A study from Norway shows that any training method that makes you stronger will also help you to exercise longer. The best way for an athlete to become stronger for a specific sport is to do strength training using the muscles in the same way that they are used in that sport. In this study, cross-country skiers did specific training training three times a week for eight weeks, using a cable pulley simulating the movements of double poling in cross-country skiing. The cable system forced them to use their muscles in the same way that they are used to push ski poles against the ground to propel them forward. This training program helped them to ski much faster.
The authors used scientific strength training methods. They asked the skiers to perform three sets of six repetitions at a workload of 85 percent of one repetition. The heaviest weight that you can lift once is called a one-repetition maximum. The heaviest weight that you can lift six times in a row is around 85 percent of your one-repetition maximum, and the heaviest weight that you can lift ten times in a row is about 78 percent of your one-repetition maximum. That means that a person who can lift 100 pounds once, can lift 85 pounds six times and 78 pounds ten times in a row.
This same principle applies to high jumping or any other movement. You jump up against gravity, so squatting with heavy weights will help you to jump higher. Lifting heavy weights over your head does not help you to jump higher. Training is specific and strengthening your muscles by using them in the same way that you use them in your sport will make you a better athlete.