How does fatigue affect the patellar reflex

Therefore, Zebis et al. The purpose of the present study was to analyze gender differences in hamstring reflex responses and TT before and after a fatigue protocol.

The muscle activity of the lateral and medial hamstrings was evaluated. It was hypothesized that, due to fatigue, reflex components of the muscles are impaired and TT is altered. We assumed that the main outcome variables would show gender-specific differences.

Fifty healthy subjects 25 males: Before testing, subjects were instructed to refrain from consuming alcohol and caffeine in the 24 h preceding the experiment and not to perform any strenuous exercise in the 48 h prior to the measurements. All persons signed informed consent. The study was conducted according to the declaration of Helsinki and was approved by the ethics committee of the University of Rostock A During the experiment, participants were examined with regard to reflex responses and TT before and after a fatiguing jumping task.

The measurements were performed using a knee arthrometer [16] , [21] — [23] Fig. In addition, before the execution of the fatigue protocol, the subjects performed isometric MVCs for the hamstrings and the quadriceps using a dynamometer. The experiment required approximately 2 h per person. A: Experimental setup, B: Measurement system 1: stopper, 2: falling weight, 3: pulley, 4: steel rope, 5: force transducer, 6: force plate, 7: visual cover, 8: linear potentiometer.

Arrows indicate the direction of the force. Posterior-anterior tibial translation was assessed by two linear potentiometers 8 placed on the patella and the tibial tuberosity. A force transducer 5 was used to measure the force transmitted to the shank.

Subjects were thereby provided with online feedback about their center of pressure. Furthermore, to avoid the influence of acoustic signals on the subjects they wore ear protection Bilsom Thunder T3.

The subjects had no information on the point in time of the perturbation. A standardized force was applied to the proximal shank of the dominant leg using a pulley system in order to induce TT. The knee arthrometer enabled us to measure the translational movement of the tibia relative to the femur in the sagittal plane. The interface pressure between the knee arthrometer and the subjects' tibia was controlled by an air pressure recorder Kikuhime, TT MediTrade, Denmark.

The interface pressure was kept constant before and after the fatiguing exercise. The locations of the subjects' feet on the force plate, the stabilizing device and the linear potentiometers were marked in order to ensure the same positions before and after the fatigue protocol.

The force sensor was placed between the stabilizing device and the pulley system. TT was elicited 15 times in order to familiarize the subject with the measurement. Thereafter, further 15 perturbations were applied before and immediately after the fatigue protocol. The electrodes were applied with a center-to-center distance of 2 cm over the muscle bellies and in line with the presumed direction of the underlying muscle fibers. The reference electrode was attached to the patella. The axis of the dynamometer was aligned with the anatomical knee flexion-extension axis and the lever arm was attached to the anterior aspect of the shank 2—3 cm above the lateral malleolus.

Straps across the waist and chest prevented excessive movements. The iMVT was tested by asking the subjects to exert isometric knee extensions and flexions against the lever arm of the dynamometer for 3 s.

For each trial, subjects were thoroughly instructed to act as forcefully and as fast as possible. They were motivated by strong verbal encouragement and online visual feedback of the instantaneous dynamometer torque provided on a digital oscilloscope HM, HAMEG Instruments, Germany. A rest period of 2 min was allowed between trials. The fatigue protocol consisted of consecutive maximal countermovement jumps [25] , each one separated by 4 s according to the sound of a digital metronome.

Rate of perceived exertion RPE was assessed using the Borg 6—20 scale. In order to analyze the data, the EMG signals of each subject were averaged. The EMG onset latencies were defined as the time between onset of TT and onset of significant muscular activity, e. Muscle activity was analyzed according to Bruhn et al. Consequently, background activity was subtracted from the reflex responses.

Maximum TT was determined based on the TT curves. In the figure, EMG data is rectified in order to visualize the different parts of the hamstring stretch reflex.

The vertical bold line indicates the onset of posterior-anterior tibial translation. Three different time intervals were analyzed 20—40, 40—60 and 60—95 ms. The torque signals were corrected for the effect of gravity and the three best maximum voluntary contractions were retained for analysis.

