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Normal bipedal walking requires symmetry existing between both legs movement which makes the gait cycle of both sides identical (4, 14, and 15). Gait is a period of cyclic movements which led to an idea of introducing time intervals of a gait period or gait cycle as a functional unit for the study of different parameters in gait (4, 7, 9, and 10). Because of these movements, the lower body must absorb both vertical and horizontal reactive forces from the ground while the foot is in contact with the ground. The upright positioning in which the gait takes place is naturally unstable. The instability caused from the trunk and appendicular skeleton being above the pelvis and legs, where the center of mass (COM) is required to remain constant over a narrow base of support. Different segments of the legs are changing constantly their position making it necessary for the trunk to balance above a moving base of support (4, 9). Relative assumption can be generalized to the reasoning of failure to absorb the shock from impact can in return, result in injury. Coventry, O'Connor, Hart, Earl & Ebersole, 2006; Racard & Veatch, 1990) One way in which these shocks can be absorbed involves a combination of muscle contraction and joint motion. The muscles and ligaments of a joint work together by constantly distributing weights back and forth in order to stabilize the joint dynamically and statically. Muscle forces, soft tissue surrounding the joint, and external loads placed on the joint work simultaneously to aid in this stabilization (Schipplein & Andriacchi, 1991). The muscle contraction that occurs at the joint helps to protect the integrity of the soft tissues, which can be susceptible to injury during walking, running, or other types of movements (Schipplein & Andriacchi, 1991). This particular method for shock absorption through muscle contraction and joint motion has been shown to be greatly affected by muscular fatigue (Christina, White, & Gilchrist, 2001; Radin, 1986). Fatigue is defined as any reduction in the force generating capacity of the total neuromuscular system regardless of the force required in any given situation (Coventry et al., 2006; Bigland-Ritchie & Woods, 1984). Additionally, a number of studies have shown the impact of fatigue on gait kinematics and kinetics of the lower extremity (Coventry et al., 2006; Christina et al., 2001; & Noakes, 2000).
Normal walking patterns are dependent on maximizing the appropriate speed that can be maintained over a given distance of controlled movement resulting in forward propulsion of the body. In addition, they must account for a vertical displacement of the body's center of mass. Current human models of normal walking patterns suggest the relationship between mechanical, physiological, biochemical, and neuromuscular factors that make significant contributions to normal walking mechanics (Cavanagh and Kram, 1985; Walsh, 2000). Ultimately, mechanics are dependent on these factors optimizing the balance between maximizing power output and duration during forward propulsion. Improving limitations within any of these systems will theoretically increase normal walking mechanics, likely through delaying or preventing fatigue if all other factors are held equal (Noakes, 2000). To understand the basis of optimizing normal walking mechanics, it is necessary to understand the components of normal walking patterns, how they relate to one another, and how they are affected by fatigue. The purpose of this study was to evaluate the effects of muscular fatigue on lower body (knee and ankle) sagittal plane kinematics while walking. These factors will be reviewed and specific emphasis will be placed on how these factors relate to neuromuscular fatigue and the effects it has on the comparison between kinematics of the ankle and knee pre vs. post fatigue measurements. It was hypothesized that alterations in both ankle and knee kinematics would be observed between the fatigued and non-fatigued states.
A total of eight college students (mean age: 26.4 Â± 3.1 years; height: 170.2 Â± 10.2 cm; and mass: 70.0 Â± 6.4 kg) volunteered to participate in the current study. Prior to their participation, it was determined that all participants were healthy and appropriately conditioned for walking. To determine the health status of the participants, a medical history questionnaire was completed. To determine the fitness levels of the participants, a survey of their level of cardiovascular training was completed. In the current study; race, gender, and body mass index were not a factor contributing to participant selection or exclusion.
All data collection sessions were conducted indoors at the University's Health, Physical Education, and Recreation building and were designed to best simulate a normal exercise setting. Testing protocols used in the current study were not approved by the University's Institutional Review Board (IRB). This was due to the fact that the current study was conducted as a class project and was therefore exempt from the standard IRB protocol. However, throughout the current study, each subject was provided information regarding the protocols to be used, both the negative and positive possible outcomes, and retained their right to withdrawal from the study without prejudice at any time. Protocol for Collecting Kinematic Data
Prior to the collection of data describing the gait cycle of the non-preferred leg, a 2-dimensional, 2-camera Peak Motus motion capture system (Vicon Peak) was set-up to collect video footage in both the frontal and sagittal plane field of view. The system was calibrated by capturing a single calibration frame in which known landmarks were digitized. The calibration frame used in the current study is shown in Figure 1.
To identify segment landmarks, a series of retro-reflective markers were placed at the following locations on the non- preferred lower extremity: (1) the base of the 5th metatarsal; (2) the most lateral aspect of the lateral malleolus; (3) the lateral base of the heel; (4) the most lateral aspect of the knee; and (5) the most lateral aspect of the greater trochanter of the femur. Angles were calculated as vector resultant angles using a manufacturer supplied computer program. These angles were defined in the distal to proximal direction starting with the foot. The angle definitions used in the current study are shown in Table 1.
