) between the origin and insertion, movement is restricted by the mobility of the joint(s). Joint movement is often three-dimensional and can be divided into the following 3 types: flexion/extension, adduction/abduction, and internal rotation/ outer rotation (medial rotation/lateral rotation). To make analyses of movement easier, these 3 types of movement should be analyzed separately by applying the principle of leverage. Essentially, muscle function involves movements that minimize the distance between the origin and insertion at the three dimensional level. Therefore, the movement of a muscle can be predicted based on the locations of its origin and insertion and the mobility of the joint(s) between its origin and insertion. As representative examples, this study describes detailed analyses of the movement of the supraspinatus, infraspinatus, latissimus dorsi, levator scapulae, serratus anterior, and gluteus medius and minimus muscles. Regarding muscle origins and insertions, it should be noted that muscles located in the same part of the body tend to have common origins/insertions, innervations, and similar functions, and such muscles are referred to as a muscle group (e.g., the muscles in the anterior forearm, posterior thigh, the hamstrings, and the adductor group). As the human body contains numerous skeletal muscles, when students start to study muscle functions, they should first learn the common features of a given muscle group, such as its origin, insertion, innervations, and functions, before proceeding to learn the characteristics of individual muscles.

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Our previous study indicated that a new tool is useful and safe for determining the sites of intramuscular injections into the deltoid muscle. However, a comparison of the measurement using tape or the new tool in a living body has not been performed. The difference of the points decided by a method using tape or the new tool between an experienced person and an inexperienced person is also unclear. Therefore, in this study, we compared the results of the measurement between an experienced person and an inexperienced person. The measurement positions by the inexperienced people using both tape measurement and the new tool were higher than those of the experienced person by 0.2 ± 1.0 cm and 0.7 ± 1.0 cm at the distance between the midpoint at the lateral edge of the acromion (a), 1.9 ± 1.2 cm and 0.3 ± 0.8 cm at the intersection point (b), and 1.4 ± 0.8 cm and 0.9 ± 0.7 cm at the lower 1/3ab point, respectively. The difference between an experienced person and an inexperienced person by using the new tool was significantly smaller than that of the tape measurement at the b and lower 1/3ab points. From these results, it is thought that an inexperienced person could measure the intramuscular injection point using the new tool the same as an experienced person.

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