Measuring hand intrinsic muscle strength: Normal values and interrater reliability

Background: The intrinsic muscles of the hand are of interest in spinal cord injury (SCI) and other myelopathies, because they are innervated by the most caudal cord segments innervating the upper limb. In addition, abnormalities of the peripheral nervous system, such as peripheral nerve entrapments, often affect hand intrinsic muscle strength of SCI patients. Therefore, measuring hand intrinsic strength may allow for early diagnosis of neurologic decline. Methods: A method was developed for measuring strength of hand intrinsic muscles with a handheld myometer. With the use of a handheld myometer, this study examined the distribution of strength measurements for second-digit abductors, fifth-digit abductors, and thumb opposers in able-bodied participants and in individuals with weakness. The quantitative measurements were compared with manual muscle test scores and interrater reliability is described for these hand intrinsic strength measurements. Thirty-one able-bodied individuals participated (17 men, 14 women; mean age = 37.7 years). In addition, 24, patients with SCI participated (23 men, 1 woman; mean age = 53.5 years; 9 with paraplegia and 14 with tetraplegia as a primary diagnosis). The Bland-Altman method was used to test for interrater reliability. Results: Mean strength of able-bodied participants was 5.0 kg for second-digit abduction, 3.1 kg for fifth-digit abduction, and 5.0 kg for thumb opposition, and the lower limits of normal were 3.0, 1.8, and 3.4 kg, respectively. The 95th percentile of interrater differences were 29.3% for second-digit abduction, 38.5% for fifth-digit abduction, and 43.7% for thumb opposition. Conclusion: Abnormal hand intrinsic strength should be suspected if values are lower than the 5th percentile values listed above or if strength change exceeds the 95th percentile for interrater differences shown above. These quantitative hand strength measurements may allow for earlier diagnosis of secondary neurologic complications and may aid in monitoring neurologic recovery in persons with SCI.