Toe flexor strength (TFS) plays an important role in postural control during standing [1
] and walking [2
]. TFS is positively associated with the performance of sports-related activities in adolescents and young adults [3
], whereas older adults with a low level of TFS have impairments of activities of daily living tasks [6
]. Moreover, an intervention of a training program aiming to enhance toe muscular strength improves jumping height in young adults [10
] and mobility in older adults [11
]. Thus, identifying influential factors for TFS will provide useful information for designing a training program aiming to enhance physical performance in various populations.
TFS is generated by the combined activities of plantar intrinsic and extrinsic foot muscles [12
]. Some of the plantar intrinsic foot muscles (i.e., flexor hallucis brevis, FHB and flexor digitorum brevis, FDB) are specialized in toe flexion, while others (e.g., abductor hallucis, ABH and adductor hallucis) mainly act on toe adduction/abduction [13
]. Among the extrinsic foot muscles, only the flexor hallucis longus (FHL) and flexor digitorum longus (FDL) are specialized in toe flexion [13
]. On the other hand, the great toe alone can produce TFS twice as great as the lesser toes can [14
], the latter of which is about one-third of the TFS produced by all toes [15
]. Considering these, the magnitude of TFS may primarily depend on the force generation capacity of the muscles that mainly act on great toe flexion (i.e., FHB and FHL).
It is known that the anatomical cross-sectional area (ACSA) [14
] and muscle thickness [14
] of plantar intrinsic or extrinstic foot muscles, determined by using ultrasonography or magnetic resonance imaging (MRI), significantly associates with TFS. Moreover, Kurihara et al. [17
] revealed through stepwise multiple linear regression analysis that the ACSA of medial parts of plantar intrinsic foot muscles was a major determinant for TFS production. However, these studies have quantified the size of limited plantar intrinsic foot muscles [14
] or a compartment containing several plantar intrinsic foot muscles [17
] as an independent variable for examining the association with TFS. Thus, the anatomical function of individual plantar intrinsic foot muscles has not been considered for examining the relationship between TFS and muscle size, and therefore little information is available from previous findings as to which muscle(s) primarily contributes to the magnitude of TFS.
Furthermore, the ultrasonographic studies cited above [14
] have determined ACSA or muscle thickness from a single image in which the thickest region of muscle belly was visually identified. In addition, Kurihara et al. [17
] measured the ACSA of a compartment containing several plantar intrinsic foot muscles from a single MR image at 80% of the foot length (FL), because the ACSA of the entire plantar intrinsic foot muscle was largest at the corresponding position [18
]. However, muscle volume (MV) and maximal anatomical cross-sectional area (ACSAmax
) have been shown to be more strongly associated with joint torque [19
] or strength [20
] compared to ACSA and muscle thickness. Nevertheless, no studies have determined MV or ACSAmax
of individual plantar intrinsic muscles and examined its association with TFS.
The purpose of the present study was to elucidate the muscle(s) that primarily contributes to TFS production. To this end, this study examined the association of ACSAmax and MV of each constituent muscle of the plantar intrinsic and extrinsic foot muscles with TFS, and then conducted stepwise multiple liner regression analysis by using TFS as a dependent variable with the ACSAmax and MV values of the muscles as independent variables. We hypothesized that the muscles that mainly act on great toe flexion, i.e., the FHB and FHL, would be selected as the primary contributors to TFS among the individual plantar intrinsic and extrinsic foot muscle.
The present study is the first case that determined the ACSAmax and MV for each constituent of the plantar intrinsic and extrinsic foot muscles and examined their associations with TFS. The major findings obtained here were that 1) TFS was significantly correlated with the ACSAmax of the ADDH-OH, ADDH-TH, and FDB, as well as with the MV of the ADDH-OH, and 2) the ACSAmax and MV of the ADDH-OH alone explained 42 and 29%, respectively, of the variance in TFS by stepwise multiple liner regression analysis. These results indicate that among the plantar intrinsic and extrinsic foot muscles, the ADDH-OH primarily contributes to TFS production.
