In his first start of the season, Jacob deGrom left the game early due to a stiff right lat muscle. He will miss his next start due to ongoing stiffness/soreness. Let's cover what the lat muscle is and its importance in pitching.
What is the lat muscle?
The Latissimus Dorsi—or "lat"—is a huge muscle located on your lower back that travels up the side of your back and inserts at the top of your humerus, the bone above the elbow. All together, the lat acts to extend, adduct (bring towards the body), and internally rotate (turns towards the body) the shoulder.
Why does lat stiffness impair deGrom's pitching?
After a pitcher winds up and cocks the ball back, the lat, among other muscles, contributes to generating force to move the baseball forward before releasing it towards the batter. In this position, the muscle is stretched out before contracting—therefore shortening the muscle—to throw the ball. A stiff muscle means that muscle is unable to stretch as far out as it should. As a result, deGrom's arm angle is changed, as well as the rest of his pitching mechanics.
A stiff muscle causes the other muscles around it to work harder because the muscle cannot create the sufficient force required to complete the task. Repeat this process for about 100 times, or however many pitches he will throw in a start, and the entire shoulder takes a beating from compensating for the stiff lat muscle.
The best course for deGrom right now is to perform stretches to make the muscle and shoulder capsule more flexible in addition to treatment from athletic trainers and physical therapists to supplement the benefits from stretching. Treatment can include hands-on techniques to loosen the muscle or modalities such as electrical stimulation and ultrasound.
At this point, it remains uncertain how long deGrom will remain sidelined, but the Mets should proceed with caution, as Steven Matz had a similar injury last season and he was out for approximately two months. deGrom reported lower back issues earlier in the spring and that could certainly contribute to the present stiffness of his right lat muscle. Furthermore, this minor injury can be a possible cause of his velocity issues over the past few weeks.