Off-season is your opportunity to get better. You would not want to mix things up during the ramp up to your competition. But your competition is over. Now is your opportunity to knock off the rust and improve upon possible weak link(s) that have built up over your season (race or races).
The stronger you are the faster you will potentially be. But let's clarify - you aren't a bodybuilder. Bodybuilders often equate the success of their workouts by how destroyed their muscles feel afterwards. Can you imagine? You are limping around stutter stepping & pulling yourself up the stairs via the railing. But you are happy - "Bro that workout was amazing!" But I am writing this assuming you are not a bodybuilding triathlete. Off-season or not, being 'this' sore will not help you be a better athlete. You want strength that will compliment your sport. Strength that will facilitate your force production: pushing harder into the ground, to be able to run faster; move your bike further faster; pull yourself further/faster in the water!
Because it is the off-season, being a little sore while your body is figuring out the moves is acceptable. Goal being, you get better at the skills of the movement & you find your "sweet spot" for the weight so that when you do go into your prep phase you can continue some of these moves to maintain strength and not be sore. Because soreness is the enemy of athletic performance.
There are many exercises to choose from. I am a big fan of simplification and hacking away at the unessential. If you love to resistance train - terrific (let's workout sometime)! But if you just want to improve your sport and work on making your body more resilient (less likely to get injured) - then find a handful of exercises you can practice - get good at those and see the benefits without taking up a second residence at the gymnasium. Every athlete will have exercises that they are better at; which could change my personal recommendations. But to keep this list short and generally applicable to most, I came up with the following list: Racked Split Squats, Pull-overs, Face-pulls, & rolling planks.
Racked Split Squats
I like these for tri-athletes because it works on leg strength, midsection bracing, core anti-rotation and some ankle stabilization.
I like these because it engages a similar movement to the "pull" of a swimming stroke. You can do this using a dumbbell or cable machine. From experience, watching people's form - I'd say for most - the cable machine is the best option. In this instance I will be highlighting the 1-arm pull over. Because I believe it will keep your movements more honest, I think it has a better carry over, and it has a slight anti-rotation component (but performing the 2-hand pull-over is acceptable provided your form is good).
I like these because they help most people with improving their posture. Posture is important and something all of us could improve upon.
I like these because they: improve your midsection strength, it is bringing your body through a version of rotation, and can inadvertently work a little of the upper body musculature.
Proper resistance selection
Your goal is to practice and get better at the movements. I the beginning there will be sort of a discovery process finding the correct resistance selection for you. The ideal weight is NOT so heavy your form breaks down (or) you have to "psych yourself up" to complete. It also is NOT so easy that you can discuss all the happenings of your day while you mindlessly go through the movements. You want to find the resistance that you have to take serious, but you can complete with crisp form. For you, it should feel "tough-ish", but completely manageable (you are in control).
Your target is 3 days of practice per week. The resistance is the same for the entire week. You practice is a/an: hard day (tough but you did it), easy day (yes that is it - go home and recover), and medium day. In that order. Take a day off between workouts. When you are comfortable with the movements and the hard day is no longer hard - go up in weight, but be reasonable. Warm-up however you warm up pre-race (maybe movement/mobility drills, body weight pause squats, mountain-climbers, light walk/jog). Practice all reps/sets of each exercise before moving on to the next. Rest as long as you need to perform the next set with good form. This could be 60 seconds, it could be 2 minutes.
Racked Split squat 5 reps each side x 4 sets
Pull-overs (single arm) 6 reps each side x 4 sets
Face-pulls 10 reps x 4 sets
Rolling planks 5 rotations each side x 4 sets
Racked Split squat 5 reps each side x 2 sets
Pull-overs (single arm) 6 reps each side x 2 sets
Face-pulls 10 reps x 2 sets
Rolling planks 5 rotations each side x 2 sets
Racked Split squat 5 reps each side x 3 sets
Pull-overs (single arm) 6 reps each side x 3 sets
Face-pulls 10 reps x 3 sets
Rolling planks 5 rotations each side x 3 sets
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