## 4 Discussion
### Study Differences and Limitations  
The JBI tool has identified some concerns, meaning that interpretation of the results requires some caution. Specifically, the tool highlighted issues related to the identification and handling of confounding factors and highlighting the specific inclusion criteria for participants. Collectively, the six studies had a paucity of female participation, which may have reduced heterogeneity. Additionally, ethnicity was not considered in any of the studies. Differences were evident across studies in the categorisation and classification of HF. The most frequently reported classification was based on the severity of symptoms using the NYHA class. Initially, we sought to compare findings between HFpEF and HFrEF participants in our meta-analysis. None of the participants in our study could be classified as HFpEF. Only one small study, which was excluded from our meta-analysis as the authors did not present the mean and standard deviation of data, examined SkM energetics in HFpEF compared to HFrEF (N = 20 HFrEF and N = 12 HFpEF, mean ejectionfraction 27 \(\pm\) 11 and 62 \(\pm\) 5% respectively) and healthy controls (N = 11) [25]. They reported a significantly longer PCr recovery time in both HF groups compared to healthy controls. Furthermore, patients with HFpEF had the largest energetic changes of all the groups studied, such as initial PCr depletion. Only one study included NYHA class IV HF patients [29]. The inclusion of class IV may provide a better insight into metabolic deficits at the end stages of the disease [33]. Two studies assessed energetics in the forearm. Although the flexor digitorum superficialis was considered'relatively unimportant' [29], it was argued in a separate study [31] that this forearm muscle would be in use despite how incapatted patients were and that focussing on this group was therefore of clinical relevance. Other studies investigated the calf muscle [28; 30; 32; 33; 34]. This heterogeneity of muscle recruitment needs to be considered in the interpretation of exercise-induced metabolic changes. In an effort to account for this, one study used various muscle groups to measure PCr recovery rates simultaneously [33].  
Differences in methodology may have impacted the validity and interpretation of findings. Variations in scanning regions, time points and field strength are likely to have contributed to result variations. Localisation has been advised in the expert consensus to avoid pitfalls in acquiring kinetics results [35]. Localisation techniques varied across studies, and three studies did not report localisation methods. Two studies utilised the smallest radio frequency (RF) coil (2.5 cm) [29; 31], thereby generating spectra from a much smaller volume than a study that used coils of a larger diameter [27; 28; 34]. However, this was appropriate, considering the smaller size of the forearm muscles in comparison to the calf muscle groups. Additionally, it has been advised by the expert consensus to use localisation to avoid pitfalls in scanning methodology and to improve the linewidth by homogenising the magnetic field (shimming).  
Each study reported the radiofrequency surface coil diameter as their initial form of localisation to the target muscle. Although three studies did not mention if they used localisation sequences, as the field strengths used ranged between 1.5 and 3 Tesla, relying on a surface coil for localisation at low field strength can therefore be considered sufficient [29; 31; 34].