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I t p a y s t o b e A B M P C e r t i f i e d : w w w. a b m p . c o m / g o / c e r t i f i e d c e n t r a l 93 STORY 5 PASSIVE AND ACTIVE INSUFFICIENCY We'll fi nish with a great party trick! First, the setup: muscles suffer diminished capabilities in positions of extreme shortness or length. This topic deserves additional consideration for bi- and multiarticular muscles such as the fl exors and extensors of your wrist and fi ngers. As we discussed earlier, a muscle —being both contractile and extensible—can assume quite a range of lengths. The extent that it can change in length is called its excursion (Image 18). Generally, a muscle can shorten or elongate by half of its resting length. Thus, if a belly's resting length is 6 inches, it can shorten to 3 inches or stretch to 9 inches. When a bi- or multiarticular muscle is in a position of maximum excursion (length), you are susceptible to encountering passive insuffi ciency. All right, now for that soiree stunt. Begin with your wrist in neutral (not fl exed or extended), and make a tight fi st. Easy, right? Now, let's try it again, but this time fully fl ex your wrist and then—without letting the wrist extend—try to make a fi st (Image 19). You can't fully clench, can you? This is because of the excursive limit of your antagonist fi nger extensors. Since they span the posterior knuckles as well as the fl exed wrist, their maximum tautness prevents your fi ngers from fully curling. You can switch the muscles' roles, too. Begin at neutral and then extend your wrist. Note how your fi ngers naturally fl ex a bit (and how diffi cult it is to fully extend them with the wrist extended). This is due to the passive insuffi ciency of the wrist fl exors (Image 20). Stretched, 9 inches length Relaxed, 6 inches length Contracted, 3 inches length To further explain: passive insuffi ciency occurs when an action is inhibited because the antagonist muscle cannot lengthen suffi ciently to allow the desired movement to occur. It is stretched too far. This is an issue mostly for actions involving bi- or multiarticular muscles because there is the possibility that the antagonist faces a stretch- intensive position at the multiple joints it crosses. There's something else at work here: active insuffi ciency. This occurs when an action is weakened or incomplete due to excessive shortness of the multi- joint agonist. Returning to your partial fi st (Image 19), the fl exion of your wrist and fi nger joints places your fl exor muscles in an ultrashortened position. They can't shorten any further (due to the limitations of the sliding fi lament mechanism). And let's not forget that in positions of such extreme shortness (or elongation), muscles become weak. Thus, your fl imsy, incomplete fi st is due to both insuffi ciencies. 18 A muscle in three phases of excursion. 19 Passive insuffi ciency as you try to fl ex both your wrist and fi ngers to make a fi st. Red line indicates pathway of extensor tendons. 20 Turn over the hand for a different passive insuffi ciency. Green line marks the pathway of fl exor tendons. Andrew Biel, LMP, is the author of Trail Guide to the Body: How to Locate Muscles, Bones and More (Books of Discovery, 2014) and the president of Books of Discovery. This is an excerpt from his forthcoming book Trail Guide to Movement: Building the Body in Motion (Books of Discovery, 2014). He lives outside of Lyons, Colorado, with his wife, Lyn Gregory, and two children, Grace and Elias.