The iMVT was defined as the highest peak torque value. Explosive voluntary muscle strength was determined by analyzing the average RTD over time intervals of 0—50, 0— and 0— ms relative to the onset of contraction. The identification of torque onset was made manually according to the method of Tillin et al. It has been suggested that this is the best method for detecting signal onsets [27]. Data were checked for normal distribution using the Kolmogorov-Smirnov test.

Differences between the values before and after the fatigue protocol were tested for significance by repeated measures ANOVA. Differences between the groups were tested for significance by the unpaired Student's t test.

Correlations between parameters were calculated using Pearson's correlation coefficient. SPSS The ES characterizes the effectiveness of an intervention. Furthermore, it is used to determine whether a statistically significant difference is a difference of practical importance.

During the fatiguing exercise, men performed RPE using the Borg 6—20 scale was The force applied to the proximal shank of the dominant leg remained constant during the pre- and post-test Table 1. The purpose of this study was to elucidate the effect of fatigue, induced by repetitive jumping, on reflex activity of the hamstrings and TT in men and women. Reflex onset latencies were enhanced in women after the fatiguing task.

The results revealed a fatigue-induced reduction of reflex responses in women associated with an increased TT. Men showed no significant differences in the parameters after the fatigue protocol. It has been assumed that increased joint laxity may contribute to increased ACL injury risk [30].

Several studies have found significant increases in anterior knee laxity, for example, after running [31] — [33] or a regular workout in volleyball [34].

However, only Kvist et al. Nevertheless, these studies measured anterior knee laxity while subjects were relaxed. In contrast, in the current study TT was measured in a functional weight-bearing situation. In a situation such as this, axial loading and forces due to muscle contraction could reduce rotation and translation compared to the passive condition [35] , [36].

The present study found an increase in TT in women but not in men after the fatiguing exercise. The ES of 0. Studies using a similar methodology have shown that an isokinetic fatigue protocol performed with a dynamometer can increase TT [13] , [16]. However, only the study by Wojtys et al. The authors reported no gender difference in any parameter. In general, knee ligamentous structures probably undergo some increase in laxity during exercise, thereby placing athletes at risk for ligamentous injury [6].

It is assumed that this is due to the fact that joint structures, particularly the ligaments, exhibit viscoelastic properties [37]. Therefore, cyclic stress of the ligamentous structures leads to time-dependent and stress-dependent modifications and therefore increased ligamentous laxity [6] , [37]. However, the muscles that cross the knee joint play a large role in maintaining physiological kinematics of the knee.

Muscle activity is able to induce large changes in strains as well as forces experienced by the ACL [38]. The fast activation of muscles by means of reflexes may play a substantial role in the stabilization of the knee joint [14]. It has been suggested that the direct reflex arc between the ACL and the hamstrings makes only a minor contribution to the biphasic reflex response in the hamstring muscles [39].

Therefore, it has been suspected that the reflex response is mainly generated by hamstring stretch reflexes [15]. In the current study, delayed reflex onset latencies for BF and ST were found in women after the fatigue protocol.

The onset latencies for BF and ST in men were not statistically different after fatigue. However, ES of 0. Similar results were reported by Melnyk and Gollhofer [16] who found an increased latency of reflex responses after submaximal fatigue.

The authors have argued that the slowing of reflex responses probably does not play a substantial role in functional knee stability. Nevertheless, the authors have not focused on gender-specific differences. The results of the present study correspond with the results of Melnyk and Gollhofer [16] who found significantly decreased iEMG values for the short latency response and medium latency response of the hamstring stretch reflex after an isokinetic concentric-eccentric fatigue protocol.

As before, gender-specific differences were not investigated. Moore et al. The authors reported a significant increase in reflex amplitude in men and a tendency to a reduction in women. They concluded that males and females might respond differently to fatigue. A reason for this could be that men and women activate their muscles differently according to the requirements of the movement task. For example, Rozzi et al. The authors concluded that the greater EMG peak amplitude and area in women might be an attempt of the nervous system to compensate for the greater joint laxity and proprioceptive deficit.

