The influence of body posture and added mass on intrinsic and extrinsic foot muscle activation and force output during common foot strengthening exercises.

Abstract

BACKGROUND Intrinsic and extrinsic foot muscles contribute to foot and lower limb function. This knowledge provides opportunities to target these muscles in exercise and rehabilitation. However, a barrier is the limited understanding of how prescribed exercises affect muscle activation and force output. This study examined muscle activation and torque production about the metatarsophalangeal joints of intrinsic and extrinsic foot muscles during common foot strengthening exercises. The secondary aim was to determine whether the addition of a postural change (leaning forward) or addition of mass can further increase muscle activation and torque. METHODS Fifteen healthy participants (9 males, 6 females; age = 36.5 ± 8 years, mean ± SD) participated. Muscle activation was measured using fine-wire electromyography (EMG) electrodes for 2 intrinsic and 2 extrinsic muscles, and surface EMG electrodes for 4 superficial muscles. Metatarsophalangeal joint torque was measured using a custom toe dynamometer. Participants attended one 90-min session and performed 16 exercises. Five included the addition of leaning forward and 3 the addition of mass (20% of participant's mass) to compare their effects on muscle activity and torque. RESULTS Muscle activation and metatarsophalangeal joint torque varied considerably across exercises. Both the addition of leaning forward and the addition of mass increased muscle activation and metatarsophalangeal joint torque, with the addition of leaning forward producing a 35% median increase in torque about the metatarsophalangeal joints. CONCLUSION Many common exercises, despite high muscle activation, produce relatively low metatarsophalangeal joint torque, raising questions about their clinical value. Adding a forward lean significantly increases toe flexor muscle activity and torque, reaching levels greater than added mass and comparable to walking.