Intrinsic Foot Muscle Activation During Specific Exercises: A T2 Time Magnetic Resonance Imaging Study

Abstract

Context: The intrinsic foot muscles maintain the medial longitudinal arch and aid in force distribution and postural control during gait. Impaired intrinsic foot-muscle function has been linked to various foot conditions. Several rehabilitative exercises have been proposed to improve it; however, literature that identifies which individual muscles are activated during specific intrinsic foot-muscle exercises is lacking.

Objective: To describe changes in activation of the intrinsic plantar foot muscles after 4 exercises as measured with T2 magnetic resonance imaging (MRI).

Design: Descriptive laboratory study.

Setting: Research laboratory.

Patients or other participants: Eight healthy National Collegiate Athletic Association Division I collegiate cross-country and track athletes (5 men and 3 women: age = 20 ± 0.93 years, height = 180.98 ± 10.84 cm, mass = 70.91 ± 7.82 kg).

Intervention(s): Participants underwent T2 MRI before and after each exercise. They completed 1 set of 40 repetitions of each exercise (short-foot exercise, toes spread out, first-toe extension, second- to fifth-toes extension).

Main outcome measure(s): Percentage increases in muscle activation of the abductor hallucis, flexor digitorum brevis, abductor digiti minimi, quadratus plantae, flexor digiti minimi, adductor hallucis oblique, flexor hallucis brevis, and interossei and lumbricals (analyzed together) after each exercise were assessed using T2 MRI.

Results: All muscles showed increased activation after all exercises. The mean percentage increase in activation ranged from 16.7% to 34.9% for the short-foot exercise, 17.3% to 35.2% for toes spread out, 13.1% to 18.1% for first-toe extension, and 8.9% to 22.5% for second- to fifth-toes extension. All increases in activation had associated 95% confidence intervals that did not cross zero.

Conclusions: Each of the 4 exercises was associated with increased activation in all of the plantar intrinsic foot muscles evaluated. These results may have clinical implications for the prescription of specific exercises to target individual intrinsic foot muscles.

Keywords: medial longitudinal arch; muscle functional magnetic resonance imaging; short-foot exercise; toes spread out.

Figures

Figure 1.

Intrinsic foot muscle exercises: A, short-foot exercise; B, toes-spread-out exercise, C, first-toe extension; and D, second- to fifth-toe extension.

Figure 2.

Representative example illustrating A, muscle heads; B, pre-exercise levels of activation, and C, postexercise levels of activation used in the analysis of muscle activation after intrinsic foot-muscle exercises.

Figure 3.

Percentage increase in activation: A, short-foot exercise; B, toes-spread-out exercise; C, first-toe extension; D, second- to fifth-toe extension. Abbreviations: ABH, abductor hallucis; ADM, abductor digiti minimi; AHO, adductor hallucis oblique; FDB, flexor digitorum brevis; FDM, flexor digiti minimi; FHB, flexor hallucis brevis; I&L, interossei and lumbricals; QP, quadratus plantae.

Figure 4.

Anatomic representation of heat maps for group mean muscle-activation increases for each exercise. A, Short foot. B, Toes spread out. C, First-toe extension. D, Second- to fifth-toe extension. Reprinted with permission from BMJ Publishing Group Ltd.2

Figure 5.

Muscle-activation increases (%), group means, and associated 95% confidence intervals for each exercise. Abbreviations: ABH, abductor hallucis; ADM, abductor digiti minimi; AHO, adductor hallucis oblique; FDB, flexor digitorum brevis; FDM, flexor digiti minimi; FHB, flexor hallucis brevis; I&L, interossei and lumbricals; QP, quadratus plantae.