## 2 Materials and Methods
### Main Outcome Measures
#### 2.3.3. Intervention Protocol  
Both the first (MI intervention) group and the second (placebo) group followed a physical therapy balance training protocol starting 2 days after the completion of Phase 1. The intervention spanned 4 weeks, aligning with the return-to-play rehabilitation period, and included a total of 6 sessions per professional football player. The balance training protocol consisted of 3 distinct parts, with a total session duration of 30 min [36; 37; 38; 39]. Details of the protocol are presented in Table 1.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Part 1—8 min \({}^{*}\)** & **Part 2—15 min \({}^{*}\)** & **Part 3—7 min** \\ \hline Running—60 s & One legged stance—3 \(\times\) 45 s hold & \\ \hline Jumping Jacks—40 s & Jump from one leg to the other with control landing & \\  & & for 4 s—3 \(\times\) 10 reps & \\ \hline Linear Knee Raise—10 reps & One legged stance on a balance board—2 \(\times\) 45 s hold & Static stretching exercises of \\  & & the lower limbs \\ \hline Squat—10 reps & One legged stance on a balance board with the knee & \\  & & flexed—3 \(\times\) 10 knee flexions & \\ \hline Leg Swing—10 reps & Squats on a balance board—3 \(\times\) 45 s hold & \\ \hline \hline \end{tabular}
\end{table}
Table 1: The physical therapy balance training protocol.  
At the end of the physical therapy balance training protocol, professional football players in the first MI intervention group individually received the MI intervention protocol. At the beginning of each MI session, athletes in both groups (MI intervention and placebo) had to complete the VMIQ-2-GR. The first MI intervention group and the second placebo group both received 6 sessions of MI and placebo instructions in addition to their physical therapy balance training protocol. The total duration of each session was 50 min including 30 min of the balance training protocol and 20 min of MI or placebo instructions. All professional football players were asked to sit comfortably in a relaxed position with low-light and quiet environmental conditions.  
The MI instructions were professionally recorded by a sound engineer in a recording studio to minimize bias. The recordings were produced using Cubase version 10 and WaveLab version 9 software, along with an NT1 fourth generation condenser microphone (Rode, Sydney, Australia). During each session, the recorded MI protocol was played on the same computer (MacBook Pro, M1, Apple Inc., Cupertino, CA, USA) and listened to using the same headphones (Keiji, HD-2400G, Zeroground, Athens, Greece) to ensure consistency.  
In the MI intervention group, professional football players first received relaxation instructions, followed by MI instructions replicating their physical therapy balance training protocol. The athletes were guided to mentally rehearse the progression of balance exercises, including the same duration and repetitions as in their physical therapy sessions. They were encouraged to vividly imagine and feel themselves performing the balance training tasks.  
By contrast, the placebo group only listened to recorded relaxation instructions without any MI intervention.