Table 2 Final multilevel model for interval endurance capacity data (377 measurements).

Fixed effects	Coefficient	SE	p Value
Constant	52.6	9.10	<0.001
Age (months/12–15 years)	6.21	1.20	<0.001
Age2	−1.83	0.363	<0.001
Boy	16.5	4.30	<0.001
Sub‐elite	0.786	2.90	0.393
Age × boy	5.27	1.33	<0.001
Age × sub‐elite	−5.11	1.39	<0.001
Boy × sub‐elite	−13.0	4.55	0.002
Percentage body fat	−0.889	0.201	<0.001
Additional training	1.092	0.324	<0.001
Motivation	4.86	1.87	0.003

Random effects	Variance	SE
Between individuals	136.0	25.43
Within boy	292.8	39.20
Within girl	105.9	16.31
Deviance	3205.6