T2-mapping MRI Detects the Anaerobic Threshold for Flexion and Extension of the Wrist: Potential Use in Setting Rehabilitation Strength Targets.

Abstract

Objectives: Upper-limb weakness significantly impairs activities of daily living in individuals with neurological disorders. Exercise at the anaerobic threshold or higher is recommended for efficient training and rehabilitation in such cases. Given a lack of standardized, non-invasive methods to assess the anaerobic threshold for small muscles in clinical practice, we used T₂-mapping magnetic resonance imaging (MRI) and cumulative frequency curves of Z values (CFZ) to detect a minimum exercise strength target for physical rehabilitation of forearm muscles above the anaerobic threshold.

Methods: T₂ values for ten forearm muscles in 13 healthy adult volunteers were obtained in the resting state and after isotonic flexion and extension of the wrist. T₂ was normalized by Z = (T_2e- T_2r)/SD_r, where T_2e is T₂ after exercise, and T_2r and SD_r are the mean and standard deviation of T₂ at rest.

Results: In the horizontal direction exercise, the sensitivity values and CFZ of four out of five agonist muscles did not increase, nor shift, showing that these muscles were contracted by the aerobic metabolism. In the vertical direction exercise, in four of five agonist muscles and in two synergist muscles, the sensitivity values increased to about 50% and the CFZ was shifted to the right, indicating that these muscles were contracted by the anaerobic metabolism.

Conclusions: Vertical direction exercise without extra weight can provide a minimum exercise strength that is higher than the anaerobic threshold. T₂-mapping MRI is useful for setting exercise strength targets in the physical rehabilitation of small muscles.