Week 11 - Flexibility BW PDF

Acute adaptations: Flexibility Learning outcomes u Define muscle and joint stiffness and thixotropy u Understand how muscle thixotropy relates to muscle stiffness u Understand the acute effects of muscle stretch u Understand the effects of long term stretching programs Muscle stiffnes...

Acute adaptations: Flexibility Learning outcomes u Define muscle and joint stiffness and thixotropy u Understand how muscle thixotropy relates to muscle stiffness u Understand the acute effects of muscle stretch u Understand the effects of long term stretching programs Muscle stiffness Muscle stiffness Stiffness: the resistance of an object or a system to a change in length u Muscle is resistant to lengthening forces, particularly over small displacements (short range stiffness) u Muscle stiffness plays an important role in maintaining limb and upright body posture u This resistance is greatly reduced with large changes in muscle ss length ne t s t iff u The resistance also decreases with J o in repeated lengthening, and returns with time at rest (see next slide) Muscle stiffness Because joint position was modified and torque recorded, this is an example of joint stiffness Stretching temporarily changes joint & muscle stiffness u Joint/muscle stiffness initially reduced by stretching u Increases rapidly as the time between successive stretches is increased (see figure) u Especially short range stiffness Muscle stiffness: thixotropy u Muscle thixotropy: the dependence of muscle stiffness on the history of muscle length change u Cross-bridge causes: u Stable bonds are broken during stretching or activity, decreasing stiffness u Inactivity increases the number of stable actin/myosin bonds, increasing stiffness u Muscle activity increases short range stiffness by ~100x, due to increased bonds Muscle stiffness: thixotropy u Muscle thixotropy: the dependence of muscle stiffness on the history of muscle length change u Non-cross bridge causes: u Primarily caused by changes in the elasticity of titin proteins within the sarcomere u Titin stiffness decreases when the sarcomere is lengthened and returns during periods of inactivity Summary u Muscle stiffness (especially short range stiffness) is important for postural regulation u Muscle stiffness is dependent on the history of muscle length change (thixotropy) u Thixotropy caused by changes in actin/myosin binding and titin elasticity Effects of stretching Stretching Stretching is contentious! Does stretching increase muscle length? Stress relaxation: u Yes. repeated stretches decreases resistive Creep: constant force muscle force application elongates the muscle/tendon unit Does stretching Before stretching decrease muscle stiffness? After 30 sessions u Yes.  Does stretching change nervous system function? u Yes. At least during the stretch. Decreases in H-reflex (H) and Tendon reflex (T) reflect a reduction in the sensitivity of the stretch reflex pathway during stretching. The lack of change in responses to cortical stimulation (MEP) suggests that alpha motor neurons are not inhibited by stretching. The lack of return of Tendon reflexes to baseline after stretching suggests that stretching decreases tendon stiffness. Guissard & Duchateau (2006) Stretching techniques u Passive stretching u Static stretching u Ballistic stretching u Active stretching u Dynamic ‘stretching’ u Proprioceptive Neuromuscular Facilitation (PNF) stretching u Contract-relax u Anatagonist-contract u Contract-relax, Antagonist- contract Stretching techniques u Passive stretching u Static stretching u Ballistic stretching u Active stretching u Dynamic ‘stretching’ u Proprioceptive Neuromuscular Facilitation (PNF) stretching u Contract-relax u Anatagonist-contract u Contract-relax, Antagonist- contract Stretching techniques u Passive stretching u Static stretching u Ballistic stretching u Active stretching u Dynamic ‘stretching’ u Proprioceptive Neuromuscular Facilitation (PNF) stretching u Contract-relax u Anatagonist-contract u Contract-relax, Antagonist- contract Contract-relax technique u Autogenic inhibition u Tension in muscle/tendon induces GTO activity u Inhibits contracting muscle for ~5 s u Reduces tonic activity within the muscle that would otherwise resist lengthening Contract-relax technique u Muscle thixotropy u Contraction disrupts stable bonds u Transition from active to inactive state may further increase the number of myosin heads placed in an OFF state (incapable of bonding) u Reduces muscle stiffness through thixotropic action u Allows muscle to be lengthened further by same external force  Antagonist-contract technique u Requires partner or strap assistance u Reciprocal inhibition theory u Antagonist contraction invokes reciprocal inhibition of agonist (to be stretched) muscle u Stretched muscle is actively inhibited, reducing unwanted activity Evidence for altered MN excitability is difficult to obtain, so it is not clear if this is true Summary u All types of stretching can increase muscle length and joint range of motion u Muscle thixotropy and changes in tendon elasticity are likely to be responsible for acute stretching effects u PNF stretching may also invoke active inhibitory process to reduce resistance to stretch Long term effects of stretching Muscle & Nerve, 2004 The study u 30 static stretching sessions u Outcome measures u 5/week for 6 weeks u Dorsiflexion range of motion u 20 x 30 sec plantarflexor stretches u Passive dorsiflexion stiffness per session u Measures of contractile function u Plantarflexors of right leg stretched (e.g. MVC, twitch response) u Measures of reflex sensitivity (H- u Plantarflexors of left leg not reflex, T-reflex) stretched Changes in joint range of motion u Dorsiflexion range of motion increased ~33% in trained leg u 0% in control leg u 74% of flexibility increase retained after 30 days without stretching Changes in joint stiffness u Dorsiflexion stiffness decreased Before stretching across entire joint range of motion u The extent of stiffness decrease was related to the amount of stretching performed After 30 u e.g. more stretching, greater sessions stiffness decrease u 78% of stiffness decrease was retained after 30 days without stretching (see next slide) u H-reflex only decreased after 20-30 stretching sessions – returned to baseline with no stretching u T-reflex decreased throughout stretching training – did not return to baseline after 30 days without stretching Summary Regular static stretching causes: u Progressive increase in flexibility u Progressive decrease in joint stiffness u Depression of stretch reflex expression that is greatest during stretching program Increases in flexibility are initially dominated by changes in muscle mechanics and later also involve changes in reflex sensitivity