THE ROLE OF EXTRINSIC FOOT MUSCLES DURING RUNNING

Abstract

Overuse injuries in running are typically associated with excessive pronation of the foot during stance. Excessive pronation may impose stress on the extrinsic muscles of the foot leading to injury. The extrinsic foot musculature includes all muscles that insert on the foot but originate proximal to the foot (e.g., soleus). The function of these muscles can be examined by perturbing the frontal plane motion of the foot during running. Understanding the functional role of these muscles may be key to understanding the etiology of overuse running injuries. PURPOSE To determine the kinematic, kinetic and EMG responses to perturbations of the foot by running in varus, neutral, and valgus-wedged shoes. METHODS Ten males ran at 3.6 m/s in varus, neutral, and valgus-wedged shoes while 3-d kinematic and kinetic data and EMG data were collected. Surface EMG data were collected from the tibialis anterior, peroneus longus, medial and lateral gastrocnemius, and soleus. Indwelling EMG was obtained from the tibialis posterior. The net joint moment, power, and total positive and negative work were calculated in each plane. EMG onset, offset, and integrated values were reported. Differences between conditions were tested using a repeated-measures ANOVA (p < 0.05). RESULTS The primary differences between conditions occurred in frontal plane kinematics and kinetics. The maximum eversion angle (varus = −5.0 ± 8.2; neutral = −6.1 ± 6.7; valgus = −8.5 ± 7.0 degrees), maximum inversion moment (varus = 12.5 ± 14.3; neutral = 14.7 ± 14.5; valgus = 21.0 ± 15.7 Nm), and total negative work done in the frontal plane (varus = 5.2 ± 2.7; neutral = 4.5 ± 1.6; valgus = 6.5 ± 2.6 J) were greatest while running in the valgus shoe and least in the varus shoe. The greater joint moment, however, was not accompanied by changes in muscle activation. CONCLUSIONS Hyper-pronation may cause excessive loading in the extrinsic foot muscles through passive mechanisms. The increased loading may be sufficient to cause injuries such as shin splints or tendonitis. Supported by the International Society of Biomechanics, Life Fitness Academy, and New Balance.