In this study, the muscle activation differed between the two exercises. The HSS strongly activates the Abd H, TA, TP, PL, ST, and Mul muscles compared with the CIS.
Perturbation-based training is effective for trunk stabilization, and it reduces pain. Schäfer et al. examined instability situations using an unstable surface or water-filled pipe or pushing from a third party and stated that perturbation-based trunk stabilization training is possibly effective in improving the physical function of the lower back in elite rowers. . Perturbation-based intervention (using soft pads, balance cushions, BOSU balls, inverted BOSU balls, Swiss balls, slashpipes, and sling trainers) for a year in adolescent athletes reduced strength imbalances and LBP intensity decreased . The Mul muscle contributes to the stability of the lumbar spine and plays a role in controlling intersegmental motion . In a previous study, the HSS demonstrated significant activation of the Mul muscle by perturbation-based intervention.
The foot is a complex structure that plays an essential role in maintaining static and dynamic posture. Intrinsic and extrinsic muscles control the movement and stability of the foot arch . Because HSS requires the center of pressure to shift forward and backward, the TP, TA, and PL, which are fundamental muscles to control the foot arch dynamically, increased the muscle activities with HSS. Exercise programs improve intrinsic and extrinsic foot muscles to help sports injuries, rehabilitation, and prevent fall risks. Short foot exercise is a proper strengthening exercise activating the foot muscles, especially the Abd H [27, 28]. Short-foot exercises can help strengthen the Abd H muscle in individuals with pes planus . Kulig et al. have reported that TP training with orthoses could improve foot functional index scores including pain and disability . Selective training for the tibialis posterior with iliopsoas stretching demonstrated prominent improvements in dynamic balance and static arch height compared with conventional towel curl exercises in participants with pronated feet . The 4 to 8 months of training for the foot exercise program was effective for running-related injuries risk for 2.42 times lower than that of the control group within 12 months for recreational runners . Further, 6 weeks of short-foot exercise intervention reduced navicular drop, foot pronation, foot pain, disability, and increment in plantar force of the medial midfoot in pes planus . Impairing balance increases the risk of falls in older adults. Foot/ankle characteristics, sensorimotor function, balance, and functional ability are associated with the risk of falls. The intervention of a program that improves the strength and flexibility of the foot muscles can reduce the risk of falls . However, the movement of the foot and ankle joints is complicated, and some people have difficulty in effectively training the internal and external muscles of the foot because of deformation or pain. Therefore, it is important to devise a method that can effectively train the intrinsic and extrinsic muscles of the foot, and the results of this study showed important data in that the HSS can train these muscle groups unconsciously.
In a present study, HSS significantly activated intrinsic and extrinsic foot muscles without the intention of moving the foot fingers by hammer perturbation.
A present study added an unstable situation with hammer perturbation during a squat called the HSS. In postural stability against perturbation, the activity of muscle groups related to the lumbar spine, foot, and ankle joint tended to be increased rather than the muscle groups associated with the hip joint and pelvis stability. The foot and Mul muscles are significantly activated without changing positions, trying to move the hip or knee joint angles or foot toes. Although this exercise shows muscle activity at BF and FB phases differently, it requires a switching function on muscle activation while swinging the hammer steadily in the squat position. Learning the timing of on and off for the muscle activation, and this movement may potentially improve the coordination of the muscles around the trunk, hip joints, knee joints, and ankle joints can develop coordination on the whole body. For clinical implications, the HSS exercise can be a candidate for core and foot stability exercise, and it could be a valuable exercise even for patients with limited joint movement.
This study had some limitations. First, we only examined participants in a single body position, while in the isometric squat position. Different body positions can lead to different muscle activation. Second, we only compared the anteroposterior direction of the hammer movement. Different hammer movements can result in different muscle activation outcomes. Lastly, we did not normalize the EMG signals because data were collected/compared in the same participant during the same session within a short period . Therefore, these factors should be considered and analyzed in future studies.
Future studies can examine muscle activities on different Hammerobics exercises by changing the weight or length of the wire or the direction of the hammers.