Furthermore, the authors argued that an interruption of this compensatory mechanism, for example due to fatigue, might increase joint laxity that may cause ligament injury. The greater muscle activity of the hamstrings when landing from a jump in females may be an explanation for the differing results in the present study between men and women regarding reflex responses and TT.

It is conceivable that the fatigue protocol used in this study, which consisted of repetitive jumps performed until exhaustion, induced more fatigue in the hamstring muscles of women and therefore impaired hamstring reflex responses. The decrease in muscle activity in distinct time intervals in women could be explained by different physiological processes: i fatigue-induced changes in intrafusal properties, ii presynaptic inhibition PSI of Ia afferents and iii changes in intrinsic properties of motoneurons.

Fatigue-induced changes in intrafusal properties are assumed to occur during sustained MVCs and probably reduce intrafusal contraction force and thereby the fusimotor-driven afferent discharge [42]. Furthermore, it has been suggested that submaximal isometric fatiguing exercise is also able to change intrafusal properties and, in turn, reflex responses [43].

These afferents are sensitive to several parameters associated with either metabolic fatigue or muscle damage [45]. It has been found that these afferents have a powerful input to inhibitory interneurons which induce PSI of Ia afferent terminals [46]. In addition, the possibility of changes in intrinsic properties of motoneurons should be taken into account [47].

It has been found that motoneurons can experience an intrinsic adaptation in firing frequency to a constant excitatory drive [48]. Therefore, it can be assumed that the exercise caused a stressful metabolic loading and thereby stimulated group III and IV afferents that probably induced PSI of Ia afferents. Furthermore, it is conceivable that the reflex inhibition of the motoneuron pool was accompanied by changes in the intrinsic properties of motoneurons. The results of the present study revealed that the fatigue protocol used in this study altered the latency as well as magnitude of reflex responses of the hamstring muscles and TT in women.

The authors of various studies have suggested that the hamstring muscles play an important role in maintaining knee stability and that they protect the ACL during movements of the tibia relative to the femur [49] — [51]. Therefore, decreased reflex responses of the hamstring muscles and in turn an increased TT might contribute to the pathomechanics of knee joint injuries.

It has been shown that female athletes have an increased risk for ACL injuries [2]. Based on our results it is conceivable that the fatigue-induced decrease in neuromuscular function with a corresponding increase in TT probably contributes to the higher incidence of ACL injuries in women.

The measurements used in this study were performed in a functional weight-bearing situation. Only a few studies have investigated the effect of fatigue on tibial translation and muscular responses with a similar methodology [13] , [16].

Therefore, comparisons with studies that used another methodology, e. Furthermore, a study by Bruhn et al. It is possible that men and women respond differentially with regard to stimulus characteristics.

In this respect, it is noteworthy that body weight may have influenced the response to the constant stimulus and in turn the observed differences in the neuromuscular response and TT. Moreover, we assume that the fatigue protocol used in this study caused a stressful metabolic loading and thereby modulated the reflex responses. Unfortunately, we have not measured metabolic data that could support this assumption.

The authors would like to thank Detlef Werner for technical support, Rike Pahnke for drawing of the measurement system and Michael Wolter as well as Daniel Lexow for their help during data acquisition. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

National Center for Biotechnology Information , U. PLoS One. Published online Feb TMT increased significantly in females following fatigue, while males showed no change. However, patellar reflex latency was significantly shorter in the weight lifters than in the distance runners.

An alteration of the patellar reflex response may be caused by several different factors, which can range from tumors in the spinal cord [7] to diseases, such as the Guillain—Barre syndrome [8] that affects the peripheral nervous system [9]. One example of a reflex is the patellar stretch reflex.

Stretching the muscle activates the muscle spindle at the end of the sensory neuron embedded in your muscle and starts the reflex. Abnormal patellar tendon reflexes can indicate neurological disease. Striking of the patellar tendon with a reflex hammer just below the patella stretches the muscle spindle in the quadriceps muscle.