Following the attachment of the retro-reflective marker set, subjects were directed to walk at a self-selected speed over an elevated platform toward a specific impact location for their non-preferred foot. They were allowed to complete as many trial passes as necessary so that they became comfortable striking target area. During these trial passes, standard athletic tape (Johnson & Johnson) was used to mark the point from which the subjects would start when completing the test trials. Once the subjects were comfortable with the motion capture protocol, a series of test trials were conducted. During testing, kinematic data describing sagittal plane movements (flexion/extension) about the ankle and knee were collected at a rate of 60 Hz. A series of three passes were collected for each subject in a non-fatigued state. Following the collection of these trials, subjects were subjected to the fatiguing protocol described below. Once the subjects were identified as being in a fatigued state, an additional three passes were collected so that their gait kinematics could be compared across fatigue levels.
Fatigue has been defined as the "transient decrease of working capacity" (Asmussen, 1979). To better understand the effect of fatigue on performance, as well as the relative importance of different components of performance, walking mechanics were compared during non-fatigued states and after fatiguing the lower extremities with an exhaustive run (on same day). Subjects were tested using the non-preferred leg prior to fatigue protocol using high speed videography recording at 60 Hz as well as digitized markers placed at joint centers of the ankle, hip and knee of the lower extremity. Subjects were assigned to practice until comfortable; striking the force plate with the non-preferred leg at subjects' desired speed. Once pre-fatigue measurements were collected, subjects were asked to complete the standard Bruce Protocol until the onset of fatigue was reported. This protocol is a standardized maximal exercise test in which the subject works to complete exhaustion as the treadmill speed and incline is increased every three minutes (Bruce, 1963). During the test, ratings of perceived exertion for each subject were collected. Once fatigue was developed, subjects were assigned to return to the testing laboratory to collect data using the same tactics as previously mentioned. Our hypothesis is ankle and knee kinematics would differ between fatigued and non-fatigued states..
The data in the current study were analyzed using the Statistical Package for Social Sciences (SPSS Inc, Chicago, IL). Prior to investigating the possible differences between states, data describing ankle and knee kinematics were analyzed for normality and the mean and standard deviation were calculated. Once the data were deemed to be normally distributed, pair sample t-statistics were calculated to identify those ankle and knee parameters that were significantly different between the fatigue states.
The ankle and knee kinematics reported in the current study were similar to those reported in previous studies and are shown in Table 2. During the gait cycle in a non-fatigued state, subjects initially contacted the ground with the heel of their non-preferred foot (heel strike) with an ankle that was in a nearly neutral position (M=0.5ï‚°, SD=2.1ï‚°) and a slightly flexed knee (M=169.8ï‚°, 4.6ï‚°). The ankle and knee kinematics observed in the fatigued state (M=0.2ï‚°, SD=1.7ï‚°) and (M=165.8ï‚°, SD=3.9ï‚°), respectively, were very similar to those observed in the non-fatigued state. As progression of the gait cycle continued and non-fatigued subjects entered the mid-stance phase, the ankle had been dorsiflexed an average of 3.6Â° and the knee extended an average of 6.0Â°. Although the ankle kinematics observed in both the non-fatigued and fatigued states were similar, knee kinematics were observed to be significantly different between the states of fatigue (p < .001). In the fatigued state, the angle of flexion averaged 169.7Â° (0.88ï‚°) across all subjects indicating that when they were fatigued, subjects were only capable of extending their knee a total of 3.9Â° degrees from heel strike. At toe off, the final stage of the gait cycle, subjects in the non-fatigued state were observed to plantar flex the ankle and flex the knee. However, when subjects were fatigued, the angle of plantar flexion at the ankle was observed to be significantly less (7.8Â°) than when they were not fatigued (8.4Â°; p = 0.04).
Descriptive statistics for ankle and knee kinematics.
Although the researchers attempted to gather both kinematic and kinetic data, the kinetic data was unusable; therefore, only kinematic data has been presented.
Bigland and Ritchie (1984), defined neuromuscular fatigue as "any reduction in the maximum force generating capacity, regardless of the force required in any given situation." Numerous studies have shown that neuromuscular fatigue can reduce the force generating capacity of muscle, as well as affect motor control and proprioception (Skinner, Wyatt, Hodgdon, Conard, & Barrack, 1986) and muscle reaction times (Hakkinen & Komi, 1986). When f fatigued, subjects were only capable of extending their knee a total of 3.9Â° degrees from heel strike to mid-stance compared to a 6ï‚° extension in a non-fatigued state. At mid-stance, knee angle was significantly lower when fatigued than when non-fatigued. At toe-off, ankle plantar flexion was also significantly lower. Both of these findings agree with Coventry et al. (2006), who suggested that while fatigued, work is redistributed to the larger proximal muscles to attenuate shock.
One suggestion for a future study is to include a reflective marker on the shoulder, introducing another joint's (hip) kinematics for analysis. Coventry et al. (2006) concluded that hip flexion increased after the fatigue protocol, which compensate for the knee and ankle motions. Another suggestion for future study is to alter the fatiguing protocol to agree with other studies. Verbitsky et al. (2006) and Mizrahi, Verbitsky, Isakov, & Daily (2000) induced fatigue by running subjects on a treadmill for 30 minutes at a speed near or above anaerobic threshold (AT), while Bisiaux & Moretto (2008) ran subjects on a 400m track at 80% maximal aerobic speed. Ensuring a consistent time between the fatigue protocol and post-fatigue data collection may also ensure that the subjects are fatigued when completing the post-fatigue walk.
In the present study, the researchers concluded that the subjects appeared to adapt their walking styles with fatigue, resulting in altered lower extremity kinematics. Changes in these kinematics with fatigue may be associated with distinguishing between the proper functioning of the lower extremities or mechanisms that may contribute to abnormal gait pathologies.