The S&P/ASX 200 (Index: ^AJXO) (ASX: XJO) has finished the week strongly to close up around 0.7% for the day and 4.5% for the week. Some companies have demolished those returns though, with the four below having an especially strong day.

Let’s look at what might be behind some of today’s price action.

Bellamy’s Australia Ltd (ASX: BAL) shares are up 4.4% to $10.48 despite the company releasing no news to the market. Shares have been on a wild ride recently as investors guess the potential impact of new taxes introduced by the Chinese state on imported goods. Bellamy’s is likely to continue growing strongly into the future, although it’s important investors remain aware of the competitive risks.

Sirtex Medical Limited (ASX: SRX) shares jumped 3.3% today as investors pile back into a company that had been sold off after it posted weaker-than-expected half-year results in February. However, markets are forward looking and the company continues to grow sales strongly, with several clinical trials progressing that may support future growth. This business is only for the more risk tolerant investors, but I would not be surprised to see shares finish 2016 far higher than today’s level of $31.

Medical Developments International Ltd (ASX: MVP) shares are up 4% to $5.70 today and up 163% over the past year as the company’s global growth prospects continue to excite investors. The company manufactures and sells the Penthrox emergency pain killer drug commonly used by emergency teams or medical professionals to treat accident victims. Given it is eyeing more expansion in the US and Europe it looks a business to watch.

Shine Corporate Ltd (ASX: SHJ) shares closed up 6% to 96.5 cents as the law firm rebounds from an approximately 70% collapse in value after it owned up to accounting issues at the start of 2016. The accounts are complicated by the fact that revenue is recognised as legal services are delivered, while there can be significant lags between recognition of income and receipt of cash. Investors then should be sure of the financials before considering committing any cash.

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Motley Fool contributor Tom Richardson owns shares of Bellamy's Australia and Sirtex Medical Limited. The Motley Fool Australia owns shares of Bellamy's Australia.

You can find Tom on Twitter @tommyr345

We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy. This article contains general investment advice only (under AFSL 400691). Authorised by Bruce Jackson.