This study demonstrated a pronounced inter-individual variation in the localization of calf muscle MPs. However, the two areas with the highest concentrations of MPs were localized in the midline close to fossa poplitea and in the proximity of the transverse plane at the maximum circumference of the calf. A computed MP heat map displayed the probabilities of finding MPs on an anatomical chart, which may guide and help the clinician in daily NMES application.
The prominent inter-individual variability in the anatomical localization of MPs on the calf muscle was the main finding of this study, which suggests that each patient should undergo a MP scan before NMES electrodes are applied. The observation of a high inter-individual variability in MP localization is corroborated by earlier research mainly of cadaveric studies on M. triceps surae, but also of one study on MP locations of the gastrocnemius muscle (14, 15, 18). To the best of our knowledge, however, this is the first study, which in a detailed and focused manner has successively searched for and identified the four best MPs in each subject examined on respectively the lateral and medial bellies of the gastrocnemius.
The observation of the most distal MP being localized at around 50% length of the calf suggests that clinicians do not have to scan for MPs below the mid of the calf when applying calf NMES. This finding is strengthened by an earlier cadaveric study by Kim et al from 2005, who found that the most distal MP was identified at around 40% of the calf length distal from fossa poplitea (15). Interestingly, however, NMES treatment may induce multiple molecular adaptations including axonal sprouting and newly formed neuromuscular junctions, suggesting that MP locations may change which warrants further studies (22).
Our identification of the most medial- and lateral MPs both being localized at around 6 cm (18%) from the MC implies a further restriction of the search area for the clinician performing the MP scan. The observation is supported by another cadaveric study by Sook Kim et al, who found the main MPs on the medial and lateral gastrocnemius at around one third and two third of the individual calf width divided with 2, respectively, from the central aspect of the calf (14).
The second main finding of this study was the localization of the two areas on the calf that exhibited the highest concentrations of MPs. The observation of one area of concentration of MPs close to the midline and some centimetres distal to fossa poplitea, would seem to be substantiated by earlier studies (18). The earlier studies did however not clearly present their suggested MP-locations in relation to a system of areas with predefined size, thus making their results harder to apply in clinical practice. In contrast, our presentation is based on specific anatomical landmarks and provides the clinician with an easy-to-use chart display, based on probabilities of a statistical significance, on where to start looking for the MPs.
The identified popliteal concentration of MPs may reflect the branching of the tibial motor nerve, which supplies the lateral and medial gastrocnemius. The other high concentration of MPs found at the maximum circumference of the calf in the proximity of the transverse plane has not been identified in earlier cadaveric studies (14, 15). Botter et al, who also used a MP scan, however identified MPs close to the maximum circumference of the calf, which would seem to support our findings (18). The higher concentration of MPs at the maximum circumference of the calf both on the medial and lateral gastrocnemius bellies is therefore new knowledge. The difference in MP-location between the previous cadaveric studies and MP scan studies are likely an effect of the differing methods used to define the MPs. Whereas the cadaveric studies defined the MPs as the location where the nerve entered the muscle, modern MP scan studies defined the MP-locations as the points on the skin that most readily induced a muscle response with NMES.
Based on our main findings we established a novel MP heat map displaying the probabilities of finding MPs in 48 predefined, 3x3 cm areas, of the calf. The heat map displayed that central and proximal areas of the calf showed a much greater probability of containing a MP than the most medial, lateral, and distal areas, which confirmed the findings of the MP1-4 map. The graphical display of the heat map moreover suggests an anatomical chart, which can be used to search for MPs in a subsequent, logical order (1).
The observation of two areas on the calf, number 4 and 29, demonstrating the highest probability of containing a MP at 50%, suggests that these areas may be good starting points for the clinician to locate MPs. Area 4 located 0-3 cm below fossa poplitea and just lateral of the midline, and area 29, located 12-15 cm below fossa poplitea at the largest circumference of the calf and 3-6 cm lateral of the midline, may be easily identified by the clinician. The finding that areas 4 and 29 exhibited significantly higher probability of identifying a MP than in 39 of the 48 areas examined on the calfstrengthen the observation that these areas are good starting points to look for MPs.
The discovery that the three areas, 9, 10 and 16, which demonstrated the second highest probability of finding a MP, 47%, shared border with each other and with area 4, implies how a manual search with MP-pens on the calf could be performed. After the search in area 4 we suggest that the search of MPs should continue in a distal and medial direction to are 9, followed by searching laterally towards area 10, subsequently in a distal direction to area 16. Then continuing the search distally towards area 29, passing areas 22 and 28 on the way.
One disadvantage with the search algorithm described so far is that only area 9 represents the medial gastrocnemius belly. This is however in line with earlier research that has also experienced problems in finding MPs on the medial gastrocnemius belly (18). If area 9 did not result in finding a MP on medial gastrocnemius we recommend proceeding with the two areas with the third highest probability of finding a MP, 37%, areas 14 and 32. After examining area 9 the clinician should continue medially and distally to area 14, and subsequently 6 cm distal to reach area 32. The search algorithm described so far is of course not unique, but is rather a suggestion on how to best use the MP heat map in order to locate the MPs of the calf in the most time efficient and optimized way.
A potential weakness of the study is that we cannot with any statistical significance say that there is a difference in probability of finding a MP between the areas that displayed higher probability than 33%. This finding, however, suggests that the two best areas have significantly higher likelihood of containing a MP than all areas with 30% probability, which is a strength of the study. Similarly, the areas with the second highest probabilities of 47%, exhibit significantly better chances of containing a MP compared to the areas with 27% probability and lower, which confirms the benefit of using the heat map. Another limitation is that some MPs were discarded when calculating the heat map, since they were situated in the same area as another MP. However, the discarded MPs were very few, and did not affect the result in any major way.