Why Muscle Slack Can Ruin Your Sports Performance

Training the ability to tension and tauten muscles can increase athletic performance.

As a strength coach, I learned early about the importance of producing force. Jumping, sprinting, cutting, hitting—these all depend on the neuromuscular system's ability to produce a high amount of force.

Not only this, but we must produce this force extremely fast to be successful.

Getting bigger and stronger in the weight room helps to increase our force-producing capacity. Training explosively with sprints, jumps and med ball throws helps to increase our capacity to produce this force rapidly.

But there's one piece many athletes are missing: Training to take up muscle slack.

What is Muscle Slack?

As Frans Bosch says, "Muscles are not located in the body 'all ready for action,'" like most of us assume. Instead, muscles hang like slack ropes when relaxed. Before they can apply force, they must be tensioned and tautened.

In many sporting actions, we only have  about 300 milliseconds to produce force. Going from slack to tense can take about 100 ms.

Training the ability to tension and tauten muscles can increase performance because less time will be wasted in this "delay phase" and more time will be spent actually producing force.

Basketball Player Jumping

Two Conventional Ways to Reduce Slack

There are two traditional methods of reducing slack, both with problems:

1. Use a countermovement

Example: Before jumping, an athlete can first dip down with their knees and hips, then jump. This stretches the muscle-tendon unit and therefore effectively reduces slack.

The problem: Many times in sport, time constraints do not allow for a countermovement. By training mainly with countermovements, an athlete gets "used to" using them and cannot get rid of slack without them. This can decrease sports performance as muscle slack must rapidly be taken up, many times without the assistance of a countermovement.

2. Use an external load

Example: By applying a load to the back in a Squat, for example, the muscle-tendon unit is stretched and put under tension, therefore reducing slack.

The problem: Many times in sport, an external load cannot be used. As with countermovements, training mainly with external loads can decrease performance because the athlete is never forced to get rid of muscle slack without the external load's assistance.

A Better Way to Reduce Muscle Slack: Pre-tensioning through Co-contractions

Example: If the calf muscles and tibialis anterior (agonist/antagonist muscles) contract simultaneously, they will stretch the muscle-tendon unit, therefore reducing the effects of slack.

Practical example: Consecutive Hurdle Hops under time pressure. As an athlete is forced to make quick ground contacts between hurdles, they are training their ability to pre-tension their muscles (through co-contractions) before producing force into the ground.

In the world of sports performance, time matters. Rapidly going from slack to tense is an ability that must be trained if athletes are to reach their potential.


Van Hooren, B. & Bosch, F. "Influence of muscle slack on high-intensity sport performance: A review." (2016). Strength and Conditioning Journal, 38(5), 75-87.

Bosch, F. "Strength Training and Coordination: An Integrative Approach." (2015). 2010 Publishers (Rotterdam, Netherlands).