Yogis maintain lumbar lordosis and then bend forward with a greater reliance on hip joint. Our results revealed a delayed onset of FRP in yogis in comparison to control group during forward bending. Also, ES muscles were activated earlier in yogis in comparison to control group during return phase. Absence of FRP occurred significantly more in yogis than control group. Lack of FRP was previously reported in some previous studies. Shirado et al. conducted a study on 25 healthy people and 20 patients with chronic low back pain to investigate the occurrence of FRP and to compare it between the groups. They showed that FRP did not occur in chronic back pain group, in contrast to healthy subjects . In a study conducted by Watson et al., absence of FRP was considered as a diagnostic criterion for individuals with chronic low back pain (93% sensitivity and 75% specificity) . Taken together, it can be concluded that the absence of FRP is an important index to predict injury and a diagnostic criterion for LBP. Absence of FRP was verified in six yogis (20 %) in our study. It might be attributed to special pattern of forward bending instructed to yogis. Similar to the rate of FRP absence in patients with low back pain, the yogis engage ES muscles for a longer duration which might predispose ES muscles to fatigue and predictable injuries. Future longitudinal studies are required to demonstrate if prolonged contraction of ES muscles among yogis could lead to injuries like LBP. Our findings might be attributed to some factors.
Maintaining lumbar lordosis during forward bending might be similar to an additional load bearing situation exerting extra load on ES muscles in yogis. Previously, it was shown that applying load would lead to a greater activation of lumber ES muscles and delayed FRP during forward bending .
Greater flexibility in yogis might be another explaining mechanism for the observed results. Shin et al. investigated the effects of flexibility on FRP and found that flexibility has a significant effect on FRP. Higher flexibility would lead to greater activity of lumbar musculature and delayed cessation of activity . Chen et al. showed that higher flexibility, measured via toe-touch-test, was associated with slower occurrence of FRP . It has been found that when performing identical deep trunk flexion, flexible people exhibited relatively delayed FRP compared with less flexible people [27, 28]. Chen et al. found that flexible participants have a larger range of motion for trunk flexion; when the trunk flexes forward to 90°, greater lumbar curvature is maintained, resulting in a lower degree of FRP .
Moreover, they found that the different FRP levels found in participants with different flexibilities can be explained by the change of lumbar curvature. The flexible participants exhibited larger lumbar lordosis from standing erect to flexing forward 90° . In contrary to their findings, yogis maintain the lumbar lordosis voluntarily when performing forward bending in our study. Thus, our results could not be attributed to lumbar curvature alteration.
It is worth noting that lumbar flexion angles had no significant difference between the groups during forward bending and return phases in both initiation and cessation of FRP. Since yogis maintain lumbar lordosis during forward bending and return phases, it appears that lumber flexion had a small contribution to these movements in yogis. Maintaining lumbar lordosis might have altered normal lumbo-pelvic rhythm in yogis. We hypothesized that maintaining lumbar lordosis while bending forward could influence the cessation and onset of FRP as verified by previous literature .
Fatigue and overuse might be other demonstrating factors for the observed difference between the groups. Da Silva et al. compared the fatigue of back muscles in young and old people with and without chronic back pain. They concluded that fatigue occurrence was more in individuals with chronic back pain in comparison to asymptomatic persons in both young and elderly groups . FRP may result in reduced back muscular energy consumption and fatigue . It can be concluded that overuse of ES muscles and its subsequent fatigue are considerable injurious factors to lumbar region. It was previously confirmed that if the hyperactivity of the ES muscles was maintained for a long term, this adaptive muscle activity pattern could be problematic since as the superficial muscles stiffen the spine via sustained and augmented compression, a continuous stimulation of nociceptors in spinal structures may predispose and result in further injury .
Delayed onset of FRP during forward bending and earlier activity of ES muscles during return phase in yogis might be attributed to prolonged activity of ES muscles secondary to special learned pattern of bending in yogis. This might lead to overuse and subsequent fatigue in ES muscles in yogis in comparison to control group. When the lumbar curvature is close to the natural standing posture of the body, the ES bears a small load; otherwise, it bears an increasingly large load which might lead to musculoskeletal fatigue.
Moreover, it should be kept in mind that fatigue usually occurs secondary to repetitive or sustained movements. Thus, learned pattern of bending might have led to habitual overuse and subsequent fatigue following many hours of practice. However, we did not evaluate fatigue in our study and this theory requires prospective studies to compare fatigue indices among yogis and non-yogis.
Observed results might be related to other factors like creeping phenomenon and repetitive movement patterns. According to Solomonow et al., creeping phenomenon can delay the cessation of activity of lumbar ES muscles during forward bending with knees straight. Nevertheless, creep occurred following sustained lumbar full flexion for 10 minutes . Since our participants maintained full flexion for only 2 s, attributing the results to immediate creep phenomenon must be interpreted with caution. However, it should be noticed if creep phenomenon might have led to permanent changes of muscle length-tension relationship in yogis. Varying amounts of creep occur through cyclic movements such as warm up and practicing in a hot yoga room . Dickey et al. concluded that repetitive forward bending can affect FRP and delay the cessation of muscle activity during forward bending, but this effect was accomplished following 100 repetition of forward bending . Three repetitions of forward bending in our study is unlikely to have immediate effects on FRP as both groups had the same number of repetitions. Plausibly, since forward bending is performed repetitively in yoga practice, this appears to have an impact on FRP.
Solomonow et al. declared that lumbar flexion and extension are governed by complex neuro-muscular system. The mechanism for the silence of ES muscles during trunk flexion has been proposed to result from stimulation of stretch receptors in the posterior disco-ligamentous tissues during the flexed posture, acting to reflexogenically inhibit the ES activity .
To the best of our knowledge, it was the first study comparing FRP between yogis and non-yogis. However, our study was not free from limitations. First, our participants were women. Thus, our results could not be generalized to male yogis. Future studies can investigate the effect of sex in FRP. Second, we did not assess fatigue indices in our study to have a direct deduction for the probable contributing mechanisms. Also, we did not evaluate creep phenomenon in our study. Future studies with the same methodology are warranted to evaluate these factors. Moreover, longitudinal studies are warranted to see whether the observed differences in FRP between yogis and non-yogis could ultimately lead to injuries among yogis and if so, to modify the pattern of forward bending and returning in yoga.