Dimensional constraints on limb movements Karl M. Newell a, * , Sue Challis a , Steven Morrison b a Department of Kinesiology, The Pennsylvania State University, 146 Recreation Building, University Park, PA 16802, USA b Grith University, Gold Coast, Qld, Australia Abstract The dimensional constraints on limb movements were investigated by asking adult par- ticipants to move speci®c segments of the upper limb (Experiment 1 ± index ®nger(s); Ex- periment 2 ± index ®nger, hand, lower arm, whole arm) with random frequency and amplitude over 2 min trials. The results showed that the amplitude±frequency characteristics of the random trials were limited to particular ranges within the respective potential workspace and with distributions that were broader than that of preferred oscillatory trials. The approximate entropy and dimension of the joint motion was higher in the random than the preferred conditions, but the random movements were shown to have a relatively low dimension. The irregularity of the random movements was enhanced in the more distal limb segments. The ®ndings reveal that the structural and functional constraints to planar motion at a single joint coalesce to provide a set of relatively low dimensional boundary conditions. The implication is that the control structure induces a more limited reduction in the dimensionality of the active system degrees of freedom than is typically postulated in theories of movement con- trol. Ó 2000 Elsevier Science B.V. All rights reserved. PsycINFO classi®cation: 2330 Keywords: Motor control; Degrees of freedom; Random movement Human Movement Science 19 (2000) 175±201 www.elsevier.com/locate/humov * Corresponding author. Tel.: +1-814-863-1163; fax: +1-814-863-7360. E-mail address: kmn1@psu.edu (K.M. Newell). 0167-9457/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 9 4 5 7 ( 0 0 ) 0 0 0 1 0 - 5