Hamstrings injuries (introduction)
 
Hamstring muscle strain injuries are common in sports that require maximum effort sprinting and acceleration. It has been reported that hamstrings injuries alone account for 16-29% of all injuries reported in soccer, Australian Rules football and rugby. Furthermore, a significant amount of playing and practice time is lost after hamstring injury (18 days on average) and the risk of re-injury is very high.
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Every muscle has a length in which is able to do its maximum strength. The length-tension relationship plays a very important role in the function of skeletal muscle. To create a length-tension curve force levels are plotted against each muscle length. 
Related to hamstrings the curve is torque (force*distance) for every angle (as bigger is the angle more lengthened is the muscle).
From this information, can be determined three regions, the ascending limb, the optimum length and the descending limb.
It has been known for some time that the descending limb is a region of instability and because of it muscle strain injuries are thought to occur when activated muscles are lengthened to grater than optimal lengths.
 
The hamstring muscles are actively lengthened during hip flexion and knee extension, which occur
simultaneously during the late swing phase in running. 
Many believe that athletes who produce peak torque at shorter muscle lengths are more likely to get injured. A shorter optimum length would mean that more of the muscles operating range would be on the descending limb of the length-tension curve. Brockett et al. explored this idea by measuring optimum lengths in athletes who have had previous hamstring injuries in one leg versus their uninjured leg. The mean optimum angle was 12.7°shorter for the injured leg, although strength values (hamstring/ quadriceps ratio) were similar. Based on these findings, are two main suggestions:
 
  • Optimum length may be a greater risk factor for muscle strain injuries than strength ratios. 
  • Hamstring injuries can be reduced if this optimum length can be increased through training 
Nowadays only exists one demonstrated method to shift the curve to the lengthened part and reduce te instable descending limb.
 
If you wanna know this method, more information will be explained in the next blog.
 
Aaron Martínez  MSC, NCSA-CSCS
 
 
Bibliography
 
  • Brockett C, Morgan D, Proske U. Human hamstring muscles adapt to eccentric exercise by changing optimum length. Medicine and Science in Sports and Exercise(2001); 33, 783–790. 
  • Brockett C, Morgan D, Proske U. Predicting hamstring strain injury in elite athletes. Med Sci Sports Exerc (2004); 36 (3): 379-87.
  • Brughelli M, Cronin J. Altering the length-tension relationship with eccentric exercise. Sports Med (2007); 37 (9): 807-826.
  • Brughelli M, Nosaka K, Cronin J. Application of eccentric exercise on an Australian rules football player with recurrent hamstring injuries. Physical Therapy in Sport xxx (2009); 1–6.
  • Friden J,  Lieber R. Eccentric exercise-induced injuries to contractile and cytoskeletal muscle fibre components. Acta Physiologica Scandinavica(2001); 171(3),321–326.
  • Gordon A M, Huxley A F, Julian  F J. The variation in isometric tension with sarcomere length in vertebrate muscle fibres. Journal of Physiology (1966); 184, 170– 192. 
  • Proske U, Morgan D, Brockett C. Identifying athletes at risk of hamstring strains and how to protect them. Clin Exper Pharmacol Physiol (2004); 31 (8): 546-50.
  • Proske U, Morgan D. Muscle damage from eccentric exercise: mechanism, mechanical signs, adaptation and clinical applications. J Physiol (2001); 537 (2): 333-45.

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