HOMINID THUMB STRENGTH PREDICTED BY HIGH-RESOLUTION MAGNETIC-RESONANCE-IMAGING AND FORCE MEASUREMENTS IN LIVING SUBJECTS

This study investigates whether in-vivo measurements of the morphology of the pollical distal phalanx (PDP) can be used to predict the maxim um force that can be exerted by the thumb in flexion at the interphala ngeal joint. A predictive equation was obtained via which measurements of fossil PDP's can be used to predict flexion force in hominids. A h ome-built insertable coil assembly, which increased the available magn etic field gradient strength from 10 to 40 (mT) m(-1), was used to acq uire high resolution T1-weighted 3D SPGR Magnetic Resonance (MR) image s of the left and right thumbs of nine volunteers. The subjects' age, sex, weight, height and self-assessed hand-dominance were recorded. Th e MR images were transferred to a computer for reformatting of section s in the transverse and volar plane (VP). Measurements were made of th e maximum length (ML), breadth (MB), tuft breadth (MT) and joint depth (JD) of the PDP. For each subject the maximum flexion force that coul d be exerted by each thumb was measured. The mean maximum flexion forc e generated was 8.21 kg (range 5.68-13.13 kg). Considerable inter-indi vidual variability was observed in the magnitude of the force differen ce between the left and right thumbs and this is greatest in individua ls with the strongest thumbs. The best model produced by multiple line ar regression analysis of all the available data has a value of r = 0. 77 and needs only the two covariates of sex and weight, i.e. FORCE = 2 .217 + 0.6651 x WEIGHT + 2.458 x SEX. However, in order to be useful f or predicting the maximum flexion force of the thumb in hominids the m odel should only contain those parameters which can be measured for fo ssil PDP's. Accordingly, the best predictive model has a value of I = 0.73 and needs only the two covariates of ML and JD, i.e. FORCE = -10. 28 + 0.2053 x ML + 1.571 x JD. The maximum force predicted for any of the hominid fossil PDP's was not significantly different from those re corded for the volunteers in the present study, although they were on average somewhat lower than those obtained for modern humans. This may be a reflection of the hominid's overall smaller body weights. (C) 19 97 Elsevier Science Inc.