If you answered to increase strength, that is a very good reason. Resistance training is the most effective way to increase strength. Strength improves performance1 and decreases injury risk.2
If you answered to increase lean muscle mass, that is also a good reason. Muscle mass is a pre-requisite for increasing strength in trained individuals. Muscle mass can act as padding in collision sports. More mass means more inertia which is good if you do not want to be moved by an opponent, but not so good when you want to start moving yourself. Once in motion, more mass means more momentum, which means more energy transferred into a ball/stick/racquet/bat/opponent, but also becomes harder to stop oneself. Theoretically, whether more mass helps or not really depends on the sport, action and body parts in question. Practically, though, more muscle is rarely a bad thing.3
The often forgotten —and arguably most important— reason to resistance train is skill acquisition and mastery. In other words to refine movement patterns, to optimize biomechanics, to improve movement efficiency and effectiveness, to maximize sport skills, and to learn to better co-ordinate. Appropriately loading a movement slows it down enough that the player can feel portions of the range of motion (ROM) that deviate from ideal technique that would otherwise be missed at game speed.
It is imperative that the players attempt to move the load as quickly as possible. This recruits more motor units and at a frequency that replicates unloaded conditions, so the efferent signal from the brain is the same as that for the sports movement they are trying to train. However, since the actual movement speed is slowed, so is the afferent feedback coming back to brain. Slow enough that it becomes much easier to sense and process. This leads to learning.
The movement patterns need to be very similar to technical sport skills and/or general athletic movement skills, with the resisted portion identical. Using squats as an example, the trunk should remain stable so that the force generated by the legs is transferred to the bar. In sport, rarely is the trunk held so rigidly, however in training it preserves intervertebral integrity under very heavy loading. It is only similar to the real-world. However the triple extension of the hips, knees and ankles, finishing in unison and with left-right symmetry is identical to a jump. Put a heavy barbell on and any deviations become more obvious to the player and coach. Fix it in the weight room, and vertical jump performance increases. Progress the loads and speed appropriately, and jump performance continues to improve.
On the other hand, oscillating only within in the mid-ROM, with the heels elevated just isn’t going to cut it. The players may get stronger, but any transfer to the court/field is blunted, because instead of improving a good movement pattern, it is creating (or reinforcing) a bad movement pattern. Due to familiarity from repeated exposure under stress (which are two stimuli that enhance learning) this bad movement pattern is likely to be selected in competition, resulting in a low jump height.
You want the resistance exercise to be identical to the portion of the sport/athletic skill you are trying to develop and improve. If it is too obscure, irrelevant, or dysfunctional, you and your players are not making good use of training time or energy. You may even be making them worse.
This is not the forum to explain the fine points of what exercise variations are good and bad for various sports and skills. We cover that throughout the curriculum.4
{Edit Sept 18. 2024 Here are a pair of articles discussing the bench press.}5
A profound understanding of functional anatomy6and sports specific skills helps a lot.
Identifying the appropriate resistance is important, as it affects the movement quality (neuromuscular adaptations) as well as the secondary physiological adaptations. Fortunately it can usually be found within a single training session, then progressed and periodized as usual. Too heavy and the “identicality” can break down. Too light, and the players can get away with anything without feeling it. Sticking in the 3-10 rep range through the annual plan will cover much of the force-velocity curve, and allow periodization to focus on mass, strength, power, and speed mesocycles. Doing too many reps means either the load is too light, or the quality (“identicality”) cannot be maintained as they fatigue.
—CG
1Magrini, Mitchel A., et al. “Can squat jump performance differentiate starters vs. nonstarters in division I female soccer players?.” The Journal of Strength & Conditioning Research 32.8 (2018): 2348-2355.
2Khayambashi, K., Ghoddosi, N., Straub, R. K., & Powers, C. M. (2016). Hip muscle strength predicts noncontact anterior cruciate ligament injury in male and female athletes: a prospective study. The American journal of sports medicine, 44(2), 355-361.
3Unlike the weight of excessive weight or equipment, muscle is functional i.e. it is the motor not the chassis or cargo. This reminds me of a colleagues’ work which would be good topic for a future insight.
4 https://goodmanspeed.com/curriculum/
5 https://goodmanspeed.com/technical/bench-press-for-speed-part-1/and https://goodmanspeed.com/technical/bench-press-for-speed-part-2/
6 https://goodmanspeed.com/cognitive/in-memory-of-joe-glenn-the-importance-of-functional-anatomy/