Guy A.M. Messa1, Mathew Piasecki2, Jörn Rittweger3,4, Jamie S. McPhee5, Erika Koltai6, Zsolt Radak6, Bostjan Simunic7, Ari Heinonen8, Harri Suominen8, Marko T. Korhonen9, Hans Degens1,9,10 1 Department of Life Sciences, Research Centre for Musculoskeletal Science & Sports Medicine, Manchester Metropolitan University, Manchester, UK; 2 MRC-ARUK Centre for Musculoskeletal Ageing Research and National Institute for Health Research (NIHR) Nottingham Biomedical Research Centre, University of Nottingham, UK; 3 Division Space Physiology, Institute of Aerospace Medicine, German Aerospace Center, Cologne, Germany; 4 Department of Paediatric and Adolescent Medicine, University of Cologne, Cologne, Germany 5 Department of Sport and Exercise Sciences, Manchester Metropolitan University, Manchester, UK; 6 Research Institute of Sport Science, University of Physical Education, Budapest, Hungary; 7 Science and Research Centre Koper, Institute for Kinesiology Research, Slovenia; 8 Gerontology Research Centre, Faculty of Sport and Health Sciences, University of Jyväskylä, Finland; 9 Institute of Sport Science and Innovations, Lithuanian Sports University, Kaunas, Lithuania; 10 University of Medicine and Pharmacy of Targu Mures, Rumania Abstract Background The ageing-related loss of muscle mass may be partly attributable to motor neuron loss and motor unit remodelling, where cycles of denervation and reinnervation may result in fibre type grouping in older muscles. The purpose of this study was to examine fibre type grouping in athletes and non-athletes of a wide age range (19-85 years) and to evaluate to which extent any observed grouping is explicable by the fibre type composition of the muscle. Since regular physical activity may stimulate reinnervation, we hypothesised that fibre groups are larger in master athletes than in age-matched non-athletes. Methods Fibre type grouping was assessed in m. vastus lateralis biopsies from 22 young (19-27 years) and 35 healthy older (66-82 years) non-athletes, and 14 young (20-29 years), 51 middle-aged (38-65 years) and 31 older (66-85 years) athletes. An ‘enclosed fibre’ was any muscle fibre of a particular type surrounded by fibres of the same type only. A group was defined as a group of fibres with at least one enclosed fibre. Results Only type II fibre cross-sectional area (FCSA) showed an age-related decrement that was greater in athletes (p < 0.001) than in non-athletes (p = 0.012). There was no significant agerelated effect on group size or group number in athletes or non-athletes, and the observed grouping was similar to that expected from the fibre type composition. Conclusions At face value these observations do 1) neither show evidence for an age-related loss and remodelling of motor units nor that 2) improved reinnervation with regular physical activity, but 3) histological examination may not reveal the full extent of ageing-related motor unit remodelling.