The purpose of this study was to investigate whether the sports performance of lower extremities during fifteen min-treadmill running at a 75% VO2 max speed was affected by wearing a mask and running time. The kinematics and kinetics results showed that there was no difference between the mask group and the nonmask group during running, and wearing a mask did not affect the joint angle or impact peaks of the lower extremities. As running time increased, hip joint flexion/extension max angles, hip joint ROM, and ankle joint plantar flexion max angles increased after nine min of running; knee joint flexion min angles and ankle joint dorsiflexion max angles decreased after nine min of running; average PVGRF increased after nine min of running. Therefore, the joint angles of the lower extremity and touchdown PVGRF were affected by exercise time and changed after nine min of treadmill running for the mask group and nonmask group.
Changes in joint angles located in the vertical position anteriorly
The study found that joint angles located vertical position anteriorly were not different between mask group and nonmask group during fifteen min-treadmill running, the joint angles of right hip average flexion max increased, right/left knee average flexion min and right/left ankle average dorsiflexion max decreased with time after nine min-running. The running gait changes adaptively under the influence of repetitive loading of lower extremities. A previous study found that runners tend to maintain the same leg stiffness as running time increases, while knee and hip more extended postures are beneficial for strengthening leg stiffness when the lower extremity touchdown [19]. In this study, the right hip average flexion max increased and the knee average flexion min decreased after nine minutes of running, which may help the runners maintain the same leg stiffness during fifteen minutes of running to adapt to the constant running speed of the treadmill. In addition, a previous study found that runners who increased the dorsiflexion angle of their right ankle by approximately 5° at any time with mechanical perturbation could still adjust their running patterns and maintain stability with a time-dependent adaptive strategy [20]. In this study, the right/left ankle average dorsiflexion max increased after nine minutes of running time, which may be the safety strategy adopted by the runners to maintain the constant speed given by the treadmill.
Changes in joint angles located in the vertical position posteriorly
The study found that joint angles located vertical position posteriorly were not different between the mask group and nonmask group during fifteen min-treadmill running, but the joint angles of right/left hip average extension max and right ankle average plantarflexion max increased with time after nine minutes running. A previous study found that knee flexion–extension muscle fatigue led to greater hip extension in the toe-off phase of running, while ankle flexion–extension muscle fatigue led to greater ankle plantar flexion in the swing phase of running [21]. In this study, the right/left hip average extension max and right ankle average plantar flexion max increased after nine minutes of running, which may be due to the increased plantar flexion angle for the running propulsive phase of stance with exercise time. In addition, past studies have found that an increased ankle plantar flexion angle during jogging can increase the push-off power of the lower extremities for forward propulsion [22]. Ankle plantar flexor stretch–shortening activity during running can store energy attached to the Achilles tendon for release early and late in the stance phase [23]. In this study, the right ankle average plantar flexion max angle of runners increased after nine minutes of running to maintain a constant running speed, which may help to enhance the elastic energy reserve and increase the push-off power of lower extremities, thereby increasing the body power for forward propulsion.
Changes in average ROM
The study found that the average ROM was not different between the mask group and the nonmask group during fifteen min-treadmill running, and the average right/left hip ROM increased with time after nine minutes of running. A previous study found that the runner hip, knee, and ankle joint ROM in the sagittal plane during the stance phase increased with fatigue at a fixed speed. The runner maintained the balance of mechanical torque and angular displacement to produce the same level of mechanical power during the running period, while the reduction in joint torque or muscle force caused by the increase in exercise time induced an increase in joint ROM [24]. Therefore, the right/left hip average ROM increased over time, which may be due to a stable strategy adopted by the runner throughout the exercise time to maintain a constant running speed in the entire exercise time. In addition, muscle tuning maintains skeletal activity on the preferred movement path, and the muscle adjusts slightly to adapt to the conditions when it is necessary to maintain the same movement conditions for a period of time [25]. In this study, the average right/left hip ROM increased with time after nine minutes of running, which may be the result of the change in the lower extremity joint angle after the adjustment of muscle activation.
Changes in the average PVGRF
The study found that the average PVGRF was not different between the mask group and the nonmask group during fifteen min-treadmill running, and the average PVGRF increased with time after nine minutes of running. In the process of running, the VGRF applied to a foot plantar surface would directly affect the load transmitted of each joint for lower extremities with musculoskeletal injuries [26]. The increase in GRF during running increases the metabolic consumption of the body to meet the output of muscle mechanical power, which accumulates the risk of injury from overuse of lower extremity joints [27]. The runners in this study maintained a constant running speed, which may lead to greater vertical ground reaction force and increase the risk of lower extremity joint injury. In addition, a past study found that running fatigue alters the lower-extremity movement patterns of novice runners, making them less prone to knee flexion on landing and more upright on the ground, which may increase the risk of lower-extremity injury [28]. In this study, the decrease in right/left knee average flexion min after nine minutes of running, that is, the lower extremities landing in a more upright manner, may reduce the ability of the knee joint to absorb shock and increase the average PVGRF. An increase in the ankle dorsiflexion angle will directly affect the ability of the lower extremities to absorb impact force and increase GRF [29]. In this study, the right/left ankle average dorsiflexion max decreased after nine minutes of running, which may alleviate the greater stress in the knee joint and control the GRF within a reasonable range to avoid the risk of lower extremity injury.
Limitations
This study has the following limitations. First, the participants of this study were male college students aged 19–21 years, so the conclusions may not be fully applicable to women of the same age or other age groups. Second, this study focused on the effect of wearing masks and running time on the joint angle and PVGRF of the lower limbs in the postpandemic era, but EMG data were not collected to observe the muscle response of the lower limbs. It is also necessary to further explore muscle activation during treadmill running to make the study more detailed.
Conclusions
Currently, the global pandemic prevention regulations are constantly updated and changed under the evolution of the new variant of COVID-19, but wearing protective masks in public will still be an important means to block the pandemic virus in the future. In this study, wearing a medical mask during 15 min of treadmill running at a speed of 75% VO2 max did not affect the sports performance of the lower extremities, but the joint angles and PVGRF of the lower extremities changed after nine min of running time. Adaptive changes in hip, knee, and ankle joint angles located in the vertical position anteriorly/posteriorly as running time increased between the mask and nonmask groups showed that the runners coped with the fixed speed of the treadmill by changing the joint angles of the lower extremities. At the same time, PVGRF also increased after a nine min running time and may be accompanied increased risk of lower extremity injury. Future research will further explore the impact of long-term exercise with medical masks on human blood biochemical values and possible safety issues in the postpandemic era.