Courtesy of Calkid
2) Frequency Potentially the most controversial, so I'll be spending a lot of time on it.
The various growth factors initiated by training all peak at around 24 hours post-workout, and than fall back to baseline by 48 hours. (2, 3, 4, and especially 6, 7) Typically, programs will sacrifice training frequency for the ability to add volume. This is counterproductive if your goal is to have bigger muscles. Given the average split of once/week, this means one will spend two days growing and five days maintaining muscle size without adding to it. This has been confirmed in the lab. One study compared the effects of a volume of weight training performed all on one day of the week to the same volume spread across three days of the week. The thrice-per-week group saw greater muscle gains as well as strength gains over 40% greater than the once-per-week group.(5)
This can also seem counterintuitive, as muscle soreness and strength often do not recover after a mere two days. In actuality, neither of those factors (soreness or voluntary strength) is related to muscle growth.
The ability to recover one's strength is directly related to muscle failure. Training to failure directly inhibits voluntary strength. Basically, training to failure fries your nerves (not the technical term

) and prevents them from being able to contract the muscles for long periods of time. So when one trains to failure and then waits until strength is recovered to train a muscle again, oftentimes the muscle has long recovered and is waiting for the nervous system to catch up.
This means that sometimes, with HST, you will be training through soreness. This is totally okay! Soreness is not harmful, and people generally find that training a sore muscle will cause the soreness to stop.
References:
1) Warren GL, Hayes DA, Lowe DA, Armstrong RB. Mechanical factors in the initiation of eccentric contraction-induced injury in rat soleus muscle. J Physiol. 1993 May;464:457-75
2) Nosaka K, Clarkson P.M. Muscle damage following repeated bouts of high force eccentric exercise. Med. Sci. Sports Exrc., 27(9):1263-1269,1995
3) Smith LL., Fuylmer MG., Holbert D., McCammon MR., Houmard JA., Frazer DD., Nsien E., Isreal RG. The impact of repeated bout of eccentric exercise on muscular strength, muscle soreness and creatine kinase. Br J Sp Med 28(4):267-271, 1994
4) T.C. Chen, Taipei Physical Education College, and S.S. Hsieh, FACSM,. The effects of a seven-day repeated eccentric training on recovery from muscle damage. Med. Sci. Sports Exrc. 31(5 Supp) pp. S71, 1999
5) McLester JR., Bishop P., & Guilliams M. Comparison of 1 and 3 day per week of equal volume resistance training in experienced subjects. Med. Sci. Sports Exrc. 31(5 Supp) pp.S117 1999
6)MacDougall JD, Gibala MJ, Tarnopolsky MA, MacDonald JR, Interisano SA, Yarasheski KE. The time course for elevated muscle protein synthesis following heavy resistance exercise.
Can J Appl Physiol. 1995 Dec;20(4):480-6.
7)Phillips, S. M., K. D. Tipton, A. Aarsland, S. E. Wolf, and R. R. Wolfe. Mixed muscle protein synthesis and breakdown after resistance exercise in humans. Am. J. Physiol. 273 (Endocrinol. Metab. 36): E99-E107, 1997