Significant differences were found in DP thickness, ML width, and CSA of the abductor hallucis muscle between feet without hallux valgus compared to feet with hallux valgus. We propose that these results may be attributed to the muscle’s changing anatomical position in feet with hallux valgus which reduces its capacity to be recruited during abduction of the hallux
. As muscle size is considered an important determinant of muscle strength, these findings may suggest that hallux valgus is associated with a reduction in strength of the abductor hallucis muscle as a result of muscle disuse. A decline in muscle function secondary to disuse is similar to that associated with increasing age
Although significant differences were found in DP thickness, and CSA of the muscle between those without deformity and those with moderate and severe deformity, differences in ML width were found only between those without hallux valgus and those with moderate deformity. It may be that the width of the muscle is not as accurate a represenation of muscle size as thickness and CSA measurements, or it may be that the width of the muscle is not affected by muscle disuse to the same extent as the thickness or area.
As the muscle size characteristics of the abductor hallucis muscle did not vary significantly between the mild, moderate and severe stages of hallux valgus it may be that morphological changes to the muscle occur early in the development of the deformity and do not change significantly thereafter. We therefore propose that intervention strategies which aim to improve the strength and function of the muscle be implemented in the mild stage of the deformity (i.e. grade 1). Although previous studies have demonstrated foot exercises can activate the abductor hallucis muscle, these studies were conducted on participants with pes planus (flatfeet)
 or asymptomatic feet
 reported that the highest electromyographic activation of the muscle occurred with isolated abduction of the hallux against resistance. However, voluntary abduction of the hallux, which is deemed to be challenging in healthy individuals
, may also prove difficult to those with foot deformity. The effectiveness of this exercise in recruiting and strengthening the muscle in those with hallux valgus is likely to be affected by the changing anatomical position of the abductor hallucis muscle. Jung
 demonstrated that eight weeks of abductor hallucis muscle strengthening with the use of short foot exercises and/or foot orthotics increased the CSA of the abductor hallucis muscle in feet with pes planus. Although short foot exercises, which involve a flexion component at the first 1MTPJ, may recruit the abductor hallucis muscle to a greater extent than abduction exercises in feet with hallux valgus, further research is warranted to determine whether short foot exercises would have the same effect on abductor hallucis CSA in those with foot deformity.
A feasibility study assessing ultrasound of lower leg and foot muscles reported that images obtained of the abductor hallucis muscle from participants over the age of 60 years displayed increased echo intensity and 18% were excluded from measurement analysis due to poor visualisation of the muscle borders
. We obtained similar findings. Verhulst
 also reported that age was the most determinant factor for echo intensity. This can be explained by increased infiltration of fat and collagen which accompanies age-related loss of skeletal muscle mass
. It is unclear whether increased adipose deposition in the abductor hallucis muscle is associated with hallux valgus, or age-related changes. However, it has been noted that muscle adipose deposition is associated with a decline in muscle strength and function in older adults suggesting such changes may not be favourable to those with hallux valgus.
[39, 41] We attempted to recruit participants of a wide range of ages in each of the four hallux valgus groups, however the mean age of participants was not equal across the groups with mean age rising as the stage of deformity increased. Although this is consistent with the identification of increasing age as a risk factor for hallux valgus, it is possible that increasing age may have contributed to the decreasing muscle size characteristics. Lower limb muscle mass has been found to decrease by 25% from age 20 to 70 years
The results of this study should be considered in light of limitations. The cause and effect relationship between abductor hallucis muscle size reduction and hallux valgus severity could not be assessed due to the cross-sectional nature of the study design. Furthermore, from the original images, 18% were excluded from the analysis because they displayed high echo intensity making it difficult to visualise the muscle boundaries for measurement analysis. This is suggestive of increased adipose and collagen tissue within the muscle which creates multiple interfaces and increased reflection of the ultrasound beam
A limitation of 2D imaging was obtaining images that represented the same cross-sectional slice of the abductor hallucis muscle across the participants. Although we used a standardised procedure to image the muscle, the placement of the probe along the reference line varied due to the individual differences in the position of the abductor hallucis muscle in relation to this line. Not only does the muscle move inferiorly in feet with hallux valgus, but individual differences have also been shown in the course of the muscle (whether arciform or straight)
. However, this limitation did not appear to greatly influence the reliability of the image measurements in the current study, despite randomisation of the order of the images being analysed, coupled with the researcher being blinded to the feet the images were depicting.
A final limitation was that foot length and width was not measured so it is unknown whether the muscle size characteristics differ between foot size.
Future work may employ the use of electromyography in conjunction with ultrasound to assess functional parameters of the abductor hallucis muscle during various strengthening exercises to determine which exercise best activates the muscle in those with hallux valgus. Future research may also examine ultrasonic size characteristics of other muscles involved in the development of hallux valgus (i.e. hallux flexors and extensors, adductor hallucis) as these muscles also lose their normal anatomical relationship with the 1MTPJ.