Positive medicine - stretching and warming up

Post-run stretching at Hackney Marshes parkrun

I've run 5-15km a week for the last 25 years (somewhere approaching 7000km) and continue to do so on the medical course.  It's a part of life now - I neither love it (apart from those times when I'm super fit) nor loathe it (aside from the repetitiousness!).  Up and down Donegal mountains on the Glover Highlander, searching new streets when growing into Maidstone or settling into Bavaria, Lancashire, Sheffield, Battersea and Derby; sweating buckets against all advice in Ghana in the midday sun (ouch!), and joining running clubs in Kent, Clapham, Nairobi, Derby and Mansfield, it's been consistent at least.  I've heard a load of received wisdom about stretching over the years, precious little of which I ever imagine has been researched by the people who have told me.  And as a medic, perhaps I should now be a little better informed...

The research consensus on stretching and warming up seems to be (1) DO warm up before exercise but (2) DO NOT stretch before exercise, but instead stretch (particularly the hips) at other times.  And - stretches, like so many things, are activity specific.  May be a surprise - so below details how stretches increase range of movement, performance, and affect injuries.  Here's the evidence...

Stretching

What is the physiological effect of stretching (i.e. what's happening in your muscles)?

Acutely:

1. Analgesia - increasing your tolerance of stretch, allowing you to tolerate a greater range of movement (ROM) than you had previously  LINK (2012) ; LINK (2006), p.5
2. Stress relaxation of muscle - reducing the passive tension in a muscle.  This tends to last < 1 hour LINK (2006), p.6
3. Elastic extension - as muscle sections (sarcomeres) slide over one another (and to a lesser extent, tendons and other connective tissues stretch) - muscle is the least stiff section so extends the most LINK (2009).  
4. Plastic deformation - short sarcomeres may be torn; connective tissue at the musculo-tendinous junction may be torn, abnormal crosslinks may be torn.  At the end of the muscle's ROM, tendinous collagen may also be torn.  May occur, can be pathological, therefore stop when it hurts. LINK (2005), p.51, 65

Chronically (adaptively):

1. Stretching stimulates protein synthesis in muscle - more sarcomeres are synthesised and added to the end of the myofibril LINK (2005), p.53
2. Ruptured collagen fibres are repaired by synthesis of further collagen which reunites the fibres, adding 'links into the chain' LINK (2005), p.55 

Effect of stretching on Range of Movement (ROM) LINK (2012) ;  LINK (2014) ;

• Static stretching increases ROM maximally if load is applied between 10 and 30 seconds LINK (2012)
• 2-4 repetitions give maximal stretch (subsequent repetitions do not increase range) LINK (2012)
• ROM increase is greatest if the muscle is contracted prior to stretch
• Foam rolling appears to have a far greater influence on ROM than stretching LINK (2014), slide 28
• Acutely, the ROM increase lasts less than an hour, whether the subject is exercising or not LINK (2009)

Effect of stretching on injuries

• Stretching does not reduce the occurrence of injury LINK (2012) ;  LINK (2014), slide 8
• Static and dynamic stretching have equivalent effects; active dynamic stretching is fine (full range), ballistic stretching (full range, bouncing intensely at the ends) increases risk of damage.
• Stability-mobility paradox: highest injury rates are in people at the top and bottom 20% of the flexibility distribution curve. LINK (2014), slide 19

Effect of stretching on power

• Pre-performance static stretching reduces power by 4-30%, LINK (2006), p.8 (this was at a duration of 120 secs+ per muscle group), lower stretching of 30s+ shows little compromise- therefore stretching before exercise is not a good idea for performance.  LINK (2009)
• Post-performance (or inter-performance) appears to elicit long term performance benefits LINK (2009)
• However, increased flexibility in general does not increase running economy, so these benefits of stretching may be activity-specific LINK (2014), slide 19

Optimal level of stretch

• Greater flexibility in the hips is adaptive for running (faster runners have this) LINK (2014)
• Less flexibility than standard in the lower leg joints (i.e. knees and ankles) is adaptive for running LINK (2014)
• However, years at sitting at desks and wearing shoes may exaggerate our inflexibility, and hence it's a probably a good idea to stretch out when you're running, particularly as you age. 

Warm-up

What is the physiological effect of a warm-up?

• temperature effects on the muscles, e.g. increased nerve conduction rate, increased anaerobic energy provision LINK (2003)
• neuromuscular effects, e.g. increased intramuscular Ca2+ LINK (2014)
• psychological effects, e.g. increased exercise enjoyment and motivation LINK (2014)

Performance effects of warm-up

• Stiffness is reduced by warmup LINK (2014), slide 21
• Reduces injuries: the maximum force of energy absorbed before failure is increased if muscle groups are warmed up before training LINK (2014)

Summary guidance on warm-ups and stretching

1. You should warm-up with low intensity exercise before high-intensity exercise
2. You should stretch muscle groups for 2-4 x 10-30s after or between exercise sessions - this helps with muscle synthesis and is likely to yield performance benefits
3. You should not seek super-flexibility; a moderate range of flexibility is optimal for most activities.  For peak performance running (if that's your only sport), flexible hips and slightly stiffer-than-average ankles is probably the best combination

Runner's World offers us an (incompletely evidenced!) guide to stretches