The results of this study indicate clear differences between elite rugby league athletes and age- and gender-matched non-athletes in tendon tap reflex scores for biceps, triceps, patellar and Achilles tendons. In each tendon, reflex scores of the elite athletes were distributed significantly lower on the range of possible tendon reflex scores than scores of the non-athletes. In a substantial proportion of cases, the athletes demonstrated median tendon reflex scores between 0 and 1.5, indicating absent or diminished reflex responses. In many of these cases, the athletes demonstrated tendon reflex scores of zero, representing an absent reflex. These differences between the cohorts were most marked in the upper limb (biceps and triceps) tendon reflexes, with the biceps tendon reflex distributions the most different between the cohorts. Possible mechanisms for these changes will be discussed later.
Both cohorts in the present study included substantial proportions of individuals with absent or diminished reflexes, despite all participants being healthy. This finding supports the conclusion by Stam & van Crevel (1989), who stated that absent reflexes may be clinically normal in the general population. However, the current study adds to these previous conclusions the important finding that absent and diminished reflexes appear far more common in elite athletes than in age-matched non-athletes drawn from the general population. Of note, no participant from either population in the current study received a tendon reflex score of 4 for any of the assessed reflexes. This finding is in agreement with clinical guidelines [
1], which indicate that a grade 4 tendon reflex response (representing muscle clonus) should be considered abnormal in populations examined in the present study.
The observed high, but not perfect, correlations between tendon reflex scores for the same tendon on the right and left sides of the body in the current study suggest that asymmetry in tendon reflex responses occurs but may not be as common in the populations sampled in the current study as previous literature [
2,
6] has suggested.
Low to moderate correlations were observed in the current study between tendon tap reflex scores and the lengths of player careers, measured in terms of both the numbers of professional NRL games played and the numbers of years spent playing professional NRL. There appears to be a possible dose response relationship with one or more factors associated with playing professional rugby league and a decrease in tendon tap reflexes. It is unclear which factor or factors associated with the length of the player’s career mediated the observed correlation between length of player career and tendon reflex responses in the current study.
Possible causes of altered tendon tap reflexes
There are many areas within the monosynaptic reflex arc where adaptation may occur with ongoing athletic activity over the length of the player’s careers, resulting in altered reflexes. A systematic search of the literature completed by the authors revealed that specific contraction types [
13], exercise intensity [
4], training history [
3] and recent stretching [
6] may be responsible for these types of changes in muscles and muscle function, and thus alter the results of a tendon tap examination.
In one study of the effects of stretching, 50 male participants from a variety of sports had Achilles tendon reflexes assessed prior to and following 3 min of passive stretching [
6]. The study’s authors found a significant 5 % decrease in Achilles tendon reflex responses and this was not affected by a subsequent 10 min treadmill run. That study showed that passive stretching has the ability to affect reflex responses in the short term. To the knowledge of the authors, there has been one study exploring the long-term effects of stretching on tendon tap reflexes [
14]. Guissard and Duchateau found a 36 % reduction in Achilles tendon reflexes following just 4 weeks of training. This reduction did not coincide with the decrease in passive stiffness pointing towards a possible neural mechanism for reduction in reflex responses [
14]. Given that professional athletes stretch regularly in order to maintain muscle compliance, chronic effects of stretching constitute one possible explanation for the diminished tendon reflex responses observed in the athletes in the current study and further research to examine the neural vs. mechanical mechanism, is warranted.
Another possible explanation is that the resistance training completed by elite athletes might be responsible for the altered tendon reflexes observed in the current study. Sixteen weeks of general resistance training did not significantly alter patellar tendon reflexes in a group of mixed gender subjects in one previous study [
8]. There is, however, contradictory evidence regarding whether particular types of muscle contractions can elicit changes in tendon tap reflexes such as those observed in the current study. Ten days of purely concentric exercise were shown to significantly decrease patellar tendon reflex responses in untrained male subjects [
13]. On the other hand, eccentric exercises in the same population did not have a significant effect on tendon tap responses [
13]. So, once again it is possible that specific concentric resistance training might contribute to diminishing tendon reflex responses in athletes but further research is required to elucidate this possibility. Kaufman et al. [
13] also noted that tendon tap reflexes were not influenced by exercise related muscular trauma or delayed onset muscle soreness (DOMS).
The current study showed diminished tendon tap responses in athletes for all muscle groups assessed. Similar findings were reported with a mixed gender study comparing endurance athletes with a non-athlete cohort, with regard to patellar tendon reflexes. Endurance athletes were found to have significantly diminished patellar tendon reflex force (42.3 %) when compared to the non-athlete cohort [
3]. It was noted that Achilles reflexes were not significantly different between the endurance and non-athlete cohorts [
3]. The similarity between these findings and those of the current study suggest that some aspects of endurance training or specific sports requirements might contribute to diminishing tendon reflex responses.
Although each theory has its merits, it is likely a combination of several factors come together to create the athletes training history. Proske and Morgan demonstrated that eccentric strength training (a component of power training) causes muscle adaption by increasing the number of sarcomeres in series and therefore causes a compliance change within the tissue [
15]. These compliance changes may alter the sensitivity of the spindle fibers and cause a reduction in the reflex response of power-trained athletes as demonstrated in the current study. It has been hypothesized that postural muscles, which are primarily slow twitch motor units, have a lower reflex threshold than other muscles due to a higher concentration of muscle spindle fibers [
5]. The postural muscles of the calf may be resistant to small compliance changes due to the higher concentration of spindle fibers and their consistent use as postural muscles on a day-to-day basis. This hypothesis may explain why the Achilles reflex in the present study was least affected by change within the athlete group. Other elite athletes like swimmers have been shown to have the opposite response with an increase in Achilles reflex response when compared to the general population [
16]. The long term microgravity that the athletes are exposed to within the pool is believed to be a contributing factor for their reflex changes [
16]. The relatively plantarflexed position of the foot while swimming may also assist in creating some compliance changes within the muscle and tendon structures.
This brief excursion into relevant literature illustrates that there are multiple interlinking factors that elite athletes encounter on a daily basis which have the potential to contribute to the diminished tendon tap reflexes observed in athlete’s in the current study. Further research is required to elucidate the effects of these and other factors that might contribute.
Clinical importance
The findings of this study indicate that traditional clinical interpretations of scores on the standard tendon reflex grading scale [
1] may not be appropriate for elite athletic populations. Clinically diminished or absent reflex scores may be normal for elite athletes and given the large proportions of athletes with diminished or absent scores, they should not be viewed as a sign of neurological injury or impairment. Tendon tap reflexes are used in the medical field to monitor patients with head injuries due to the fact that that their reflex responses will be facilitated [
17]. Clinically normal reflex responses in an NRL athlete may be facilitated by a central nervous system injury. Animal models have shown that concussive head injuries have the ability to increase reflex responses above baseline [
18]. Research is needed in this field in order to determine if this relationship does exist in elite sport.
Tendon reflexes are a protective mechanism which limit rapid movement and stretching of joints and surrounding tissues [
19]. As mentioned previously, tissue compliance changes with training have been shown to modify tendon tap reflexes. With this in mind, it is possible that there is a relationship between injury risk and altered tendon reflexes.