The picketers alleged that a truck delivering Adidas sportswear to the warehouse hit the strikers as it collided with a GMB promotional trailer. The union said it would press for charges against the lorry driver.
Hundreds of staff were striking over pay and working conditions at the sports chain, in the third day of strikes over the past two weeks.
The GMB union said that many of the strikers earn little more than the national minimum wage of£5.35 an hour, before bonuses. Union negotiators are seeking a higher basic rate of pay for staff and are calling for access to a bonus scheme.
JJB has 430 stores in the UK and executive director David Whelan also owns Wigan Athletic Football Club.
The strikers have organised a 'yellow card' protest by Wigan Football Club fans on Saturday, when the team plays Charlton at the JJB Stadium. The union will also hold events at various JJB outlets in cities across the UK this weekend.
GMB officials have complained that employment agencies illegally supplied staff during last week's strike.
Regional secretary Paul McCarthy said: 'GMB members know full well that JJB is collaborating with a range of employment agencies to illegally break the strike. GMB intends to mount a vigorous campaign to expose these illegal plans.'
This includes installing cameras on the picket lines to collect evidence that is admissible in court, if GMB decide to prosecute the employment agencies.
Activist and singer Billy Bragg will play at a benefit concert for the Wigan strikers in December.
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