The dynamic control ratio and its equilibrium point:  A preliminary study of isokinetic fatiguing internal-external rotational effort of the shoulder joint in healthy subjects

Journal of Electromyography and Kinesiology

Purpose  A number of studies have looked into the glenohumeral joint-related dynamic control ratio (DCR), an isokinetic strength ratio derived from the division of the peak eccentric moment of external rotators (ER) by the peak concentric moment of the internal rotators (IR). However, given the shape of the moment curves of and the non-coincidence, anglewise, of the peak moment, the DCR itself a continuous function and not by a single value. This study aimed therefore to explore the variation of the ER-IR related DCR at a high angle resolution and explore its behavior under fatiguing conditions. 
Methods  18 young adults, split between those with experience (E) and those without (NE) in overhead sporting activities completed two separate series of 45 ER eccentric and 45 IR concentric isokinetic contractions at 120°/s. The analysis focused on contractions 1-3 (TR1), 21-23 (TR2) and 41-43 (TR3).
Results   The relative fatigue values for IR, in E and NE and for ER, in E and NE subjects were 40, 32, 25 and 27%, respectively with significantly higher fatigue resistance in ER (E) compared with IR (E) and with IR (NE). All DCR traces varied linearly with the progression of internal rotation of the forearm while significant differences (p<0.001) were found in their values within group between TR1,TR2 and TR3 and between E and NE groups. Reaching the equilibrium point (DCRe = 1) was  common in all instances, for both groups, only during TR3 and while no significant differences were noted for the DCRe angle, upon pooling the two groups, a significant progressive decline in the moment at DCRe took place. 
Conclusions  Given the current findings the use of the DCR as a single value isokinetic parameter for depicting the antagonistic synergy between the GHJ rotatory muscles should be discouraged. Instead, and especially under demanding conditions e.g. due to fatigue, the use of the angle-based DCRs and the angle-moment values of the DCRe, may provide a much wider view while serving as effective clinical markers for the integrity of this synergy. However, proving these contentions requires comprehensive basic and clinical research.

Suzuki K, Okada T, Hakkaku T, Katz-Leurer M, Dvir Z

vol.70, Jun 2023