The unique findings of this study provide objective evidence confirming the effects of toe exercise on metatarsalgia. Insole was the standard treatment for metatarsalgia, and its effect was reported to improve VAS by an average of 1 point [14]. In this study, the average improvement was 2.7 points, which indicates less pain than that associated with insoles. Previous reports have evaluated TGS between the sitting and upright positions [15]. They found that there was no difference in the TGS between the upright and sitting positions. Contrary to the previous report, TGS in the sitting position was significantly lower than that in the upright position. The push-type toe-grip strength meter in the current study measures the pressing force applied to the ground by the toes, whereas the pull-type meter used in the previous study measures the toes’ pulling force in the proximal direction, similar to the hand-grip dynamometer. As the shape of the toes is different for each person, it is questionable whether the pulling bar of the pull-type device fits all foot shapes. Furthermore, we placed emphasis on the force that presses or steps on the ground as a function of the toes. In that sense, we believe that the push-type device reflects the function of the toes more clearly.
The current study also demonstrated that the affected foot has significantly weaker TGS in the sitting position compared to that in the unaffected foot, but no significant difference is observed in the upright position. It is unclear why the difference in the upright case is not manifested; it may possibly be due to some compensatory function working in the upright position to maintain the TGS.
Primary metatarsalgia is considered an abnormality that is related to the anatomy of the metatarsal bones as well as to the relationship between metatarsal bones and the rest of the foot which leads to overload [9]. Metatarsal bone-length discrepancy has received the most attention so far [16]. The present study showed that patients with index-minus accounted for most of the participants, suggesting that the second metatarsal was relatively long, and the metatarsal length seemed to be involved in the onset of metatarsalgia. However, as the length of the metatarsal bone cannot be changed except for surgery, another viewpoint is required for conservative treatment.
During walking, the load increases to about 2 times the body weight before toe-off at the MTP joints [17] due to the combined effect of forward-falling and ground reaction force loads applied to the forefoot. When the toes lose the ability to functionally push off the ground, it puts an increased load on the metatarsal area. This repetitive overloading in the metatarsal area causes metatarsalgia.
We observed an improvement in PUM, indicating that exercise not only improves strength but also enhances toe function. Therefore, metatarsalgia may be relieved because of an improvement in toe function.
Patients with long durations of metatarsalgia (more than one year) did not have improved VAS scores to a great extent in this study. The multivariate generalized linear model showed that the foot with plus-minus morphology had a significant improvement in AOFAS scores compared to that in the foot with minus morphology in the upright and sitting positions. Regarding VAS scores, improvement was worse in patients with a long history of disease and high BMI. In terms of the long duration of metatarsalgia, chronic pain is generally considered difficult to cure. Although there is an opinion that it is difficult to create a temporal boundary in terms of the difference between acute and chronic pain, generally, pain lasting 12 weeks or more is regarded as chronic pain [18]. The cause of pain in metatarsalgia is unclear, but it is presumed that the acute phase involves nociceptive pain. The improvement in pain was worse in chronic cases, suggesting that mixed pain, involving nociceptive, nociplastic, and neuropathic pain, is involved in chronic disease [19]. For patients with hallux valgus, waiting for elective surgery has been associated with less improvement in physical function outcomes following surgery [20], which supports our findings. Our results suggest that a different treatment strategy may be needed for chronic pain in metatarsalgia lasting more than 1 year.
We found that an increased BMI positively correlated with worse improvement in VAS scores. Moreover, previous reports showed a high BMI was positively correlated with elevated opioid consumption rates [21]. However, this result may stem from doctors prescribing a high number of opioids because of the patient being obese. The relationship between obesity and pain is still unclear.
The strength of this study is in the assessment of the toes using reproducible absolute values of TGS, showing that weak toes tend to promote metatarsalgia. This device measures TGS by measuring the strength used to press the toes into the floor. The force of toe plantar-flexion is the strength of sustaining one's weight, shared with the ball area in the toe-off phase. Therefore, this movement directly reflects the power of the toe-off phase by pressing off the ground. This push-type toe-grip strength meter is a useful device to detect toe weakness in the clinical setting.
The current study has certain limitations. First, the participants were recruited from a single institution; hence, the results may not be generalizable. Second, we have no objective proof of overloading in the metatarsal area in people with weak toe grip. Proper walking not only needs a certain level of plantar-flexion strength but also the ability to move the toes in order to work effectively. The ability of toes to control the loading warrants further investigation. Third, pain may deteriorate due to the natural course of metatarsalgia progression. Since we did not include a control group, further investigations in the form of randomized controlled studies, are required to investigate the effectiveness of toe exercises in metatarsalgia.