Table 1 Animal experiments: overview, set-up and analysis details

1. General and weight analysis

* 5 to 8-week-old male and female C57Bl/6 J and Frzb^−/− mice

* Primary outcome: 5-week-old mice body weight, Fig. 1a

* Total sample size: n = 29; WT C57Bl/6 J: n = 15 and Frzb^−/−: n = 14

* Secondary outcome: 8-week-old mice body weight, Fig. 1b

* Total sample size: n = 31; WT C57Bl/6 J: n = 20 and Frzb^−/−: n = 11

2. Muscle analysis

* 5 to 6-week-old male and female C57Bl/6 J and Frzb^−/− mice

* Primary outcome: Mice muscles’ weight, Fig. 1c

* Total sample size: n = 30; WT C57Bl/6 J: n = 16 and Frzb^−/−: n = 14

* Secondary outcome: Myosin heavy chain composition, Fig. 1d

* Total sample size: WT C57Bl/6 J: n = 3–2 and Frzb^−/−: n = 3–4

* Secondary outcome: Hanging time, Fig. 1e

* Total sample size: n = 16; WT C57Bl/6 J: n = 7 and Frzb^−/−: n = 9

3. Catwalk analysis

* 8-week-old male and female C57Bl/6 J and Frzb^−/− mice

* Total sample size (8-week-old): n = 36; WT: n = 20, Frzb^−/−: n = 16

* Primary outcome: Stand, Fig. 2a

* Secondary outcome: Swing phase, Fig. 2b

* Secondary outcome: Step cycle, Fig. 2c

* Secondary outcome: Stride length, Fig. 2d

4. Chronic exercise protocol

* 5-week-old male and female C57Bl/6 J and Frzb^−/− mice

* Total sample size: n = 16; WT: n = 9, Frzb^−/−: n = 7

* Primary outcome: Mice body weight, Fig. 3a

* Secondary outcome: Soleus CSA and histology, Fig. 3b-c

* Secondary outcome: Tibialis anterior fibre type composition, Fig. 3d

5. Cardiotoxin injection

* 10-week-old male and female C57Bl/6 J and Frzb^−/− mice

* Total sample size: n = 39; WT: n = 19, Frzb^−/−: n = 20

* Primary outcome: Hematoxylin and eosin stained Tibialis anterior, Fig. 4a

* Secondary outcome: Hematoxylin and eosin stained Soleus, Fig. 4b

6. Satellite cell isolation

* 4-week-old male and female C57Bl/6 J and Frzb^−/− mice

* Total sample size: n = 14; WT: n = 7, Frzb^−/−: n = 7

* Primary outcome: Satellite cells and myotubes immunofluorescence analysis, Fig. 5