Jan Gadd, programme leader for physical education at the University of Wales Institute in Cardiff, (UWIC) said knowledge gaps were made worse because too few local authorities employed specialist advisers whom they could consult once in post.
Ms Gadd, who is also a consultant for the PE and School Sport (PESS), said PE training was vital to help all teachers handle health and safety issues -especially with increased emphasis on outdoor play in the foundation phase.
But she said: "Money is tight and we've now got HE institutions offering fewer hours for PE.
"At UWIC, primary trainees in their first year were getting 20 hours in 2005 -this year it is 15.
"Even secondary trainee specialists have seen a reduction from 192 hours contact time last year to 147 hours this year."
She said more teachers were turning to PESS staff, drafted in to encourage children to become active, for health and safety advice.
Last week, inspection body Estyn warned that local authorities could be left in a vulnerable position if schools were not getting the help they needed.
Inspectors found seven local authorities in Wales do not have a specialist PE adviser, and five do not have an outdoor activities point of contact for schools.
Jonathan Moody, a PESS co-ordinator in Torfaen, said: "We find many new teachers with less experience of PE. We do our best but we are not specialist advisers."
Rhys Williams, of the NUT Cymru, said: "Extra-curricular activities enrich learning. We have always encouraged our members to engage in them. Now we feel our position has been weakened."
Susan Lewis, chief inspector, said: "I am concerned that some schools and authorities are leaving themselves vulnerable by failing to provide specialist advice."