## 3 Results
### General Characteristics of the Studies
\begin{table}
\begin{tabular}{c c c c c c c c c c c c} \hline \hline  & \multirow{2}{*}{**Group**} & \multirow{2}{*}{**Type of HRV-g**} & \multirow{2}{*}{**Duration**} & \multicolumn{2}{c}{**Training Distribution (\% Time)**} & \multicolumn{2}{c}{**Training Volume**} & \multicolumn{2}{c}{**Training**} & \multicolumn{2}{c}{**Training**} \\ \cline{4-11}  & & & **Z1** & **Z2** & **Z3** & \multicolumn{2}{c}{**(Hours)**} & \multicolumn{2}{c}{**Volume (km)**} & \multicolumn{2}{c}{**Frequency**} & \multicolumn{1}{c}{**Training Load**} \\ \hline Da Silva & HRV-g & Ref: 10-day average ARSSD, If MSRD & \multirow{2}{*}{8 weeks} & - & - & - & - & - & 3 & - \\ \cline{2-11}  & PT & & \(<\) mean MSRD-ISP-ID: If hot-HT & & & & & & 3 & & \\ \hline Javaloys & HRV-g & SWC of MSRD-ID: If MSRD/STD & \multirow{2}{*}{8 weeks} & 66 & 24 & 10 & 9.3 \(\pm\) 2.8 & - & - & - \\ \cline{2-11}  & et al. [11] & PT (TP) & & & & & 64 & 27 & 9 & 8.8 \(\pm\) 2.8 & & - & - \\ \hline Javaloys & HRV-g & SWC of MSRD-ID: If MSRD/STD & \multirow{2}{*}{8 weeks} & 49 & 39 & 12 & 11.1 \(\pm\) 3.1 & - & - & 1033.3 \(\pm\) 312.5 au. \\ \cline{2-11}  & et al. [10] & PT (BT) & & & & 54 & 33 & 13 & 11.4 \(\pm\) 3.1 & & & 1028.8 \(\pm\) 214.5 au. \\ \hline \multirow{3}{*}{[PERSON] et al. [27]} & HRV-g-I & Ref: 10-day average SD1, HRV-g & \multirow{3}{*}{8 weeks} & & & & & \(\sigma\)58 \(\pm\) 0.2 & \(\sigma\)518 \(\pm\) 09 (390 \(\pm\) 42) \\  & & SD1 \(\geq\) SD1 \(\sigma\)/SC1 \(\downarrow\) SD SD1 & & & & & & \(\sigma\)58 \(\pm\) 0.3 & TRMPS \(\times\) week \\ \cline{2-11}  & et al. [27] & HRV-g & ref: HD/SD1 \(\downarrow\) 2 consecutive days. & \multirow{2}{*}{8 weeks} & & & & & \(5.0\pm\) 0.3 & 314 \(\pm\) 6 TRMPS \(\times\) week \\  & & rest: HRW \(\leftarrow\) HRW-I but only VT if if & & & & & & & \(\sigma\)53 \(\pm\) 0.6 & \(\sigma\)692 \(\pm\) 0.31 (343 \(\pm\) 107) \\  & & SD1 \(\leq\) SD1 \(\sigma\) & & & & & & \(\sigma\)50 \(\pm\) 0.8 & TRMPS \(\times\) week \\ \hline \multirow{3}{*}{[PERSON] et al. [5]} & HRV-g & Ref: 10-day average HP power, If HF - & - & - & - & - & - & \(36\pm\) 4 & - & - \\  & et al. [9] & & HF \(\leftarrow\) 1 load J, HF HF \(\downarrow\) 2 consecutive days & \multirow{2}{*}{4 weeks} & & & & & \(\sigma\)529 \(\pm\) 0.4 TRMPS \(\times\) week \\ \cline{2-11}  & HRV-g & & & & & & & & \(38\pm\) 6 & & 529 \(\pm\) 0.4 TRMPS \(\times\) week \\ \cline{2-11}  & et al. [12] & & & & & & & & & \(\sigma\)58 \(\pm\) 0.2 & - \\ \hline Nunttila & HRV-g & \multirow{2}{*}{1 LT if QRT was higher than ref} & \multirow{2}{*}{8 weeks} & \(\mathbb{S}2\pm 8\) & \(15\pm 6\) & \(3\pm 3\) & \(5\pm 2\) & - & \(6.3\pm 1.4\) & - \\ \cline{2-11}  & et al. [12] & PT (BT) & & & \(84\pm 7\) & \(12\pm 5\) & \(4\pm 3\) & \(6.0\pm 1.9\) & & \(6.1\pm 0.4\) & \\ \hline [PERSON] et al. & HRV-g+SH & \multirow{2}{*}{1 LT if U or \(\leftarrow\) 1 load; HF \(\downarrow\) 20\% \(\downarrow\)} & \multirow{2}{*}{15 days} & - & - & - & - & - & 3365 \(\pm\) 425 au.u. \\ \cline{2-11}  & et al. [22] & PT+SH+I & load; HF \(\downarrow\) 2 consecutive days & \multirow{2}{*}{8 weeks} & \(\mathbb{S}3\pm 27\) & \(14\pm 25\) & \(3\pm 5\) & \(6.5\pm 2.8\) & \(42\pm 22\) & \(6.1\pm 1.8\) \\ \cline{2-11}  & et al. [26] & PT (BT) & outside the SWC: low intensity or not & \(84\pm 12\) & \(13\pm 10\) & \(3\pm 4\) & \(6.3\pm 2.5\) & \(41\pm 20\) & \(5.6\pm 1.6\) \\ \hline \multirow{3}{*}{A.u.: arbitrary units; HF: high frequencies; HT: high-intensity training; MT: moderate training Ref: reference; MSRD: root of the mean squared differences of successive R-R-intervals; MSRD/TD: 7-day rolling average of vagal-mediated square; QRT: quick recovery test using MRSD; SWC; smallest worthwhile change; TRMPS: training impact; VT: vigorous training.} \\ \cline{2-11}  & & & & & & & & & 3481 \(\pm\) 179 au.u. \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Characteristics of the training intervention of studies included in the meta-analysis.