ABSTRACT PURPOSE:The slump test is used to assess adverse neural tension in patients with low back or hamstring injuries. Hamstring stretching with the addition of neural tension causes acute strength loss (McHugh et al 2013). The purpose of this study was to assess whether rugby players presenting with a positive neural tension sign had associated hamstring weakness. METHODS:The slump test and hamstring strength were assessed in 30 rugby players in preseason (age 26±5 yr, height 183±8 cm, weight 93±12 kg). The slump test was performed in sitting using cervical and thoracic flexion, and ankle dorsiflexion to increase neural tension during active knee extension. A positive sign was defined as pain or discomfort during knee extension in the slump position with or without dorsiflexion. Isometric hamstring strength was assessed in sitting, with the thigh flexed to 45º and the trunk flexed to 90º relative to the horizontal (Biodex System 2). Two contractions were performed at 100º, 80º, 60º and 40º knee flexion. Torque was corrected for limb mass and passive muscle tension. History of prior hamstring strain was documented. The effect of adverse neural tension and previous hamstring strain on angle specific hamstring strength was assessed by angle by group anova. RESULTS: Ten of 39 players had a positive neural tension sign (6 bilateral = 16 positive neural tension signs). Twenty players reported a previous hamstring strain (6 bilateral = 26 previous strains). Six of 26 previously injured hamstrings had a positive neural tension sign vs. 10 of the 42 uninjured hamstrings (P=0.99). Hamstring strength at longer muscle lengths was significantly lower in limbs with a positive neural tension sign versus the limbs with negative tests (angle by group P=0.015); 16% lower at 40º (long), 7% lower at 60º, 4% lower at 80º, 2% higher at 100º (short). Previous hamstring strain did not affect hamstring strength (angle by group P=0.61; group effect P=0.91). CONCLUSIONS: These rugby players had a high prevalence of adverse neural tension (26%) and previous hamstring strain injury (53%). While previous injury did not impact hamstring strength, adverse neural tension caused marked hamstring weakness in the lengthened state. These results highlight the importance of identifying adverse neural tension in athletes at a high risk of hamstring strain injury.

(email mchugh@nismat.org)

INTRODUCTION The slump test is used to assess adverse neural tension and has been shown to be positive in rugby players with recent hamstring strains (Turl and George, 1998). Kornberg and Lew (1989) demonstrated superior outcomes for rehabilitation of grade 1 hamstring strains when adverse neural tension was addressed and treated. More recently hamstring stretching in the slump position was shown to result in marked post-stretching strength loss (McHugh et al 2013). These results indicate that adverse neural tension can result in hamstring weakness. Therefore, the purpose of this study was to assess whether rugby players presenting with a positive neural tension sign have associated hamstring weakness. The hypothesis was that adverse neural tension results in strength loss when muscle contractions are performed with the muscle in a lengthened position.

References

1.  Kornberg, C., & Lew, P. (1989). The effect of stretching neural structures on grade one hamstring injuries. Journal of Orthopaedic & Sports Physical Therapy, 10(12), 481-487.

2.  Brockett, C. L., Morgan, D. L., & Proske, U. (2004). Predicting hamstring strain injury in elite athletes. Medicine & Science in Sports & Exercise, 36(3), 379-387.

3.  McHugh MP, Tallent J, Johnson CD. The role of neural tension in stretch-induced strength loss. J Strength Cond Res. 2013 May;27(5):1327-32.

4.  Turl, S. E., & George, K. P. (1998). Adverse neural tension: a factor in repetitive hamstring strain?. Journal of Orthopaedic & Sports Physical Therapy, 27(1), 16-21.

METHODS cont. Strength Testing Procedures For isometric strength tests, subjects were seated with the thigh flexed 45° above the horizontal, and the seat back at 90° to the horizontal (Biodex System 2) (Fig. 2). Subjects performed 2 maximal voluntary contractions to determine strength throughout ROM at 100° (short), 80°, 60° and 40° (long) knee flexion. Baseline passive torque due to limb mass and passive muscle tension was subtracted from the measured torque to provide a measure of contractile torque production. The average of the 2 contractions at each angle is reported. When the isometric test was completed on the first test leg, the protocol was repeated on the contralateral leg with the same technique. History of prior hamstring strain was documented and passive straight leg raise (SLR) ROM was measured bilaterally.

