Voluntary activation deficits of the infraspinatus present as a consequence of pitching-induced fatigue
Gandhi J, Elattrache NS, Kaufman KR, Hurd WJ. J Shoulder Elbow Surg. 2011 Aug 9. [Epub ahead of print]
Neuromuscular fatigue is an unfortunate consequence of athletic participation. It places athletes at risk for injury by decreasing dynamic stability of joints and altering normal kinematics. Baseball players are often at a high risk of injury due to the repetitive nature of pitching which causes neuromuscular fatigue of the external rotators. However, fatigue inhibition of the infraspinatus has never been examined following a game simulation. Therefore, the purpose of the study was to examine the voluntary activation and external rotator strength before and after a game simulation. Twenty-one uninjured high school baseball pitchers had external rotation strength assessed bilaterally before and after a simulated game on a dynamometer. Voluntary activation of the infraspinatus was also assessed during strength testing with a modified burst superimposition (electric stimulation). This technique imposes a supramaximal burst of electric stimulation to the infraspinatus on top of the voluntary maximum contraction of the player to determine the amount of neuromuscular inhibition. Pitching velocity and self-reported fatigue levels were also assessed. The simulated game consisted of six innings with 10-15 pitches per inning. Each pitcher threw between 75-90 pitches. There were no bilateral differences for external rotation strength or voluntary infraspinatus activation. Following the simulated game voluntary activation was significantly decreased and external rotation strength approached a significant decrease. Pitch velocity and self-reported fatigue were also significantly decreased following the game.
This study demonstrated that baseball pitching causes neuromuscular fatigue of the infraspinatus in high school baseball pitchers. The fatigue causes a decrease in the voluntary activation of the muscle, which raises the risk of injury. Neuromuscular facilitation exercises like rhythmic stabilization and plyometric training at the shoulder may be useful preventively in baseball players to enhance neural endurance and decrease fatigue related injury. Since pitching is a unilateral sport, the throwing arm commonly undergoes several anatomic and physiologic adaptations to the stress of throwing. It would have been expected to have significant differences in voluntary activation bilaterally although none were found in this population. This suggests that healthy high school players do not undergo an adaptation of the voluntary activation of the infraspinatus on the throwing arm. It would be interesting to again examine the voluntary activation at the end of the competitive season to determine if chronic fatigue may lead to deficits in infraspinatus activation. In addition, this study found that pitching velocity and self-reported fatigue are good clinical tools for determining neuromuscular fatigue in baseball pitchers and should be used to evaluate when the pitcher is truly fatigued and should be removed from the game. What do you do clinically to prevent fatigue related injuries in baseball players? Are there specific exercises? What are your indicators that a pitcher is fatigued and does your coach respect your opinion during a game?
Written by: Stephen Thomas
Reviewed by: Nicole Cattano
When baseball pitching is thought of as high intensity eccentric activity, then this decreased voluntary activation of the infraspinatus is not an abnormal finding. After eccentric activity, such as that experienced during deceleration of the arm in baseball pitching, there is a decrease in strength of the involved musculature. This has been considered to be as a result of the micro-tear muscle damage that typically follows. However, according to Prastartwuth et al., this decrease in strength is not due to DOMS, but an activation deficit at the motor cortex. This means that there is a disconnect between the brain and the muscles. Again, this can be explained by the hyperactivity of the mechanoreceptors. Sherrington’s Law of Reciprocal Inhibition illustrates that tightness in one motion can identify a weakness in the opposite motion. In other words, if internal rotation is tight, then external rotation is weak. Reinold et al. have demonstrated losses of up to 11 degrees of shoulder total arc rotation following a bout of pitching. The weakness discussed may then be related to a disrupted motor recruitment pattern and motor programming. Research on eccentric muscle activity has shown that in some cases after training or performance, muscle strength declined by almost 40% and continued 10 days post-activity with only slight recovery.
Thanks for the comment. You bring up some interesting points. We do commonly see significant decreases in strength following eccentric exercise or a simulated game, however it is interesting that the researchers only found the decreases in strength to approach significance. Yet the activation levels were significantly different. This illustrates to me that like you stated this fatigue is not being produced by muscle damage but more importantly from the neuromuscular system. It is difficult to determine if it is at the motor cortex, spinal cord, or the neuromuscular synapse. Regardless it is clear that it is mainly neuro fatigue and may be more difficult to identify clinically.
I'm not sure if reciprocal inhibition plays a role in this situation. If tightness contributed to the results then they would have found bilateral differences in the players prior to the game simulation. Since baseball players commonly have bilateral motion deficits.
Thanks again for your comment.