**Tessy Luger \*, Robert Seibt, Monika A. Rieger and Benjamin Steinhilber**

Institute of Occupational and Social Medicine and Health Services Research, University Hospital and University of Tübingen, Wilhelmstraße 27, 72074 Tübingen, Germany; robert.seibt@med.uni-tuebingen.de (R.S.); monika.rieger@med.uni-tuebingen.de (M.A.R.); benjamin.steinhilber@med.uni-tuebingen.de (B.S.) **\*** Correspondence: tessy.luger@med.uni-tuebingen.de; Tel.: +4970712984364

Received: 31 January 2019; Accepted: 2 April 2019; Published: 6 April 2019

**Abstract:** We investigated whether physical requirements and motor variability decreased over days in novices during a repetitive screwing task. Fifty-seven subjects performed one hour of repetitive screwing and fastening on three days, separated by 2–7 days. The average physical requirement and relative cycle-to-cycle variability (coefficient of variation, i.e., CV) were calculated from continuous recordings of electromyography of four arm muscles (biceps brachii, triceps brachii, flexor carpi radialis, extensor digitorum), forearm acceleration, and electrocardiography. Muscle activity levels, heart rate, and forearm acceleration decreased from day 1 to day 2 (range: ~4% to ~20%) and/or 3 (range: ~4% to ~28%). Not all muscles showed a similar pattern. Activity of the extensor digitorum and biceps brachii decreased already between days 1 and 2 (range: ~6% to ~13%), whereas activity of the flexor carpi radialis and triceps brachii decreased between days 1 and 3 (range: ~13% to ~20%). No changes in physical requirement were detected between days 2 and 3. Relative motor variability did not change across days, except that variability of forearm acceleration increased from day 1 to 3 (~5%). This study found consistent changes in physical requirements and indicated that several arm muscles show earlier decreases of muscular activity, like the extensor digitorum, compared to other body parts, like the flexor carpi radialis. Moreover, movement strategies may develop differently than muscle activation strategies, based on the different developments of physical requirements and motor variability. The development of physical requirements in industrial tasks is part of daily living and starts at task onset, highlighting the importance of task familiarization and the randomization of experimental conditions in scientific studies.

**Keywords:** manual materials handling; electromyography (EMG); motor control; experience; electrocardiography; kinematics