Statistical Analysis Effect of neural tension on hamstring strength was assessed using Angle (100°, 80°, 60°, 40°) x Slump Test (positive/negative) mixed model ANOVA. Effect of previous hamstring strain on hamstring strength was assessed using Angle (100°, 80°, 60°, 40°) x Hamstring Strain (yes/no) mixed model ANOVA. Prevalence of positive neural tension sign in previously injured hamstrings was assessed using Fisher’s Exact Test.

Figure 2: Example of athlete during isometric testing at 60° lknee fexion.

The Effect of Adverse Neural Tension on Hamstring Strength in Rugby Players

Michael B. Fox1, Dominick Mazza1, Susan Y. Kwiecien2, Malachy P. McHugh, FACSM2 1Sports Therapy and Rehabilitation, New York, NY; 2Nicholas Institute of Sports Medicine and Athletic Trauma, Lenox Hill Hospital, New York, NY

METHODS Experimental Protocol The slump test and hamstring strength were assessed in 39 rugby players in preseason (age 26±4 yr, height 183±8 cm, weight 94±13 kg). Slump Test Procedures The slump test was performed in sitting with 4 different levels of neural tension (Fig.1): 1. Seated knee extension in spinal neutral position with ankle plantarflexed. 2. Seated knee extension in spinal neutral position with ankle dorsiflexed. 3. Seated knee extension in the slump position with ankle plantarflexed. 4. Seated knee extension in the slump position with ankle dorsiflexed. A positive test was defined as pain or discomfort during knee extension in the slump position with or without dorsiflexion.

Figure 1: Example of the 4 levels of neural tension during the Slump Test.

80°

60°

40°

100° DISCUSSION A positive neural tension sign in otherwise asymptomatic rugby players is not an innocuous finding. These athletes have associated hamstring weakness that is most apparent with the hamstrings in a stretched position (Fig. 3&4).

Brockett et al (2004) demonstrated that athletes with a history of hamstring injury elicit a shorter optimum length for active tension. Specifically, the angle of peak torque occurred at 12º greater knee flexion on the previously injured versus uninjured side. As a result previous hamstring strain was associated with a leftward shift in the length-tension curve. However, the present data do not support this observation as previous injury was not associated with leftward shift in length-tension curve (Fig. 5).

Conclusion These rugby players had a high prevalence of adverse neural tension (26%) and previous hamstring strain injury (53%). While previous injury did not impact hamstring strength, adverse neural tension caused marked hamstring weakness in the lengthened state. Assessment of adverse neural tension may be an important component of preseason screening of rugby players.

Practical Relevance/Future Directions The present results raise the following questions: 1.  Is risk of hamstring strain recurrence related to adverse neural tension? 2.  Can adverse neural tension be resolved with flossing exercises? 3.  Does correction of adverse neural tension improve hamstring strength? 4.  Is weakness in a lengthened state a risk factor for a hamstring strain?

RESULTS Ten of 39 players had a positive neural tension sign (6 bilateral = 16 positive neural tension signs). Twenty players reported a previous hamstring strain (6 bilateral = 26 previous strains). Six of 26 previously injured hamstrings had a positive neural tension sign vs. 10 of the 42 uninjured hamstrings (P=0.99). Hamstring strength at longer muscle lengths was significantly lower in limbs with a positive neural tension sign versus the limbs with negative tests (angle by group P=0.015; Fig.3); 16% lower at 40º (long), 7% lower at 60º, 4% lower at 80º, 2% higher at 100º (short). For limbs with a positive neural tension sign knee flexion torque at 40º was 79% of peak torque compared with 90% for limbs with negative tests (Fig.4).

RESULTS contd. Hamstring flexibility as assessed by SLR tended to be reduced in legs with a positive slump test (81±5°) versus 85±7°, P=0.09). Previous hamstring strain did not affect hamstring strength (angle by group P=0.61; group effect P=0.91). These analyses indicate that there was no residual weakness associated with previous injury (group effect P=0.91) and that there was no evidence of selective weakness at longer muscle lengths (angle by group P=0.61).

Figure 3: Effect of positive neural tension sign on absolute knee flexion torque. Angle by Group P=0.015.

Figure 4: Effect of positive neural tension on relative knee flexion torque (% peak torque). Angle by Group P=0.018.

Fig. 5: Effect of previous hamstring strain on knee flexion torque at different muscle lengths (angle by group P=0.61; group effect P=0.91).

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