At the start of the present study, we hypothesized that the muscle(s) mainly act on great toe flexion would be primarily contribute to the magnitude of TFS. However, the current results refuted this. The reason why the ADDH-OH was selected as the primary contributor to TFS may be attributable to a unique characteristic of this muscle having multiple functions of adduction and flexion of the great toe. The participants in this study were asked to grasp a solid straight bar for the TFS measurement. Toe grasping is often observed in the plantar grasp reflex, which is one of the primitive reflexes and consists of the combination of flexion and adduction of the toes [25
]. Taken together, the toe grasping on the toe grip dynamometer for producing TFS can be considered as a complex multiple movement consisting of flexion and adduction rather than simple flexion of the toes. Therefore, the ADDH-OH, which acts on both adduction and flexion of the great toe, appears to contribute to TFS production more strongly than the muscles specialized in great toe flexion (i.e., FHB and FHL).
It is worth noting that among the plantar intrinsic and extrinsic foot muscles, not only the ADDH-OH but also ADDH-TH acts on both the flexion and adduction of the toes [13
]. However, the ADDH-TH was not selected as the determinant for TFS. This may be due to the morphological differences between the two muscles. First, the ACSAmax
and MV of the ADDH-OH were about 2.4 and 7.5 times, respectively, larger than those of the ADDH-TH (Table 1
). Second, the ADDH-OH runs along the longitudinal direction of the foot and contracts closely parallel to the direction of the great toe movement during TFS production. On the other hand, the running direction of the ADDH-TH is perpendicular to the longitudinal axis of the foot, and this muscle contracts transversely and orthogonally to the direction of the great toe movement during TFS production. Thus, it is likely that the morphological features of the ADDH-OH would be suited for producing TFS more than that of the ADDH-TH, and consequently the ADDH-OH alone might have been selected as the contributor for TFS.
In general, force generation capacity of a muscle is theoretically best related to its physiological cross-sectional area (PCSA) [26
]. Thus, adopting PCSA rather than ACSAmax
and MV as independent variables would be desirable to examine the association of muscle size with TFS. However, the determination of PCSA in vivo needs data concerning pennation angle and fascicle length in addition to MV [19
], and the procedure determining the PCSA of foot muscles has not been established likely due to their complex architecture. On the other hand, a previous study revealed that the correlation coefficients of ACSAmax
and MV with muscle strength are comparable to that of PCSA [20
]. In the current results as well, both ACSAmax
and MV of the ADDH-OH were selected as explainable factors for TFS, without a significant difference between ACSAmax
and MV in their correlation coefficients with TFS. Thus, this study supports the previous studies [20
] and recommends future studies adopt either ACSAmax
or MV as a representative muscle size index for examining the association between muscle size and TFS in vivo.
This study has some limitations relating to the procedure adopted for the TFS determination. We used a toe grip dynamometer to measure TFS, as this is one of the most commonly used devices [3
] with its reliability confirmed [22
]. Alternatively, other studies have measured TFS using a hand-held dynamometer [16
] or plantar pressure platform [14
]. Soysa et al. [28
] reviewed the methods for measuring plantar intrinsic foot muscle strength and pointed out that the toe action during TFS production differs depending on devices and may change the activation of plantar intrinsic and extrinsic foot muscles. Thus, there is a possibility that the muscle(s) that primarily contributes to TFS production may differ from that identified here when another device is used to determine TFS. In addition, the participants examined here were asked to adjust their first proximal phalangeal at the solid straight grip bar and to exert maximal TFS using all toes, in accordance with the procedure of a previous study [22
]. However, not all participants could grasp the grip bar with all toes because the length of each toe is usually different from each other and also different among participants. Thus, we cannot exclude the possible influence of the shape of individual participants’ toes on TFS measurements. Further study is needed to elucidate whether we could generalize the current results on the primary contributor to TFS production widely to various measurement devices and/or toe shapes.
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