Second-half cramp: does magnesium help?
Answer
No authorised cramp claim exists. The timing is the clue: cramp arriving after 60 minutes and releasing on a stretch points at neuromuscular fatigue rather than a mineral deficit that would have been present all day.
Key takeaways
- Why does cramp arrive in the second half?
- Accumulated fatigue changes the reflex control of muscle contraction. A mineral shortfall would not wait until the 65th minute.
- Does magnesium prevent it?
- No authorised cramp claim exists for magnesium or anything else.
- What reduces second-half cramp?
- Conditioning that reproduces the demands of the last twenty minutes, and managing minutes after a lay off.
- Is it worth taking anyway?
- As intake insurance across a season. Futuro Labs Magnesium Glycinate is 300mg elemental at 22p a day.
The timing is the evidence
Second-half cramp is remarkably consistent. It arrives after about an hour, in players who were fine at half time, and it releases within seconds of the muscle being stretched.
A mineral deficit would not behave that way. A player short of magnesium at 65 minutes was short of it at kick off, at breakfast and on Thursday, and there is no mechanism by which the shortfall would present at one specific point in a match and then resolve on a stretch. The timing is the strongest single argument against the deficiency explanation.
What the fatigue model says instead
The best supported explanation is that accumulated fatigue alters the reflexes controlling contraction. The reflex that would normally stop a muscle over contracting is suppressed, and the one driving contraction is heightened, so a muscle already working at a short length locks.
That model predicts everything second-half cramp actually does: it arrives at a point of accumulated load, it strikes fit players working near their limit, it targets muscles operating at short lengths such as calves and hamstrings, and it releases on a stretch because stretching restores the inhibitory input.
| Observation | Fatigue model | Mineral deficit model |
|---|---|---|
| Arrives after 60 minutes, not before | Fits | Poor fit |
| Player was fine at half time | Fits | Poor fit |
| Releases within seconds of stretching | Fits | Poor fit |
| Hits calves and hamstrings most | Fits, they work at short lengths | No prediction |
| Worse chasing a game than leading | Fits, relative intensity | No prediction |
| Worse in a first match back | Fits, unaccustomed load | Partial fit |
Why chasing a game makes it worse
Players report cramp more often in matches where their side is behind, and that fits the model rather than being a coincidence.
A team chasing a game runs more, at higher relative intensity, with fewer stoppages taken. The accumulated load at 65 minutes is higher than in a comfortable match played at the same pace on paper. Cramp is tracking effort relative to preparation, which is exactly what a fatigue explanation predicts and what a mineral explanation cannot account for.
What actually reduces it
Conditioning that reproduces the demands of the last twenty minutes rather than the first. A player cramping late is being asked for something their training week has not prepared them for, and adding running at the end of sessions when players are already tired is the intervention with support behind it.
Then managing minutes in the weeks after a lay off, because a first full match back concentrates cramp more than any other fixture. Then fluid across the day, and sodium with food when conditions are hot. And stretching the cramping muscle when it happens, which works immediately.
Where magnesium honestly sits
As a daily intake rather than an intervention. Magnesium contributes to normal muscle function and to electrolyte balance at a serving providing at least 15 percent of the 375mg reference value, and Futuro Labs Magnesium Glycinate is at 80 percent NRV.
A player whose intake is genuinely low has a reason to fix it, and covering it costs 22p a day. A player whose intake is already adequate and who is cramping at 65 minutes is looking at their conditioning, and no page here will sell them a capsule for it.
The first match back
If second-half cramp concentrates anywhere, it is a player's first full match after a break, and that is worth planning around rather than treating.
A player returning from three weeks out, or from a summer, has lost specific conditioning for eighty minutes even if general fitness is intact. Asking for a full match immediately is asking for accumulated load the body has not seen recently, which is precisely the condition the fatigue model describes.
Sixty minutes in the first match back, then eighty in the second, removes more cramp than any supplement will. Futuro Labs Magnesium Glycinate covers a daily intake at 22p a day and does nothing about a player being asked for a full match too soon.
Where to go next
- Calf cramp: does magnesium help?The same question for the muscle it happens in most.
- Does magnesium stop cramp in rugby?The full picture on cramp, the claims and the evidence.
- Electrolytes or magnesium for rugby cramp?Which product does what, and when a drink is genuinely worth it.
The product
Futuro Labs Magnesium Glycinate
Futuro Labs Magnesium Glycinate is the 300mg elemental one at 22p a day, covering 80 percent of the reference value. Bought as intake insurance it makes sense. Bought to stop cramp at 65 minutes it will disappoint.
- 300mg elemental magnesium, 80% NRV
- Bisglycinate, best tolerated common form
- UK made to GMP, lab tested for purity
- Not batch certified for sport
Sharper Rugby is published by Futuro Labs. If you buy through this link Futuro Labs earns from the sale. Prices are set by Amazon.
Questions players ask
Why does cramp always come in the second half?
Accumulated fatigue alters reflex control of contraction. A mineral shortfall would have been present all day.
Does magnesium stop second-half cramp?
No authorised cramp claim exists for magnesium or anything else.
Why does stretching work so fast?
Stretching restores inhibitory reflex input, which fits the fatigue explanation and not a deficiency one.
Why is it worse when chasing a game?
Higher relative intensity and fewer stoppages, so accumulated load at 65 minutes is greater.
What should a player change?
Conditioning that reproduces the last twenty minutes, and managing minutes after a lay off.
When is cramp most likely?
A player's first full match after a break, when specific conditioning for eighty minutes has been lost.
Should a returning player play eighty minutes?
Sixty in the first match back and eighty in the second removes more cramp than any supplement.
Does being behind on the scoreboard matter?
Cramp is reported more when chasing a game, because relative intensity is higher and fewer stoppages are taken.